Many Little Hammers
Wise management of weeds in an organic cropping system involves integration of many separate management tactics. Which tactics you use will depend on the weed species present, the crop, the time of year the crop is planted, the type of equipment you have available, other crops in the rotation, and other site- and operation-specific factors. This is why understanding how weeds operate as species is so critical: Only through this understanding can you effectively match your tactics to the weed problem at hand.
Most of the individual tactics discussed in the following sections cannot control weeds by themselves. Instead, they shift the population dynamics of the weeds so that mortality increases by some percentage and consequently fewer individuals grow into the next size class. Matt Liebman and Eric Gallandt have referred to the use of multiple tactics as the “many little hammers” approach to weed management (Liebman and Gallandt 1997). Instead of attacking the weeds with a single, big hammer like an herbicide that is intended to kill off all the weeds at once, many tactics, each of which may be relatively ineffective when used alone, can be used together to accomplish successful management. For example, tactics that each reduce the number or size of weeds present by only one-half can prove highly effective in combination if they are cheap, easy and compatible with the overall cropping plan. In some cases, one tactic may enhance the effectiveness of another (synergism), so discovering multi-tactic, synergistic combinations will prove particularly effective for managing your weeds.
We introduce cultural tactics in this chapter before discussing tillage and cultivation in the following chapter because organic farmers too often place excessive emphasis on cultivation for weed control. Cultivation is an important part of weed management on many farms, but it can damage soil structure and cause crusting and compaction. If used thoughtfully, cultivation will complement cultural practices so that both become more effective.
Crop Rotation and Weed Management
Rotation between spring, summer and fall planted crops is an important strategy to reduce overall weed problems because it interferes with the life cycles of weed species that have preferred seasons of germination. (see the “Season of emergence” section in each species chapter.) For example, spring germinating weeds will be destroyed during seedbed preparation for summer planted crops, and relatively few individuals of those species will subsequently sprout because the season is not favorable for their germination. During the summer, fall and winter, some of the dormant seeds of the spring germinating species will be eliminated by accidental germination deep in the soil and because earthworms, carabid ground beetles and other soil organisms will consume them. (see the “Seed longevity” section in each species chapter.) Hence, a summer planted crop decreases future pressure from spring germinating weeds.
Similar processes occur when rotating between spring and fall planted crops or between summer and fall planted crops. Fall planted crops like spelt and winter wheat are well established and growing vigorously by the time spring weeds emerge. Thus, spring germinating weed species tend to suffer from severe competition in a fall seeded grain. Furthermore, you will usually harvest a winter grain before the spring weeds set seed. If the field is cleaned up after harvest, for example by light disking and planting a cover crop, seed production by spring germinating weeds can be prevented. Alternatively, if the winter grain was interseeded with forage or a clover cover crop, combining the grain will cut off immature flowering stalks of many spring germinating species, and the interseeded crop will compete heavily with the weeds as they attempt to regrow.
Some crops are hard to keep weeded and others are relatively easy. Also, the best method for weeding varies between crops. Row crops can be intensively cultivated whereas cultivation in grain crops is largely limited to harrowing. Hilling potatoes or mounding soil around the base of corn stalks in a timely fashion can kill most of the weeds present by uprooting those in the inter-row and burying those in the row. In contrast, hilling would ruin lettuce and many other short-statured vegetable crops. Late germinating weeds in corn, cotton or onion rows can be killed by flame weeding, whereas few other crops will tolerate flaming. Some crops like soybeans are naturally highly competitive and effectively suppress weeds after they are well established, while other crops like onions require regular weeding throughout the growing season. Consequently, alternating crops in which you use different weed control tactics varies the types of pressure you can apply to weed populations. This prevents strong dominance by a single hard-to-control species and lowers the abundance of most of the species present.
The length of the crop-growing period is also a critical factor affecting the promotion or control of weeds. Short cycle crops like lettuce, radishes, spinach and mustard greens that are harvested only a few weeks after planting tend to reduce weed populations if you clean up the area after harvest. They are in the ground for so short a time that the weeds do not have a chance to go to seed and thus act in a manner similar to a tilled fallow. In contrast, weeds commonly go to seed in winter squash and field corn, which have long growing seasons and are difficult to weed late in the growth cycle. Consequently, using short cycle “cleaning crops” after long cycle crops in which weeds go to seed helps keep weed populations under control.
Rotating grain and vegetable crops with forage crops can be effective in reducing weed populations. Annual forages like triticale and sorghum-sudangrass cut for silage or hay have been shown to reduce wild oat populations in a subsequent field pea crop, since cutting prevented the wild oat from going to seed. Only annual forages that continued growing the full season, however, were effective for reducing broadleaf annuals. Perennial forages such as alfalfa are particularly effective for suppressing many weed species because the repeated mowing prevents most annual weeds from going to seed and tends to deplete root reserves of many perennials. A survey of Canadian grain fields found that a wide range of weed species were less abundant when grain followed alfalfa than when it followed another grain crop (Ominski et al. 1999). Field pennycress and dandelion, however, were more abundant following alfalfa because field pennycress had time to set seed before the first cutting and because dandelion established easily in alfalfa and tolerated mowing. A study of long-term organic cropping systems in Maryland found that seed banks of smooth pigweed and common lambsquarters sampled before corn were lower following hay than following soybeans or wheat, but that the seed bank of prostrate-growing annual grasses was greater following hay (Teasdale 2018). The weed community on land rotated between perennial hay and annual crops appears to undergo a cyclical change in species composition, with many species decreasing during the hay part of the rotation while others increase. Whether a species increases or decreases depends on its biology but also on whether or not it has an opportunity to set seed during the forage establishment year. Thus, the utility of forage crops for suppressing weeds in a crop rotation will depend on the identity of your problem weeds and on your management practices.
This discussion has highlighted that the diversity of field operations associated with specific crops is as important as the diversity of crops themselves (Teasdale 2018). Each crop has a unique set of tillage, planting patterns, cultivation, fertility, harvest and other operations that suppress those weed species that are not adapted to those operations. Likewise, operations are conducted at different times for each crop, thereby adding to selective suppression of species whose life cycles are unlike that of the crop. Therefore, planning a diverse rotation should also include planning for the diverse set of associated management operations that will target and suppress the most problematic weed species. Carefully planned rotations of crops and operations can be an effective preventive approach that will lower the initial weed populations faced in rotational crops and increase the potential effectiveness of the weed tactics used within each crop.
Weed control should not be viewed as simply a set of tactics designed to kill or impair unwanted weedy plants but rather as an integrated set of cultural practices that give the species being grown (the “crop”) a competitive advantage over competing species (the “weeds”). A large proportion of weed control comes from the weed having to compete with the crop. For example, a study with field corn found that the weight of velvetleaf and Pennsylvania smartweed was reduced 96% and 99% by the presence of the crop (Jordan 1979). You can easily demonstrate the effect for yourself by leaving two patches of ground unweeded for a few weeks in the middle of the season: one in the crop and one that is left unplanted. Both will become weedy, but the weeds in the patch with the crop will likely be much smaller. Thus, crop competition is the foundational element of weed management, and all practices that favor crop growth, from seedbed preparation and selection of vigorous seed, to density and spacing, fertility and water management should be considered part of a weed management program.
The usefulness of crop competition in managing weeds depends on weed density. If weeds are abundant many will likely escape control methods and, in this case, managing crop competition becomes critical. In contrast, if the density of weed seeds and perennial storage organs in the soil is low, then the competitive ability of the crop is less important for preventing yield loss. However, strong crop competition can still be useful for reducing reproduction of the few weeds present.
Crop Vigor and Uniformity
Producing a competitive crop begins with a vigorous, uniform crop stand. Planters should be kept in good repair and adjustment so that seeds are placed at a precise depth without skips. Remember that any skip in the row provides an opportunity for weeds to grow with reduced competition. Similarly, if seeds are placed too deep or too shallow, they may emerge slowly with reduced vigor, a situation that favors weeds. In addition, planting too shallow or too deep, or at an inconsistent depth will cause variation in crop emergence, leading to challenges in timing cultivation for optimum crop size. Generally, grain drills and planters with double disk openers provide more accurate seed placement than those with single disk openers.
Always use high quality seed. Old, moldy or damaged seed will produce a crop stand that has skips, overall low density and slow-growing plants that compete poorly with the weeds. As discussed in Chapter 4, rapid crop growth is important not only for direct competition of the crop with weeds but also for maintaining a size difference that allows for effective mechanical weed control. If you produce your own seed or save seed from a previous year, you will want to test it for viability and speed of germination before you plant. Germination speed is a good indicator of seed vigor and tends to indicate seedling growth rate. Small flat corn seed has the most rapid emergence, but medium flat seed tends to produce faster-growing plants after emergence. Large round corn seeds can be damaged during harvest and consequently have low vigor. And even if undamaged, they take up water more slowly than flat seeds because of less surface area relative to seed volume.
For vegetable crops, be sure transplants are vigorous. Growing mixes vary greatly in nutrient availability, and finding the right mix for each crop can pay off in improved vigor, weed suppression, yield and quality. Also ensure that the transplants are at the optimum growth stage when the time comes to set them out. If transplanting will be early or delayed, try adjusting the temperature in the greenhouse to ensure that the plants are the right size at transplanting. Ensure that the plugs are wet before you put them in the ground, and give them water during or immediately after transplanting to ensure a good start. Adjust press wheels so that 1) the plug is well covered with loose soil to ensure that the spongy potting soil does not wick away the water, and 2) that soil is firmed in around the plug so that roots are not blocked by air pockets. Good planting practices ensure faster crop growth and better competition with weeds. Rapid establishment of the crop also allows earlier and more effective cultivation (see Mechanical and Other Physical Weed Management).
Some evidence indicates that crops tolerate weed competition better when steps are taken to maintain soil quality. For example, in the long-term cropping systems study at the Rodale Institute in Pennsylvania, corn in the organic treatments, which received cover crops and compost, suffered less yield loss from a given level of weed competition than the conventionally managed system that lacked those soil building inputs (Ryan et al. 2010). Similarly, few crops tolerate poor soil drainage, but some weed species like yellow nutsedge and barnyardgrass thrive under such conditions. Hence, practices that improve drainage and water infiltration will tend to shift the competitive balance in favor of your crops.
High crop density provides early leaf canopy closure and more intense competition against weeds within the row than a sparse planting. Note in this regard that plant density recommendations are always developed in weed free conditions. If your field usually has moderate to high weed density, typical recommendations may be inappropriate. In a survey of the many studies on competition between a wide range of crops and weeds at various crop densities, we found that increasing crop density nearly always reduced the growth of weeds regardless of the crop (Mohler 2001 and, for example, Figure 3.1).
Not all crops, however, can tolerate high densities. Crops that produce a single unit (corn, root crops and crops producing heads) may have lower yield quantity and quality at higher than recommended densities. Root crops will tend to make small roots if planted too closely, and this can lead to poor yields in marketable size classes. Similarly, the size of lettuce, cabbage, broccoli and cauliflower heads will shrink with increasing density, and the sprouts of Brussels sprouts will be too small for use if the plants are closely spaced. As the great Dutch agronomist C.T. DeWit once commented, “Brussels sprouts grown at high density are collards.” Sweet corn will tolerate small increases in planting density, but ear size shrinks, and the frequency of barren plants increases with increasing density. Also, a high-density planting can encourage disease or cause lodging. Evaluate where your problems lie!
Any crop that makes multiple units of produce on a single plant (for example, wheat, soybeans, squash or tomatoes) and most leafy greens (for example, chard, leaf lettuce or kale) can be planted at higher than recommended rates without yield loss. Each plant will yield less, but the higher density will compensate. For crops that tolerate high density, yields generally continue to increase with planting density when the field is weedy, whereas yields typically plateau in weed free conditions. The extent to which the extra yield compensates for the extra cost of seed depends on the crop, weed pressure, how effectively the crop can be cultivated and other factors. In our experience, a 50% increase over recommended densities is often worthwhile (see also Strydhorst et al. 2008). A North Carolina study in which soybeans were planted in 30-inch rows at 75,000 per acre to 225,000 per acre and in which the weeds were managed with cultivation found the lowest weed cover and highest net returns consistently occurred at the 225,000 per acre planting density (Place et al. 2009). Even if a higher crop density does not improve net income during the current year, the decreased weed seed production will benefit subsequent crops.
Weed suppression by cover crops increases substantially with dense plantings. Sowing recommendations for cover crops are usually derived from use of the same species for forage or grain. When the crop is used for cover, however, higher densities are possible, and these give substantially better weed suppression. The principle upper limit on cover crop density is the cost of the seed. Winter cover crops sown at very high density (e.g., 10 times a normal rate), however, may suffer winter damage due to the weakness of individual plants.
As a general rule, the closer crop plants come to a square grid arrangement, the more competitive they are against weeds (Figure 3.1). This means that, in principle, narrowly spaced crop rows provide better competition against weeds than do widely spaced rows. In row crops, the need to provide space between the rows for cultivation severely limits options for narrowing row spacing. Work with small grain crops, however, has shown that narrow row spacing often improves both weed control and yield, and that the benefits are magnified further by moderate increases in sowing density (Olsen et al. 2005). Both narrow row spacing and wide row spacing, plus inter-row cultivation, improve weed control in small grains relative to using neither of these practices. One way to halve the row spacing in a small grain is to link two drills such that the second is offset from the first.
In the northern United States, conventional soybeans are usually planted in narrowly spaced rows with a drill. Providing weeds are well controlled, soybeans in closely spaced rows out yield those in widely spaced rows due to more optimal resource use. In contrast, organic soybeans are typically sown with a planter in rows spaced 2.5 feet (76 centimeters) apart to allow room for inter-row cultivation. However, with an adequate cultivator guidance system and narrow shovels, cultivating more closely spaced rows is possible. Rapid canopy closure would reduce the number of inter-row cultivations to one instead of the usual two and would reduce weed growth in the row by decreasing penetration of light from the side. As a general rule, weed suppression from narrow row spacings will be most effective when combined with increased crop density.
Several mathematical models of orchard and row crop canopies have shown that during most of the growing season in temperate latitudes, crop plants intercept more light if the rows are oriented in a north-south direction rather than in an east-west direction (Smart 1973). Late season and winter crops, however, capture more light if planted in east-west rows. The reason north-south rows capture more light during the summer growing season is because, in the early morning and late afternoon, sunlight strikes the side of the row rather than shining on the bare ground between rows (Figure 3.2). Moreover, this is the time of day when light is most limiting to crop growth. The effect increases as one moves south from 55° to 25° latitude.
By increasing light capture by the crop, north-south planting increases shade cast by the crop onto weeds, which should decrease their growth. Clover interseeded into oats had lower density in north-south oriented plots, especially in the crop rows. A study of cover crops interseeded into corn showed that, when rows were oriented east-west, a zone of high light on the south side of rows reached to the base of the crop, whereas with north-south orientation shade was most intense in the row. This result is important since weeds are more easily controlled in the inter-row area than in the rows.
In contrast with summer crops, which should be more weed suppressive in north-south oriented rows, winter crops should be more weed suppressive in east-west oriented rows since the sun will be low in the sky through most of the period of active crop growth. As predicted, winter wheat and barley in Western Australia had greater light interception and usually had greater yield and lower weed biomass in east-west rows than when planted in north-south rows (Borger et al. 2010).
The effect of row orientation should increase with the size of the crop species and the distance between rows. For short crops in closely spaced rows, the benefit will probably be small, whereas in larger crops in widely spaced rows, they should be noticeable. For example, a study of apple orchards calculated a 24% increase in light interception with north-south planting (Jackson and Palmer 1972). The potentially small weed control benefits from adjusting row orientation should be considered relative to potential consequences on operational efficiency, cost and soil conservation.
Many factors enter into variety selection. When selecting varieties, most farmers consider market demand, produce quality, yield and disease resistance more important than ability to suppress weeds. Nevertheless, factors such as vigorous early growth, speed of leaf canopy closure, height and foliage density should be considered as qualities contributing to weed suppression. All crop varieties differ in capacity for weed suppression. If you produce several varieties of a given type of crop, consider planting the more robust, rapidly growing and competitive varieties in weedier fields and planting the less competitive varieties in cleaner fields. For example, the Russet Burbank potato produces more shade and suppresses hairy nightshade better than Russet Norkotah (Hutchinson et al. 2011) and suffers less yield loss in weedy conditions (Colquhoun et al. 2009), so Russet Burbank would be a better choice for the weedier of two fields. Similarly, butternut squash varieties are more competitive than delicata type squash, and Danvers type carrot varieties are more competitive than Nantes type varieties.
Modern cereal grains have been bred for a high harvest index by shifting allocation of plant resources from the stem to the seeds. This increases yield in weed free conditions, but if a short-statured crop is overtopped by weeds, the higher yield potential may not result in greater actual yield. Consequently, some organic growers are experimenting with older, tall-statured grain varieties. Even among short-statured, high yielding varieties, weed competitiveness can vary substantially. Specific recommendations are necessarily constrained by the need for good adaptation to regional climate and soil conditions.
Because seed size affects the rate of growth shortly after emergence and the rate of leaf canopy closure, crop varieties with larger seeds tend to be more competitive than varieties with smaller seeds. Some experiments have shown that when grain and soybean seeds are screened into size classes, planting the large size seeds resulted in a crop that competed more effectively against weeds (Stougaard and Xue 2005). Note that if the seed is larger then more pounds of seed must be planted per acre to achieve the same planting density. Some work, however, has shown that the increased yield from the larger seeds can more than compensate for the increased seed cost (Stougaard and Xue 2005). In principle, a grower could invest in either larger seeds or in more seeds per acre. Since 1) the mechanisms increasing competitive ability may differ between seed size and seed density and 2) the incremental effects of both factors decrease as seed size and density increase, the optimum strategy may be to increase both seed size and density moderately rather than concentrating investment on one tactic or the other. Thus, for example, a study on spring wheat competing with wild oat showed a 12% yield increase with higher seed density, an 18% increase with larger relative to smaller seeds and a 30% yield benefit from using both a high density and larger seeds (Stougaard and Xue 2004).
Use of Transplants for Small Seeded Crops
Most annual weeds have very small seeds and consequently they establish relatively slowly. If the crop is also relatively small seeded (for example, cole crops, lettuce, tomatoes, leeks, etc.) then growing the crop in weed free soil and transplanting after the plants are well established gives the crop a substantial head start over the weeds. Few experienced organic growers direct seed small seeded crops on any substantial scale if the crop tolerates transplanting.
If the bed was prepared before the transplants are ready, the surface soil should be tilled thoroughly to a depth of 1–2 inches with a harrow or rotovator to kill any weed seedlings before transplanting. This should be done even if weed seedlings have not yet emerged: they are on their way up and you always want to give the transplants the maximum head start over the weeds. The depth of tillage depends on the depth that the dominant weeds in the field are likely to emerge from (see the “Emergence depth” section in each species chapter). You want to kill those weeds that have already germinated and are likely to emerge, while minimizing the number of new seeds that are brought to a shallow depth where establishment is more likely.
Every season has weeds that are well adapted to the prevailing weather conditions at that time of year, and every weed has a period of the year when it is most likely to emerge and grow. This optimal emergence period can vary with location and seasonal weather variability (see the “Dormancy” and “Season of emergence” sections in each species chapter). Generally, crop species will be most competitive when planted during a period when the dominant weed species in a field are less likely to emerge and grow vigorously. This can both reduce the number of weeds emerging and the vigor with which emerged weeds will compete with the crop. Several strategies discussed in this book, including rotation, cover cropping and stale seedbed, will disadvantage weeds by manipulating crop planting and tillage dates relative to weed emergence dates.
Many of our crops have their origin in the tropics or subtropics, for example corn, cotton, beans, squash, tomatoes, peppers, etc. Consequently, they grow most vigorously when the soil and air are warm. As a result, pushing warm season crop species to get an exceptionally early or late harvest puts the crop at a disadvantage relative to the weeds. You may find harvesting warm weather crops outside of their usual season is worth the additional effort but anticipate weed problems and take measures to compensate for them. For example, use low or high tunnels, or ridge planting to promote early soil warming.
Corn is a crop that illustrates this point. Conventional field corn growers in the Northeast and the upper Midwest get maximum yields by planting long season hybrids in late April or early May. Growers transitioning to organic practices often encounter difficulties if they continue planting so early. Wet soil often prevents timely cultivation, and slow emergence and growth of the corn makes it relatively uncompetitive with weeds that are favored by cool spring conditions. Moreover, without fungicidal seed treatments the seeds often rot in the cold soil, leaving skips in which weeds grow out of control. Consequently, many organic growers plant corn two to four weeks later than their conventional neighbors. Although the organic corn yields are often lower than the best conventional yields, the higher soil quality on organic fields often compensates for the shorter season varieties and results in yields above the county average. Organic sweet corn growers who want to produce an early crop increasingly transplant their corn. This allows the young sweet corn to establish in optimal conditions and assures a uniform stand.
Another way to increase the competitiveness of crops is to plant them in mixtures. Some mixtures may be less competitive against weeds if the crops compete more with each other than they do with the weeds, so the mixtures must be chosen carefully.
- Clover or alfalfa overseeded into winter grain in the spring or planted simultaneously with a spring grain establishes too slowly to compete with the grain crop but slows down weed growth when the grain matures and light penetrates through the crop canopy. Similarly, red fescue planted in the fall with winter wheat does not reduce wheat production but helps suppress quackgrass (Bergkvist et al. 2010). Conversely, when grain and hay are sown together, the fast, early growth of the grain suppresses weeds that would otherwise compete heavily with the slower-establishing hay species.
- Lettuce is harvested much sooner than tomatoes if they are planted at the same time. Consequently, a row of lettuce next to a row of tomato plants competes with the weeds when the tomato plants are small and is harvested before it is overtopped by the tomatoes. Similarly, kohlrabi can fill the space between young Brussels sprouts and is harvested about the time the Brussels sprouts start to shade the whole bed.
- Light can penetrate the leaf canopy of sweet corn and allow weed growth. Interplanted winter squash or pumpkin vines that run under the corn will compete with the weeds and improve late season weed control. Squash or pumpkin yield will be greatly reduced (for example, by 50–75%) under the corn, so the total planting should be increased accordingly. You need to carefully plan the planting date and maturity of both crops to avoid trampling the vines when you harvest the corn.
- On intensive vegetable farms, when skips occur in a row due to poor establishment, some other crop can be planted to fill in the gap. If you do not have a crop that fits appropriately into the space, you can sow a rapidly growing cover crop like oats or buckwheat (see “Summer Cover Crops”). You may find it worthwhile to keep a supply of cover crop seed on hand for such emergency purposes.
Nutrients and Water
Several studies have shown that weeds are often better equipped for taking up mineral nutrients like nitrogen, phosphorus and potassium than are the crops with which they compete. Not only do many weeds produce root surface area at a faster rate than typical crops, but they also concentrate nutrients in their tissues (see the “Response to fertility” section of each species chapter). Consequently, highly available forms of nutrients like chemical fertilizers and rapidly decomposing organic fertilizers like blood meal tend to favor weeds relative to crops. In contrast, the slow release of nutrients from green manures and compost tends to favor crops relative to the weeds. Particularly for long season crops, the slow release from organic materials may slow early top growth slightly but encourage a stronger root system and an overall healthier, more productive crop by harvest time.
Most of the mineral nutrition of the crop should come from soil organic matter built up by feeding the soil with green manure and compost. Some crops, like field corn, will often benefit from an additional dose of starter fertilizer banded next to the row, and some heavy feeding crops like broccoli may yield better if given an additional nitrogen source like composted chicken manure after they are well established. Applying such supplements in a band next to the row will avoid feeding inter-row weeds before the crop can reach the fertilizer. Foliar application of a soluble fertilizer like fish emulsion can similarly direct nutrients specifically to the crop, provided weeds are not already established within the crop row.
Similarly, drip irrigation, which applies water next to the crop plants rather than sprinkling it over the whole field, will favor crop plants relative to weeds. This is especially useful for managing weeds along the edges of vegetable beds where they usually receive less shade and root competition from the crop. If the wetting zone is too narrow, however, roots may fail to spread throughout the bed, leading to inefficient use of nutrients released from soil organic matter and incorporated amendments.
Sowing a cover crop into or after the final crop of the year can be an effective tactic for managing weed populations. They compete with weeds during periods when cash crops are not grown, thereby suppressing growth and seed production of weeds that otherwise could replenish their seed banks. Soil preparation for planting and terminating cover crops also can destroy weeds that would otherwise have established and produced seeds. In addition, cover crops provide several advantages to the agroecosystem. Benefits for the soil include prevention of erosion, reduced leaching of nutrients and increased favorable biological activity. The dense root systems and the organic material that is incorporated into the soil in the spring also improve soil structure. As explained in the section “Essential Concepts of Mechanical Weed Management,” maintaining good soil properties by using cover crops makes weeding easier.
Winter Cover Crops
Winter cover crops usually are planted and establish in late summer or fall, have the capacity to survive winter conditions and complete their life cycle or are terminated before cash crops are planted in spring. Winter cover crops directly compete with weeds, particularly species that thrive in cool weather, like common chickweed, shepherd’s purse and quackgrass. They may provide little benefit for control of the warm season weeds that will infest the succeeding cash crop, however, when the crop rotation moves to a cool season vegetable crop or winter grain, the reduced input of cool season weed seeds to the soil may prove helpful. Because some cold tolerant weeds like common chickweed are relatively shade tolerant, ensuring a highly competitive cover crop through high planting density and optimal planting date may be required to effectively control seed production.
Various crop species are harvested on different dates, and this will determine to a large extent the type of cover crop that can follow those cash crops. In the northern states, the only readily available annual cover crops that survive the winter well are winter wheat, spelt, grain rye, triticale, annual ryegrass, hairy vetch and Austrian winter peas. In more southern regions, the list of winter covers increases substantially to include crimson, berseem and subterranean clover, winter barley, winter oats, black oats, and brassica species. Although these are useful in many situations, the cold adapted species are among the most popular cover crops in southern regions of the United States as well as in the north.
Hairy vetch, grain rye and annual ryegrass can all become severe weed problems in winter grain crops, though their potential for weediness seems to vary geographically. Caution should be exercised when using these species as cover crops if winter grains are part of the crop rotation. Regardless of the rotation, no cover crop should be allowed to go to seed without having a plan for managing volunteer plants growing from this seed. Hairy vetch usually has a small percentage of hard seed that does not germinate the year of sowing and thus can infest a field in later years even if the cover crop does not set seed.
Summer Cover Crops
Weeds grow rapidly during warm weather and consequently, large seeded cover crops that quickly produce leaf area will suppress weeds best. Common summer cover crops with large seeds include buckwheat, sorghum-sudangrass hybrid (sudex), cowpeas (also called blackeyed peas) and soybeans. If a crop will not be planted until mid-summer and no winter cover crop was planted the previous year, you can use a spring sown cover crop to protect the soil, suppress weeds and add organic matter to the soil until you are ready to plant. Many organic growers use oats, grain rye, field peas or bell beans (a relatively small seeded variety of the normally large seeded broad bean) for this purpose. On grain farms, mustards can be sown in early spring for a weed- and disease-suppressing cover crop before early-summer-sown soybeans or dry beans. We do not recommend the use of mustard cover crops on farms that produce cole crops since frequent planting of members of the mustard family in the crop rotation can promote disease and insect pests.
Several of the cover crops mentioned above have special properties that are worth noting. White clover forms a low growing sod that works well between beds of long season vegetables on plastic. To prevent the white clover from becoming very weedy, sow it with oats (spring planting) or buckwheat (summer planting) and then mow the nurse crop after the clover is well established. Because they have large seeds, a thick sowing of bell beans, cow peas, field peas or soybeans will completely cover the ground within two weeks after emergence and effectively smother annual weeds. Similarly, buckwheat has large horizontal leaves that cast dense shade and is thus especially effective at suppressing annual weeds. It is relatively short, however, and thus can be overtopped by tall or vining perennial weeds that survive on belowground reserves until they reach sunlight. Sorghum-sudangrass hybrid (sudex) is an extraordinarily competitive cover crop. It grows 6 feet tall or more and can thus compete effectively even with most perennial weeds if you sow it thickly. Height, however, has disadvantages. A tall stand of sorghum-sudangrass can shade neighboring crops, and the long stems are difficult to incorporate when you are ready to plant the next crop. Also, vigorous stands of sorghum-sudangrass require relatively high nitrogen fertility, and when N is in short supply, the cover crop is likely to be thin and weedy. Part of the weed suppression by sorghum-sudangrass is due to allelopathy (Weston et al. 2013). Because of its allelopathic potency, direct seeding of crops should be delayed 10–14 days after incorporating the residue (see the section “Cover Crop Management”).
Cover Crop Mixtures
Grasses are often more competitive against weeds than are legumes, possibly because grasses develop a more extensive and competitive root system and extract nitrogen from the soil profile (Teasdale 2018). Grass-legume mixtures are sometimes more competitive than the grass or legume component alone; for example, rye-hairy vetch mixtures can be more suppressive than either rye or hairy vetch alone. Multi-species mixtures of up to eight species are being investigated as a means for achieving more consistency in productivity and ecological benefits across a range of climatic conditions. Such mixtures often show lower variability and improved productivity compared to the average performance of individual species, although even these multi-species mixtures often produce no greater biomass than the most productive individual species (Teasdale 2018). Mixtures that pair complementary species, such as nitrogen-fixing legumes with non-legumes or highly winter-hardy with partially winter-hardy species, can potentially achieve multiple benefits with greater resilience over a broader range of conditions. However, if a cover crop is used for a single purpose, such as weed suppression, then the single species that functions best for that purpose will often be the best choice. For example, the legume component of a mixture can dilute the weed suppressive ability of the more competitive grass, causing the mixture to be less competitive than the grass alone (Mohler and Liebman 1987, Brainard et al. 2011). Thus, the grass component may be the better choice for pure weed suppression, but the legume component would be important for achieving a combination of nitrogen fertility and weed suppression goals.
Cover Crop Management
Termination of the cover crop has important consequences for weed suppression in the following crop. Usually, you will want to mow a cover crop before incorporating it. This is particularly important if the cover crop is taller than 8–12 inches or if you are using an implement other than a moldboard plow. If the cover crop is not well incorporated, it may recover and compete severely with the subsequent cash crop. Flail mowers work best because they cut the material into short pieces and leave it uniformly distributed on the ground. Rotary mowers tend to bunch the crop residue into clumps that are difficult to incorporate. If the cover crop has grown tall and you are using a sickle bar or disc mower, cut the top first and then cut closer to the ground. Rye stems longer than 12 inches will make rotary tillage completely impractical. Long rye stems also collect on shanks of chisel plows and field cultivators. Hairy vetch is easier to incorporate, but by late spring the stems will become tough and will wrap on implements. Most experienced growers incorporate their cover crops 10–14 days before they plant. This gives the material time to rot and for allelopathic compounds that can harm sensitive crops to decompose (see “Allelopathy”). Many damping-off and root rot fungi thrive on fresh green organic matter, and allowing the green manure to rot for a week or two allows time for these to be replaced by beneficial fungi (Hoitink et al. 1996). A lag between incorporation and seedbed preparation also gives many weed seeds an opportunity to germinate in response to tillage and be killed by final seedbed preparation (see “Seed Germination: Why Tillage Prompts Germination”). Ultimately, the goal is for rapid establishment of a uniform and competitive cash crop.
Cover crops also can be terminated with the residue left on the soil surface as an organic mulch. This option provides a great deal of flexibility for conventional no-till producers who can kill the cover crop with a burndown herbicide at the optimum time for planting spring crops. In addition to herbicides, cover crops can be terminated by mechanical means with equipment such as a flail mower that shreds vegetation and drops it in place, a sickle bar mower that drops intact residue in place, a light disk set to slice over the vegetation and leave it on the surface, an undercutter that severs roots from stems just below the soil surface and a roller-crimper that crushes vegetation. The success of a mechanical approach to terminating cover crops requires waiting until the cover crop is flowering, otherwise it will usually recover and regrow. The requirement to wait until flowering may require delaying cash crop planting. However, delayed plantings can also be beneficial for weed management purposes (see “Planting Date”). Finally, no-till planters need to be fitted and adjusted to plant through the dense layer of cover crop residue on the soil surface. An alternative is strip-till, which allows the cash crop to be planted in narrow tilled strips but retains the cover crop residue on the untilled area between crop rows. When accomplished successfully, no-till production with surface cover crop residue offers multiple environmental benefits besides suppression of emerging weeds, including protection from soil erosion and nutrient runoff, improved rainfall infiltration, reduced evaporation of soil moisture, and increased soil organic matter.
Rotational No-Till Cropping with Rolled Cover Crops
The capacity of cover crop residue to suppress weeds provides an opportunity for no-till production of cash crops without herbicides. Organic growers are often faced with the tradeoff that multiple pre-plant tillage and post-plant cultivation operations are required to manage weeds, but this can lead to loss of organic matter and destruction of soil structure. In theory, cover crops can provide a solution to this dilemma by utilizing a uniform, dense layer of cover crop residue on the soil surface to suppress weed emergence, while at the same time providing the benefits of no-till, adding organic matter to the soil and releasing nutrients to the crop. Recent research has identified the roller-crimper as the tool of choice for cover crop termination because it is fast and leaves the cover crop tissue intact, thus slowing decomposition and maximizing weed suppression. It is typically designed with blades welded to a drum that simply roll down the cover crop stems in one direction and kill the cover crop by crushing the stems where the blades pass over them. The cash crop is usually no-till planted in the same direction to facilitate seed placement.
The production of a uniform, dense layer of cover crop residue is required to maximize suppression of weed emergence. This means that tillage to prepare a seedbed is usually needed to achieve rapid establishment of a uniform cover crop stand. This explains why this approach is referred to as “rotational no-till,” because it relies on tillage within the rotation for maximizing cover crop production, while the cash crop is produced using no-till techniques. Maximum cover crop biomass is then achieved by planting the cover crop early enough in the fall for good establishment before winter and by allowing sufficient growing time in spring to maximize production before termination at flowering. As a rule of thumb, a cover crop residue biomass of at least 7,200 pounds per acre needs to be left after crimping to provide consistently good weed suppression (Teasdale and Mohler 2000). Biomass levels less than this do not completely cover the soil but leave small openings through the dead residue where weeds can receive light and emerge (see discussion of “How Much Mulch” in the “Organic Mulch” section). Achieving such a high production of biomass often requires a cover crop mixture such as a legume and rye.
One drawback to reliance on cover crop residue for weed suppression is that if weeds do escape because of deficiencies in the mulch coverage, the dense mulch and un-tilled soil can hinder post-planting cultivation operations to destroy these weeds. High-residue cultivation techniques can be used, whereby a coulter cuts through residue in front of a flat cultivator shank that moves just below the soil level, severing weed shoots from roots with minimal disturbance of the residue. Generally, cultivation in no-till soils with high residue is not as efficient as cultivation of tilled soils. Thus, cultivation and no-till cover crop mulches are not complementary weed control tactics but instead tend to antagonize each other (Teasdale 2018). On the other hand, growing the cash crop at high densities can be complementary with a cover crop mulch. Research has shown that the cover crop residue reduces and delays weed emergence enough to permit a crop such as soybeans to develop a leaf canopy faster than weeds and enhance the competitiveness of the crop relative to weeds. In addition, reduction of the weed seed population and elimination of perennial weeds in rotational crops prior to no-till crop production can complement cover crop mulching and can greatly enhance the weed-suppressing efficacy of rolled cover crops.
There are several constraints on crop performance in this no-till system, including the difficulty of planting through a dense layer of cover crop residue, the short growing season in northern areas, low soil temperatures under the dense residue, destructive insect populations sheltered by the mulch, and nutrient release that is poorly synchronized with crop needs. The need to plant cover crops early and terminate late to achieve high biomass levels can conflict with the need to harvest and plant cash crops within a recommended time frame. A fall grain crop or a relatively short-season vegetable crop can be planted during the preceding year to allow time for optimal establishment of the cover crop in late summer/early fall. In addition, a sufficiently long growing season is needed after the cover crop flowers and is terminated to grow a productive cash crop. These requirements generally mean that the rotational no-till system works best in the southern or middle latitude states, but usually insufficient time is available to fit a long cover crop growing period into rotations in northern areas. If these constraints can be overcome, this rotational no-till system offers the opportunity for reducing tillage operations in organic crop rotations.
In addition to growing a mulch in place by killing a cover crop before planting, organic materials can be brought in from other locations to mulch the soil surface surrounding crop plants. Mulches of organic materials are highly effective for suppressing emergence of small seeded (that is, less than 2 milligrams) annual weeds. Since most agricultural weed species are small seeded annuals, the use of mulches is broadly effective against many species. Mulches are ineffective, however, for controlling perennial weeds because these have sufficient energy stored in the roots or rhizomes to push shoots up through even very thick layers of mulch. For example, we once observed hedge bindweed sprouting up through an 18-inch-thick pile of bark mulch that was waiting to be spread around ornamentals. Large seeded weeds (for example, greater than 5 milligrams) may also emerge through substantial layers of mulch, though a sufficiently thick and dense mat can suppress all but the largest-seeded annual weeds. Grass weeds can usually emerge through more mulch than can broadleaf species with the same size seeds.
How Much Mulch
Two attributes of a mulch mat are critical for weed suppression (Teasdale and Mohler 2000). The first of these is the number of layers of mulch particles, for example leaves or stems, that cover the soil surface. The second is the fraction of space in a mulch mat that is occupied by solid material rather than air. The importance of the first of these is that more layers of material block more light from reaching the soil surface and a seedling must twist and turn more as it grows up into the mulch. The importance of the second factor is that when the mulch material is more tightly packed, seedlings have difficulty finding gaps in which to grow upward. Also, the more space is occupied by solid material, the less light is reflected from particles and down into the mat.
Since both factors are important, specifying how thick a layer of mulch material needs to be to suppress weeds depends on the type of mulch as well as the species of weeds present. Larger seeded weeds require more mulch for suppression than do smaller seeded species (Mohler and Teasdale 1993, Figure 3.3). A 5-inch layer of loose straw, hay or leaves that subsequently settles to about 2–3 inches is generally effective against most small seeded annual weeds. Because compost is denser, about 2 inches is usually enough to suppress most weed seedlings. Applying compost at such a high rate, however, is likely to create excessive fertility and stimulate weed growth in subsequent years (see “Nutrient Use”). Compacted hay or straw used as slabs from a bale is highly effective at thicknesses of about 1.5–2 inches. Often weeds emerge between the slabs if they do not cover the ground completely. To compensate for this, thin slabs of mulch (say 1 inch) should be placed over the joints between main slabs to obtain full coverage.
Although dense material like compost and slabs of hay or straw are very effective at blocking growth of weeds up from the soil, they also hold water well and provide a good medium for germination of windblown seeds like dandelion or seeds that are present in the mulch itself. Hay and straw become progressively more prone to support windblown or resident seeds as these mulches rot.
When to Mulch
The optimal time to mulch a crop depends on the crop and the season of planting. Large seeded and transplanted crops can be mulched almost immediately after planting. For summer planted vegetable crops, a heavy cover crop can be mowed or rolled, or new mulch material can be laid and the vegetable crop transplanted through the mulch. For spring plantings, however, you may want to delay application until the soil warms. Also, removing the first flush of weeds prior to laying organic mulch is often helpful. Although a thick mulch eliminates most light at the soil surface, even a homogenous-appearing mulch layer has partial “windows” through which some light penetrates. If weeds have already emerged before laying mulch, more weeds will be positioned to exploit such windows and emerge through the mulch. In contrast, if the first flush of weeds has been removed then fewer weeds will successfully penetrate the mulch.
Weed Seeds in Mulch
The most commonly used mulch materials are straw, hay, compost, tree leaves and bark chips. Regardless of the material used, you should thoroughly check it for weed seeds. Hay, and particularly late cuttings of grass hay, often contain mature weeds and perennial grasses that you do not want in your fields. Straw is generally free of weed seeds but may have thistle seed heads and some grain seed heads that did not get picked up by the combine. Tree leaves are generally free of weed seeds but may contain acorns and other tree seeds that subsequently germinate and compete with crops.
Applying straw or hay mulch is often unpleasant. Doing so by hand is backbreaking, and it is usually scratchy. It is also usually dusty enough that you should wear a dust mask. Bale choppers and blowers are available, however, which can automate the process. The gun type blowers that are used to apply mulch in landscape seedings need to be modified for most work in vegetables. They are useful for blowing mulch between plastic covered beds, provided the beds are swept off afterwards (either by hand or with a rotary brush). Adding a flexible tube allows mulch to be blown in under established crop plants, but you may have to modify the fan speed. Wetting loose organic mulch with sprinkler irrigation before it is subjected to high winds will help keep it in place.
Sources of Mulch
In grain growing regions, grain straw is often plentiful and cheap. The source of straw should be checked to ensure the grain crop was not grown with phenoxy herbicides, which can later volatilize off the straw and injure sensitive crops like tomatoes. In areas with dairy farms, straw will be in demand for bedding, but spoiled hay will likely be cheap and readily available, at least in some years. Growers should assess available sources of organic materials in their region for creative ways to mulch soils while also improving soil quality (Box 3.1).
In hilly regions, many vegetable farms use the best bottom land soils for vegetables but have upland parts of the farm that are underutilized. These upland areas can produce a steady supply of mulch for weed control and nutrient supply for the vegetable fields, provided you replace mineral nutrients (especially P, K and Ca) exported with the mulch. These minerals will end up in the vegetable production soils after the mulch rots or is incorporated, and the soil should be monitored to avoid nutrient imbalances. In particular, build up of K to excessive levels can be a problem when organic mulches are used to the exclusion of other weed management methods.
Notes on Particular Mulches
Some mulches pose special problems and advantages. Bark and wood chips can pose problems in vegetable cropping systems because their high C:N ratio encourages decomposer microbes to take nitrogen from the soil, thereby starving the crops. The problem is not during the year the mulch is applied, since the microbes only have access to the mulch at the mulch-soil interface. But when the soil is eventually tilled, N will be tied up by the decomposing wood.
Unlike hay and straw, tree leaves do not tangle into a mat and thus sometimes blow about and smother small crops. Applying the leaves after the crop is established helps hold them in place. Chopping the leaves also helps prevent movement.
Rye straw commonly releases allelopathic compounds (see “Allelopathy”) that are toxic to other plants. Since small seeded weeds are more susceptible than large seeded or transplanted crops, this can be advantageous. However, in some circumstances, the toxins may also slow crop growth. For example, early growth of corn can be slowed by a rye mulch, possibly due to an interaction between soil cooling, N immobilization and allelopathy. However, application of rye straw after sweet corn is well established seems to be a safe practice.
Continuous No-Till Vegetable Production Using Organic Mulches
Although continuous no-till cropping is generally impractical without the use of herbicides, a few small-scale intensive vegetable farms have developed continuous no-till systems that rely on mulch and hand weeding. Mulch is used to prevent emergence of weed seedlings, and the few that do emerge are hand pulled to prevent reproduction and competition with the crop. After several years, the near-surface seed bank is depleted, and the amount of mulch and hand weeding required can be reduced.
The approach is illustrated by Jay and Polly Armour’s Four Winds Farm in Gardiner, N.Y. (www.fourwindsfarmny.com). This 24-acre farm has four acres in permanent raised vegetable beds, with the rest of the land used to produce grass-fed beef cattle. The cattle manure that accumulates in the barn over the winter is composted with horse manure from a nearby farm to make compost. The temperature in the pile is monitored during composting to ensure that weed seeds in the manure are killed. The compost forms the principal mulch material used on the vegetable beds. When beds are first brought into production, compost is spread 2 inches thick and seeds or transplants are planted directly into the compost. The thick layer of compost effectively isolates the soil surface from the cues that prompt weed seed germination (see “Why Tillage Prompts Seed Germination”). The few weeds that arrive by wind-blown seeds, particularly dandelion, are hand pulled. After nearly 20 years without wheel and foot traffic, tillage or weed seed production, soil on the beds has a high tilth and a very low seed bank near the surface. These factors allow beds to be planted without tillage and with compost applied only to meet the nutrient demands of the crop.
Although repeatedly applying compost at high rates can be expected to create excessive phosphorus concentrations in the soil, if weed seed production is prevented in the early years, the high P may not exacerbate weed problems. In principle, other mulch materials could be used, at least for some crops, once soil tilth is sufficiently good to allow planting without tillage for seedbed preparation.
Box 3.1. Mulched Beds for Onion Seedling Production
Relinada Walker farms 67 acres of organic vegetables and grains near Sylvania, Ga. One of her specialties is organic onion transplants, which she ships to various locations in the South and Northeast. Mulch is a key component of her onion transplant production system.
The first step is to create a clean seedbed. When growing transplants for the late fall market in the Southeast, she starts by plowing under a cover crop of brown-top millet around September 1. She lets this break down for 2–3 weeks before rotovating and creating beds with a bed shaper. This break between operations not only gets the soil ready for making beds but also allows time for a flush of fall weeds to come up and be killed during seedbed preparation. She then ensures that the beds are moist, if necessary by irrigation, and spreads 1 inch of compost as a mulch over the beds. The compost is made of some mixture of vegetable waste, cotton gin trash and peanut hulls, depending on what is available. She adds bark fines to the finished compost to create a fine textured but fibrous material that spreads uniformly over the beds. Although the mulch layer is relatively thin, the pores between mulch particles are small so that no light reaches the soil surface. She sprinkle irrigates the mulch to wet it down and then lets it set for one day, or ideally two. This allows her to be sure that the compost mulch will not reheat. The onion seeds are planted into the mulch with a gang of four EarthWay seeders pulled by a tractor. The mulch holds moisture to facilitate good seedling establishment while suppressing the weeds. Of course, eventually some weeds do come through the mulch, but the mulch keeps the soil loose so that these are easier to hand weed. With this system the onions are ready for transplanting around Thanksgiving.
She uses a similar system for more northerly markets. For shipping to the mid-Atlantic states, she plants in October and pulls the seedlings from late January through early March. Before plantings in January and early February she has used clover and rye as cover crops but is concerned about the possible allelopathic effect of rye on tiny onion seedlings. Consequently, she is still experimenting to find the best cover crop to use with these mid-winter plantings. January plantings are ready to pull in April for buyers in Pennsylvania.
Walker grows one to two acres of onion transplants each year, depending on the volume of advance sales. Using mulch allows her to keep hand weeding costs down and thereby grow a product that other organic farmers can afford. The increased organic matter from the mulch and the lack of weed seed shed during the production of onion seedlings also create a good location for direct seeded rotation crops like carrots.
Many synthetic mulch materials are marketed for use in vegetable production, including plastic films of various colors, spun and woven cloth that is permeable to water, and plain and oiled paper. All of these can control weeds, but they all pose problems as well.
Plastic films come in various colors. Clear plastic warms the soil better than black plastic, but it allows weeds to grow. Normally it is used in conjunction with residual herbicides. Infrared transmitting films warm the soil about as well as clear plastic but suppress weeds by blocking light that triggers weed germination. Plastic films warm the soil for early production and can be highly effective for suppressing weeds in the crop row, where they are difficult to control with cultivation. Plastic films are especially useful for weed control in onions, which grow slowly, and in full season vegetable crops like peppers and winter squash that tend to get weedy late in the season (Figure 3.4). They are also particularly useful for vegetables like trellised tomatoes, where the supports interfere with cultivation and even with hand hoeing.
Weeds frequently grow in the planting holes and may be sufficiently competitive to require laborious hand weeding. In particular, perennial weeds that sprawl or vine will grow toward light entering at planting holes and will thereby be directed onto the crop.
Spun cloth ground covers are similar to floating row covers but are colored brown or black to block light from weeds. These are reasonably effective at preventing the growth of annual weeds. Many perennials, however, can penetrate these materials. Pulling these weeds pulls on the cloth, and that may disturb crops planted in holes in the material. Moreover, great masses of quackgrass and other perennials will cling to these ground covers when they are collected, thereby increasing the expense of disposal.
Woven landscape fabrics are generally much heavier than the spun fabrics (5–7 ounces per yard as compared to 2 ounces per yard for the spun materials). Woven fabrics effectively block growth of perennial weeds for several years. They are much more resistant to tearing than plastic film or spun fabrics and are suitable for long-term installation around grape vines and fruit trees. In this application, the fabric should be covered with bark mulch or rounded pebbles to prevent deterioration by ultraviolet light. Avoid crushed stone, which has sharp edges that can puncture the fabric. After a few years, weed seeds blown in from adjacent areas will germinate in the organic mulch or in soil that naturally accumulates on the fabric. Whereas shoots cannot push up through the fabric, fine roots can penetrate it, allowing establishment of the weeds. In addition, weeds that spread by runners may also begin growing on the fabric. Consequently, the period of effective weed control is often substantially shorter than the lifetime of the fabric.
Organic standards currently require that synthetic mulches be removed annually. Woven fabrics can be reused repeatedly for growing annual crops, but removing debris from the fabric can be laborious. While these durable fabrics are initially substantially more expensive than other synthetic mulches, they can be used for multiple years.
Brown kraft paper can be used as a biodegradable mulch, but it presents several challenges. The rolls of paper mulch are heavy, bulky and difficult to handle. Paper alone is a poor mulch material because it tears easily during installation, and this problem is made worse by wind. Usually, paper mulches will not endure long enough to provide weed control during the entirety of a long season. Kraft paper treated with vegetable oil or other stabilizers is somewhat more durable, but still compares unfavorably with plastic film. Organic certifiers generally allow paper mulch, including non-colored newsprint, to be incorporated into the soil at the end of the growing season.
Although paper is relatively ineffective as a mulch, paper covered with an organic mulch material can be more effective than using either the paper or the organic material by itself. The paper provides a thin but dense layer that blocks weed growth while the organic material on top helps hold the paper down and intercepts light that would pass through the paper.
Recently, high-performing, biodegradable, starch-based plastic films have become available as substitutes for polyethylene plastic mulches. These require careful field application to optimize their performance. The thickness and polymers in the mulch affect biodegradation rates, and weather conditions affect the speed of degradation. The initial area of mulch breakdown is usually along the edge where the mulch is covered with soil. Some starch-based, biodegradable mulches are not approved for soil incorporation on certified organic farms.
Weeds Along Edges of Synthetic Mulch
All synthetic mulches must be anchored along all edges to prevent the material from blowing in the wind. Normally, this is accomplished by pressing soil along the edge of the mulch material, usually with hilling disks attached to the mulch layer. This soil is, of course, above the mulch and so tends to become weed infested. Although the weed roots cannot grow directly down through the mulch, they can grow around the edge and into deep soil. Also, getting cultivating tools close to the mulch without snagging and cutting it is a problem. One way to cope with weeds along the edges of the mulch is to use vegetable knives that are set deep enough to reach in under the mulch without catching it. Alternatively, a rolling spider gang cultivator with the gangs tilted upward toward the bed can be run along the edges. Another approach is to mulch the inter-bed areas, including the anchoring soil along the edges of the plastic, with an organic mulch such as straw. In addition to suppressing weeds, the organic mulch will protect the soil and makes a mud-free path during harvest. A fourth alternative is to apply a band of a natural product burn-down herbicide along the edge of the plastic to kill young weeds (see “Natural Product Herbicides”). Multiple applications may be required for a long-season crop. The effect of these herbicides on various mulch materials has not been well studied, and some may speed deterioration of certain synthetic mulches. We suggest you check with the manufacturer of the mulch or apply a normal application rate to a small area of stretched mulch for a season before using any herbicide extensively on mulched beds.
A different approach to the use of synthetic soil covers for weed control involves covering the soil with a large, opaque tarp for several weeks and then removing it prior to planting. Unlike solarization (see the next section), in which the objective is to achieve soil temperatures sufficiently high to kill weed seeds, the objective of tarping is to smother emerged weeds to create a stale seedbed and to shift soil properties in ways that are detrimental to seed persistence as described below. Tarps are typically 6 mil black polyethylene plastic, which is tough enough to be rolled up and reused repeatedly. Tarps can cover multiple beds, but tarps larger than 30 by 100 feet can be difficult to handle (Maher and Caldwell 2018). Often, beds are prepared prior to tarping, in which case the tarp is used to create a stale seedbed (see “Stale Seedbeds” in Chapter 4). Alternatively, tarps can be used to smother existing weeds, harvested crops or cover crops prior to no-till planting. In the latter application, any living plant biomass should be mowed prior to tarping to help manage residues. Tarping durations are typically 3–10 weeks depending on the cropping plan, though tarps can be applied in the fall as a way to preserve beds and control soil moisture prior to early spring planting. They can also reduce fluctuations in soil moisture and conserve moisture during dry periods. Tarping periods less than three weeks are unlikely to completely kill emerged weeds. Tarps can add crop management flexibility by suppressing weeds and holding beds idle when time, equipment or field constraints limit other types of bed preparation.
Tarping is a relatively new practice and consequently has received little systematic study. Weeds vary in susceptibility to tarping. As might be expected, perennial weeds are generally resistant to tarping but can be stressed by extended tarping periods. Annual weed species also vary in their susceptibility. The mechanisms whereby tarps affect weed seeds in the soil is unknown. The primary mechanism may be the stimulation of germination that results in death due to lack of light. The magnitude of this effect could depend on soil conditions prior to and during tarping, including the intensity of pre-tarp soil disturbance, soil moisture and the time of year. The covered soil is, on average, usually a few degrees warmer than adjacent uncovered plots, even during cool spring weather in the northeastern United States (Maher and Caldwell 2018). Also, since growing plant roots are not present to take up nitrate and water cannot percolate through the soil to wash away nitrate, nitrate levels rise with the period of tarping (Ryberger et al. 2018). Both elevated temperatures and the presence of nitrate in the soil are known to stimulate the germination of many weed species, and the resulting seedlings would then die in the soil or after emergence under the tarp. Elevated soil nitrate after tarp removal could also favor some weeds in the following crop. Biological activity that is detrimental to weed seeds may also occur under tarps, but such effects have not yet been demonstrated.
Most synthetic mulches pose significant end-of-season disposal problems. The labor and cost of disposing of large amounts of dirty, and probably wet, material should be considered when contemplating the use of these materials. If biodegradable mulches are not gathered and composted, they can be difficult to fully incorporate and fragments can blow about, leaving the farm an unsightly mess. Biodegradable mulches vary in how quickly they decompose. Combining paper mulch with an organic mulch material like straw or compost improves decomposition of the paper and helps hold it in place during the growing season (see “Biodegradable Mulches”). Organic standards currently require complete annual removal of most synthetic mulches other than paper, even if they are biodegradable.
Weed Management During Transition to Organic Production
Organic vegetable farms are often established on old hayfields or pastures. This shortens the time until certification since semi-abandoned land often receives no chemical fertilizers or pesticides. Such fields, however, often have severe infestations of perennial weeds and dense seed banks of annuals.
Avoid planting vegetables the first year when you are starting vegetable production on very weedy ground. Instead, till, plant a cover crop, till in the cover crop before perennial weeds get large or annuals go to seed, and repeat this at four- to six-week intervals throughout the summer. This will reduce the weed seed bank and exhaust the storage organs of perennial weeds, while simultaneously building up soil organic matter and soil tilth (see “Tilled Fallow” and “Soil Tilth and Cultivation”). Oats or barley planted early in the spring at a high seeding rate make suppressive cover crops for the beginning of the season. Buckwheat and sorghum-sudangrass are fast growing, competitive summer cover crops. Near the end of hot weather, plant oats or an oat-field pea mixture on land that will be planted early the next year. This cover crop will compete with weeds in the fall, but frost kill in most regions, leaving the field ready for early planting. On fields that will be planted to vegetables after the last frost in the spring, plant hairy vetch, rye or a mixture in early fall (see “Winter Cover Crops”). To compete effectively with weeds, sow them all at high density (see “Crop Competitiveness”).
Although there are costs to keeping fields in cover crops/fallow for a year before planting, these costs are often less than the labor required to control weeds in relatively uncompetitive crops. Since most growers bring fields into production on a staggered basis as their operation grows, usually opportunities exist for getting the weeds at least partially subdued before planting cash crops.
You have several good options if weed problems seem likely following the fallow/cover crop year. One is to plant short cycle transplanted crops like lettuce and kohlrabi that can be cultivated throughout their growth cycle and are harvested before most weeds get a chance to shed seeds. A second option is to plant sweet corn or potatoes. They can be cultivated with a tine weeder before and after emergence to reduce the density of annuals. They can also be cultivated aggressively between the rows and will tolerate having a lot of soil thrown into the rows to bury small weeds. Unless you can hoe or flame weed the corn, some weeds are likely to go to seed. Further weed suppression in potatoes can be obtained by mulching with straw, and in warmer climates this will keep soil temperatures in a favorable range for tuber production. Both the short cycle crops and intensively cultivated sweet corn or potatoes can continue the cleaning of the soil you began the previous year if they are managed well. A third option is to plant a highly competitive crop like winter squash. The squash can be cultivated until it starts to run, and it will tolerate the weeds that come up later. Unless you mulch heavily, you will likely have substantial weed seed production with this option, but at least you can get a crop off the field. Finally, you can grow a crop that you would produce on plastic for cultural reasons. You will probably have to hand weed the planting holes and possibly the edges of the plastic (see “Synthetic Mulch”), but with only moderate effort you can get a crop and still reduce the weed problem. Laying straw mulch between the plastic strips is initially laborious but saves labor later. If at all possible, avoid planting a slow growing, poorly competitive crop like onions, carrots or parsnips on the field until you have the weeds under control. Onions, however, can be transplanted into plastic mulch, which makes their production in high weed pressure conditions more practical.
Grain and Mixed Grain and Livestock Farms
Grain fields transitioning from conventional agriculture often have two weed management problems. First, the soil may have relatively low tilth, and this can make cultivation relatively ineffective. To understand why low tilth interferes with cultivation and how to improve your soil, see "Soil Tilth and Cultivation" in Chapter 4. The other problem is that you may have high densities of a few weed species that the previous herbicide regimen did not control well. Study the sections on those particular weeds and expand on the general strategy below accordingly.
If you have livestock, the best crop for transition is alfalfa or a grass-alfalfa mix. Repeated mowing will bring many perennials under control, and natural seed mortality will destroy part of the seed bank of annual weeds. Grass or clover will have similar benefits for reducing populations of annual weeds, but the less frequent mowing may not effectively suppress some perennials. If you do not have livestock, perennial sod crops have the same advantages for weed management, but you will export nutrients in the hay or haylage you sell. If your P or K are excessive, that may be desirable since excess nutrients can promote certain weeds. If P and K are very low to medium, however, exporting them in forage could set back your overall transition.
From a weed management perspective, soybeans are also a good crop to begin transition since 1) it is a competitive crop and 2) it can be cultivated aggressively early in the season with a tine weeder or rotary hoe and then later with a row-crop cultivator. From a nutrition perspective, soybeans are also a good crop to start transition since it is a nitrogen fixer and therefore does not require external nitrogen inputs. If the farm has been largely cropped in the past with summer row crops and spring grains, a winter grain crop early in the transition will help reduce weed populations. Due to the history of spring crops, the field will have few fall germinating species, and harvest and post-harvest operations will kill spring germinating species before they set seed (see “Crop Rotation and Weed Management”). For most farms a spring grain crop is a poor crop to start transition because the options for weeding are limited to early harrowing and many weeds will certainly go to seed before harvest. The Farming System Trial at Rodale showed that transitions beginning with corn tended to be weedier than a transition beginning with soybeans or winter wheat (Liebhardt et al. 1989), probably because corn grain is harvested late in the season and because the relatively open canopy allows light to penetrate to late maturing weeds. The low nitrogen status of most soils at the beginning of transition will also reduce the growth potential of high-N-requiring crops like corn, further reducing their competitiveness with weeds. Generally, corn harvested for grain causes more weed problems than silage corn or sweet corn because many more weed seeds will mature before grain has dried sufficiently to harvest.
Soil solarization is an approach for killing weed seeds located near the soil surface. It is suitable for regions with warm climates and intense sunlight. Typically, the soil is tilled, firmed to create good soil-seed contact, irrigated and then covered with a clear polyethylene tarp for several weeks. To maximize heating, the tarp is laid close to the soil surface and the edges are covered with soil. The plastic transmits and traps sunlight, thereby heating the surface soil. Clear plastic is more effective for heating the soil than black plastic (Standifer et al. 1984). The soil is irrigated prior to covering because 1) moist, biologically active seeds are more susceptible to heat damage than are dry seeds, 2) moist soil conducts heat better than dry soil and 3) moistening the soil increases biological activity of microorganisms that attack seeds. Although solarization will kill many dormant seeds, much of the action appears to be against seedlings that result when seeds are prompted to germinate by warm, moist conditions under the tarp. Generally, seedlings are more susceptible to heat stress than are the seeds they come from.
Most weed species adapted to the warm climates where solarization is practical can tolerate temperatures up to 122°F, but few tolerate prolonged or repeated exposure to temperatures higher than that. Many studies have shown the effectiveness of solarization for achieving lethally high surface soil temperatures in warm climates. In an experiment in India, soil temperatures at 2 inches deep under clear plastic exceeded 132°F on 23 out of 32 days and exceeded 140°F for 7 days, whereas temperatures at the same depth in uncovered soil never exceeded 122°F. Similarly, in Mississippi, soil temperatures at half an inch reached 149–156°F under clear plastic, but only 103–122°F in the uncovered soil (Egley 1983). Daily maximum temperatures at 2 inches averaged 18°F higher under the plastic than in bare soil.
The many studies on soil solarization indicate several important points. First, because solar heating of the soil acts primarily near the soil surface, usually the seed bank is substantially reduced only in the top few inches. For example, a study in Louisiana found that annual bluegrass and barnyardgrass seeds were completely eliminated from the top 1.2 inch and substantially controlled down to 2.4 inches, although some reduction in the barnyardgrass seed bank occurred down to 6 inches (Standifer et al. 1984). Since most weed seeds need to be close to the soil surface to produce seedlings, the minimal damage to deep seed banks is not critical for weed control in the crop that immediately follows solarization. However, since viable seeds may remain just below the cleaned soil layer, minimizing soil disturbance before, during and after planting is critical for the success of this method.
Second, the soil must remain covered for several weeks to effectively kill weed seeds (Figure 3.5). This factor largely precludes use of solarization in regions with relatively short growing seasons, even if mid-summer is hot and sunny. Similarly, the long periods of hot, sunny weather required for effective solarization largely limits use of the procedure to late-summer and fall planted crops.
Finally, species differ substantially in how well they are controlled by solarization. For example, pigweed and morningglory species were controlled by three weeks of solarization whereas grasses and horse purslane required longer periods of treatment (Figure 3.5). Purple nutsedge, a perennial emerging from tubers, was not significantly affected by solarization at all. Generally, perennials are not effectively controlled by solarization because they can emerge from large storage organs deep in the soil where the killing effect of the solar heat does not penetrate. Similarly, large seeded annual species can sometimes emerge from below the depth of the well-cooked soil.
In addition to killing weeds, solarization can kill soilborne plant pathogens, mobilize nutrients and increase crop yields. However, the high cost of the approach, the long amount of time the field has to remain covered and the ineffectiveness of the method against some weed species makes solarization only appropriate for selected regions, fields and cropping systems.
Natural Product Herbicides
Several natural product herbicides are approved for use on organic farms. Except for corn gluten, all registered materials are "burn-down" type herbicides that kill or damage only the green tissues they contact. Destruction of the roots of well-established plants requires multiple applications, which eventually exhaust the plant's belowground reserves. Note that these burn-down products are non-selective, meaning that they will kill green crop tissue as easily as weed tissue. Therefore, direct applications away from crop plants.
Acetic acid, the active ingredient in vinegar, is the best-known natural product herbicide. Note that organic standards require that the acetic acid be derived from natural fermentation. Generally, it is most effective against small annual broadleaf weeds that do not have a waxy or densely hairy leaf surface. Apparently, acetic acid does not stick well to waxy surfaces, and the dense leaf hairs on plants like velvetleaf prevent the acid from reaching vital tissues before it evaporates. Tests in several states have shown that acetic acid concentrations of 5–10% are relatively ineffective against even small annual broadleaf weeds, and that effectiveness in the 15–30% range increases with concentration (Brainard et al. 2013). High application volumes whereby weeds become visibly wet are required for consistent control. Grasses tend to resprout after getting burned back. Control is improved when temperature and relative humidity are higher at the time of application (Brainard et al. 2013).
Several other burn-down products based on clove oil, lemongrass oil, citrus oil and capric plus caprylic acid are also available. Trials with these products indicate that concentrations and application rates similar to those for acetic acid are required to achieve reasonable control, but they may be more or less effective against specific weeds or growth stages. OMRI-approved herbicidal soaps are available, but their use is not allowed on organically certified crop land. They do provide a way to burn down weeds along fences and around building foundations where mechanical management can be difficult.
Because natural product herbicides require several to many gallons of concentrate per acre, they are an expensive approach to weed management. Consequently, they are most cost effective when applied to intermittent patches of weeds, such as along the edges of mulch, or in a band, such as to the row just before emergence of slowly emerging crops like carrots. In these applications, inter-row cultivation can be used to inexpensively remove weeds between the crop rows. Natural product herbicides may also find some use in preparation of stale seedbeds for high value crops. In such applications, however, high rates of the active ingredient will be necessary to avoid escapes that then have a substantial head start on the crop.
A corn gluten meal product is currently the only OMRI-approved pre-emergence herbicide. Corn gluten that is not specifically OMRI approved is likely to have been made from genetically modified corn. The active principles in corn gluten are very short chain proteins. The material thus also doubles as a nitrogen fertilizer (10% N). Corn gluten kills a substantial percentage of most weed species during germination. It inhibits root growth, and the seedlings die of drought stress. It is not effective against perennial weeds. A drawback to the material is that it is toxic to most crops when they are germinating and, consequently, the manufacturer recommends waiting 4–6 weeks before planting seeds. Whether corn gluten is more effective for weed control than a 4–6-week tilled fallow seems doubtful. The material can be used safely, however, with transplanted crops. Several crops, including broccoli, cauliflower and strawberries, have been transplanted into soil recently treated with corn gluten without harm, and it is probably safe for many other transplanted crops as well. Since it is a legitimate fertilizer, the material can be used in the many states where it is not registered as an herbicide. The recommended application rate is 446 pounds per acre, which makes it a bulky and expensive way to manage weeds. Other seed meal byproducts from processed mustard and soybean oil crops have shown potential for controlling weeds, but their bulk and cost represent major obstacles to adoption.
Livestock for Weed Management
Farmers have used cattle, sheep and goats to control weeds for many centuries. Most common agricultural weeds make nutritious forage. Of course, some are toxic (Burrrows and Tyrl 2006), and others become unpalatable when they mature (see “Palatability” and the “Palatability” section of each species chapter for details). The most common way growers use large livestock is to clean up fields after harvest. A brief period of intensive grazing is most effective for this purpose since the animals then trample any weeds that they do not eat. Intensive grazing will kill most annuals and set back perennials, but most perennials will resprout after the livestock are removed.
Cattle feed almost exclusively on grasses and herbs and can therefore be used to manage ground vegetation in orchards, though care must be taken to avoid damage to tree roots. You can run sheep in orchards too if the trees are tall, but they will nip buds and young branches from dwarf trees. They will also strip bark if preferred forage is not available. Goats prefer woody browse over most herbaceous plants, so they are inappropriate for use in orchards. For the same reason, however, goats can very effectively reclaim brushy pastures. We once watched a flock of goats attack a patch of the introduced Himalayan blackberry in Oregon. They ate it with relish despite the large thorns that covered the tall, tough canes. Sheep do not eat as wide a range of woody browse as goats do, and they cannot reach as high, but sheep can effectively control many nuisance shrubs in cattle pastures, including red cedar, multiflora rose and speckled alder.
Pigs can root out perennial weeds that are otherwise difficult to control. They are especially effective against species like quackgrass, perennial sowthistle and the nutsedges, where most of the rhizomes, storage roots or tubers are in the plow layer. Breaking the soil, for example with a chisel plow, will allow the pigs better access to the weed storage organs. To minimize damage to soil structure, avoid both prolonged grazing by pigs and grazing when the soil is wet.
A flock of chickens makes an excellent adjunct to a vegetable operation. Since they essentially spread raw manure as they graze, they should not be put into the field before a short season crop. The National Organic Program requires 90 days between manure application and harvest of crops in which the edible portion does not touch the soil and 120 days for crops where the edible produce does touch the soil. Following this standard protects public health regardless of whether or not your farm is certified organic. Chickens can be released into the field after harvest, however, and will pick out weed seeds, clean up perennial weeds and also eat slugs and insect pests. Chickens relish dandelion, quackgrass and most other weeds but will reject some members of the mint and parsley family. They can also help weaken a cover crop in preparation for incorporating it or speed the decomposition of a cover crop after it is mowed prior to incorporation. A low, temporary fence coupled with clipping the wing feathers keeps them confined to the area where you want them. Chickens should not be left in the same area for long periods, however, because their constant scratching will ruin soil structure. Their potential for damaging the soil is particularly great if the soil is wet and the ground is mostly bare.
Geese can also be useful for weed management. They are true grazing animals. They selectively eat grass and a few other weeds (chickweed, horsetail) but avoid most broadleaf species, including most fruit and vegetable crops. Goslings can be trained to eat a wider range of weeds by feeding them the weeds with little choice of other forage when they are young. Although geese have been used successfully in vegetables, particularly potatoes, their droppings constitute a potential health risk that most growers will want to avoid. Geese were widely used in cotton, however, prior to the development of herbicides. They can be helpful in controlling weeds in nurseries and in other non-food crops like cut flowers. They have also been used successfully in perennial crops including orchards, grapes, strawberries, brambles, blueberries and asparagus. These crops often have severe problems with perennial grass weeds, and geese can help manage these weeds. To avoid health risks, the geese should be removed four months before harvest, which in most crops restricts their use to postharvest weeding. Geese should be fenced, and electrified mesh is probably the best choice for simultaneously confining the geese while keeping out predators. Goslings are generally preferred over adult geese for weeding since they are more active and less likely to trample crops. Two to six goslings per acre are sufficient to keep weeds under control if they are released against the weeds early in the season. The optimal number depends on weed density and whether inter-row areas are cultivated. Placing water and supplemental food at opposite ends of the pen encourages the geese to walk through the whole area, weeding as they go (Geiger and Biellier 1993).
Very few farmers will keep livestock just for weed control. Sustainable farming, however, is based on ecological integration. Using your livestock to help control weeds is one more way to achieve that integration.
Preventive Weed Management
Minimal Weed Competition Versus Preventive Management
Farmers take one of two approaches to weed management. Most seek to keep weed populations sufficiently low that they can obtain good yields but otherwise do not worry if their fields have some weeds present. Ecological theory predicts that even minor weed infestations reduce crop yield, and that the impact per weed is actually greatest when weed density is low (Cousins 1985). Variation in yield from year to year and field to field is so large, however, that weed abundance usually has to be surprisingly high before yield loss can be detected statistically. The bottom line is that although weeds usually hurt yield, the impact on profits from a moderate stand of weeds may be too low to matter most of the time. But, given their great powers to reproduce, even maintaining weeds at moderate levels usually requires substantial effort and diligence. Moreover, a moderate density of weeds can hurt your crop in a year when growing conditions prevent timely cultivation or if the crop is stressed.
The other approach to weed management is to consistently minimize weed reproduction through a program of preventive management coupled with attack on soil seed banks and reserves of perennial roots and rhizomes. Farmers who follow this strategy may eventually have fields with low weed populations that allow them to greatly reduce weed management efforts. Their small seed banks and low populations of perennial weeds also buffer them against yield loss to weeds in years when weather makes timely cultivation difficult.
Preventive management often requires more precise cultivation with a wider array of implements. It also usually involves extra hand pulling, flame weeding and hoeing of weeds to prevent seed set, even though the impact of the weeds on the immediate crop may be negligible. Although hand weeding is usually associated with vegetable production, see profiles of Carl Pepper and Paul Mugge in Chapter 5 for examples of applying this approach cost effectively in field crop production. Many growers who follow a preventive management approach use tilled fallows to flush seeds from the soil (see “Tilled Fallow” in Chapter 4) and intensive cover cropping to prevent seed production when a cash crop is not present. If nothing else, seedbed preparation for a cover crop after harvest kills weeds before they can make additional seeds. Preventive weed management sometimes even involves sacrificing a crop by tilling it under rather than allowing heavy weed seed production. The loss of the crop has an immediate cost, but it is a cost that can be partially offset by planting another (often different) crop. Farmers following a preventive management strategy sustain a cost they understand and can manage compared to the unknown costs of future weed problems.
Neither strategy is "correct" (Brown and Gallandt 2018). Farmers following each of these strategies have been highly successful for many years. With the more common strategy of weeding enough to get a good yield in most years, weed management costs vary moderately from year to year, with occasional spikes to bring weed problems under control after bad years (Figure 3.6a). On average, however, the cost of weed management remains about the same over the long run. With this strategy, costs due to yield losses from weeds are generally low but occasionally become substantial in bad years.
With a preventive management strategy, costs of weed management are relatively high in the early years but drop with time along with weed populations (Figure 3.6b). If the preventive strategy is highly successful, the farmer may eventually be able to relax management in later years and actually have lower management costs than the grower following the standard "weed enough to get a good yield" strategy. Greatly reduced weeding may never prove possible, however, if any of a variety of factors prevent perfect control in every year. The great benefit of a preventive strategy is that yield losses due to weeds decline essentially to zero after a few years. Most importantly, the large yield losses due to weeds in occasional bad years disappear.
Cleaning Up After Harvest
Even if you do not follow a strict program of preventive management, steps to reduce seed production will help keep weeds under control. Probably the most important of these is to clean up the field as quickly as possible after harvest. Once the crop is removed, it no longer competes with the weeds and they will then grow and mature rapidly. Depending on the crop, the weeds and the season, delaying cleanup by a week may increase seed shed by several fold; delaying cleanup by a month may increase seed production by a hundred-fold. Cover crops are another important tactic for reducing seed production. Because they reduce weed density and slow growth and maturation of weeds, a competitive cover crop can mean the difference between no seed production and substantial seed production prior to the next crop. Tillage associated with cover crop establishment also can help eliminate weeds that would otherwise mature after harvest. Experienced growers recommend keeping a supply of frequently used cover crop seeds on hand so that cover crops can be planted without delay after crop harvest.
Reducing Seed Shedding During Combining
Combine harvesters normally spread weed seeds throughout a field. Most weed seeds are dispersed with the chaff rather than with the straw. Consequently, several companies have developed systems for collecting the chaff, usually in a wagon pulled behind the combine. In a Canadian study on spring wheat, chaff collection reduced the number of wild oat seeds dispersed by the combine by 77% (Shirtliff and Entz 2005). Although 50–60% of the seeds had already fallen from the weeds by harvest, chaff collection still captured roughly one third of the seeds produced. Chaff collection also greatly reduced the average distance weed seeds were spread and would thereby tend to keep weed problems more localized. As an alternative to chaff collection, a Harrington Seed Destructor has been developed in Australia that mechanically kills seeds in the chaff before the chaff is released onto the field (Walsh et al. 2012). Mechanically killing the weed seeds avoids the need to handle large volumes of chaff.
A related approach is to direct the straw and chaff into narrow windrows that are then burned. This has the disadvantage of destroying crop residues that would otherwise support soil health, and it is not allowed in some areas due to air quality regulations. Nevertheless, the practice is widespread in some regions.
Depending on the weed and the crop, the combine can be set to collect weed seeds along with the grain. The weed seeds can then be separated by subsequent cleaning, thereby removing them from the field. We know a grower who regularly removes wild mustard seed from small grain fields in this way. He sells the seeds to an artisanal mustard producer. Computer models show that removing a substantial portion of weed seeds during harvest can tip the balance between an increasing weed problem and a decreasing one.
The benefits of removing weed seeds during harvest depend on the crop, your particular weed problems and other factors. Most weed seeds have already dispersed by the time corn or soybeans are harvested, though a substantial proportion of seeds of some species like giant ragweed, waterhemp and common lambsquarters remain on the plant well into the fall. In contrast, collecting weed seeds during the harvest of any spring grain will substantially reduce seed dispersal by many weed species. In winter grains, it can often reduce the number of weed seeds reaching the soil from a moderate number to almost none, but the effects will depend on how fast the grain and the weeds mature. The captured weed seed and chaff can be fed directly to poultry, ground with a hammer mill to destroy viability and then fed to other livestock or used for biofuel.
Promoting Weed Seed Predation After Seed Dispersal
Predation of weed seeds can substantially reduce seed populations. A summary of 10 experiments estimated that short-term weed seed predation rates averaged 52%, with the potential for considerably higher losses (Davis et al. 2011). Generally, loss rates from seed predation at the soil surface are higher than losses would be from aging and decay if seeds were buried in the soil. In one study, seed removal by predators was responsible for a 38% reduction in seedling emergence and an 81% reduction in weed biomass in the subsequent season (Blubaugh and Kaplan 2016). However, seed predators do not affect the abundance of seeds already buried in the soil, suggesting that additional tactics would still be needed for long-term seed bank management (Blubaugh and Kaplan 2016).
Both invertebrates and vertebrates contribute to weed seed predation. The most prominent invertebrate predators include carabid beetles and crickets. Invertebrates are generally most active during warmer months in mid to late summer and have a foraging range of tens of yards. The most prominent vertebrate predators are rodents and birds, which can be active throughout the year and have a foraging range of hundreds of yards or more. Generally, the larger the predator, the larger their weed seed size preference. Thus, invertebrates tend to consume smaller seeded weeds and vertebrates prefer larger seeded species. From a broad ecological perspective, seed predators should be considered as part of a larger food web, so their behavior not only reflects search strategies for food sources but also avoidance of predators that can consume them. The preference of most grain-eating beetles and rodents for denser vegetation, and grain-eating birds and ants for open patches, is probably related to their relative need for predator avoidance.
Maintaining fields in an untilled condition is the most important management practice for encouraging weed seed predation because it keeps weed seeds on the soil surface where predators can more easily find them. This practice is most advantageous for reducing a high density of seeds in a year with poor weed control; once seeds are incorporated into the soil seed bank, the opportunities for predation are limited. Leaving seeds on the soil surface is most beneficial when seed shed occurs at or before crop harvest. For example, peak activity by seed predators often occurs in late summer, which coincides with weed seed production in full season crops but is not well synchronized with crops harvested in mid-summer. Caution is required, however, to ensure that leaving fields untilled after harvest does not permit weeds to grow and produce additional seeds, thereby undoing the benefits of predation.
A second practice that usually enhances weed seed predation is maintenance of high vegetation cover (Meiss et al. 2010). Including forage crops, cover crops or dense plantings of row crops in rotations can increase vegetative ground cover and the potential activity of seed predators in fields. Although an abundance of living vegetative cover enhances seed predatory activity, dead crop residue on the field has little influence. In addition, some research has indicated that seed predation is higher near the edge of fields and that increasing the field-edge-to-area ratio may enhance seed predator activity, but other research shows high local variability and the lack of a definitive response to border management.
Generally, seed predation is controlled by a complex interplay of environmental and biological interactions that occur locally within individual fields and can, therefore, be a very dynamic and ephemeral process. Although weed seed predation can provide substantial weed management benefits, weed seed predation should not be a primary factor driving crop management decisions.
Maintaining Clean Field Margins
A final step to help reduce weed reproduction is good management of field edges and driveways. Weeds often survive in a field primarily by seeds entering from untilled land adjacent to the field. Although some individuals within the tilled field may set seed in some years, that reproduction may not be sufficient to maintain the population without supplementary seeds from the field margins. For example, we believe that curly dock and broadleaf dock populations are often maintained in this way in regularly tilled fields. Similarly, field edges may be critical for maintaining populations of creeping perennial weeds and those with wind-dispersed seeds. Your management of the field may be continuously eliminating the weed, but roots or rhizomes grow into the field each year and are then spread about on tillage implements the next season. If a species is more abundant along the long edge of a field than it is toward the middle, this indicates that increased management of the field margin may prove worthwhile. Weeds are often more abundant on headlands due to soil compaction and gaps in the stand as well as seed input from the field margins.
To avoid spread of weeds from the field edges, keep grassy margins and driveways mowed frequently enough to prevent seed production. Regular mowing will also force perennials to put energy into shoots rather than into roots or rhizomes that spread into the field. Where a grass way abuts to a hedgerow or woods, keep the brush trimmed back so that you can mow consistently along the edge. When using a rotary mower along field edges, drive around the field in the direction that throws the cuttings, which contain weed seeds, away from the field rather than into it. Eliminate irregular edges along tilled fields so that the edge is straight or smooth. Old rock piles or encroaching brush that prevent mowing or tillage also create pockets where weeds thrive and then spread.
Preventing the Arrival of New Weed Species
Most farmers who know their weeds eventually observe a new species that has not been on the farm previously. Invasive species from other continents and regions are spreading through the United States and Canada. Other long-naturalized species are shifting their ranges as the climate changes. When a new species shows up on the farm, prudence dictates that you attempt to eradicate it during the year it arrives. Once it has multiplied and spread, eradication becomes much more difficult, and you will likely have to learn to live with the new weed species. Consequently, learn to recognize at least the most problematic species found in your region even if they are not yet common, and keep an eye out for new species as you work. Many resources are available for helping you identify weeds (see Box 3.2).
New weed species can arrive on the farm by any of the means discussed in the “Dispersal” section. You can protect yourself against most of the ways new species arrive.
Weed Seeds in Forage, Cover Crop, and Grain Seed
Corn, soybean and small grain seed produced off the farm should be confirmed free of weed seeds before it is sown, even if you are just using it for a cover crop. Because they have very large seeds, corn and soybeans can usually be cleaned completely. Small grain seed may still have a low level of contamination that is sufficient to start a weed infestation, even after cleaning. Blue tag certified grain seed is usually safe. To give perspective, for grain sown at 110 pounds per acre, a 0.01% contamination translates to about 5,000 1-milligram seeds per acre, though often the contamination level is much lower than the maximum indicated on a bag of certified seed. Always visually inspect non-certified seed for contamination. We have frequently found grain rye being sold for cover crop seed that was severely infested with noxious weeds.
Forage seed often contains weed seeds because many weed species have seeds that are similar in size and density to various forage species. Always inspect forage seed for weed contamination. Be especially cautious of seed produced in other regions, since it is more likely to contain new weed species you want to avoid.
If contaminated seed has weeds of a species that is already common on your farm, calculate how the level of contamination you are observing translates into the number of weed seeds you are sowing. Remember that your soil probably already contains at least several hundred weed seeds per square foot, so it is only the unusual weeds you have to worry about. Resources for visual identification of weed seeds include Davis (1993) and Ohio State University’s OARDC Seed ID Workshop (www.oardc.ohio-state.edu/seedid/), although the latter requires you to guess at the seed’s identity. Alternatively, grow some of the weed seeds out in flowerpots to see what comes up: If you cannot identify the weeds as species already on your farm, then you probably have a potential problem.
Box 3.2. Resources for Weed Identification
Weed identification guides are available for most regions of the United States and Canada.
Alex, J.F. 1992. Ontario Weeds: Description, Illustrations and Keys to their Identification. Consumer Information Centre, Ontario Ministry of Agriculture and Food: Toronto, Ontario.
Barkley, T.M. 1983. Field Guide to the Common Weeds of Kansas. University Press of Kansas: Lawrence, Kansas.
Bouchard, C.J., R. Néron and L. Guay. 1999. Identification Guide to the Weeds of Quebec. Centere ARICO Direction des services technologiques MAPAQ: Quebec, Quebec.
DiTomaso, J. 2007. Weeds of California and Other Western States (2 volumes). University of California Dept. of Agriculture and Natural Resources: California.
DiTommaso, A. and A.K. Watson. 2003. Weed Identification, Biology and Management. Two CD set. HRC Photo.
Gains, X.M. and D.G. Swan. 1972. Weeds of Eastern Washington and Adjacent Areas. Camp-Na-Bor-Lee Association, Inc.: Davenport, Washington.
Georgia Cooperative Extension Service. 1987. Weeds of the Southern United States. Cooperative Extension Service, College of Agriculture, University of Georgia: Athens, Georgia.
Hall, D.W. 1994. Weeds of Florida. University of Florida, Cooperative Extension Service, Institute of Food and Agricultural Sciences: Gainesville, Florida.
Haragan, P.D. 1991. Weeds of Kentucky and Adjacent States: A Field Guide. The University Press of Kentucky: Lexington, Kentucky.
Harrington, H. and R. Zimdahl. 1974. Weeds of Colorado. Colorado State University Extension: Fort Collins, Colorado.
Kinch, S.S. 1975. South Dakota Weeds. South Dakota State Weed Control Commission: South Dakota.
Muenscher, W.C. 1955. Weeds, Second Edition. Comstock: Ithaca, New York.
Stearmar, W.A. 1941. Weeds of Alberta. A. Shnitka, King’s Printer: Edmonton, Alberta.
Stucky, J.M., T.J. Monaco and A.D. Worsham. 1981. Identifying Seedling and Mature Weeds Common in the Southeastern United States. North Carolina Agricultural Research Service Bulletin No. 461: Raleigh, North Carolina.
University of California Statewide Integrated Pest Management Program. Weed photo gallery. http://ipm.ucanr.edu/PMG/weeds_intro.html University of Missouri, Weed ID Guide.
University of Missouri, Division of Plant Sciences. https://weedid.missouri.edu/
Uva, R.H., J.C. Neal and J.M. DiTomaso. 1997. Weeds of the Northeast. Cornell University Press: Ithaca, New York.
XID Services. 2012. 1200 Weeds of the 48 States and Adjacent Canada. Interactive CD. XID Services: Pullman, Washington.
XID Services. 2012. 1000 Broadleaf Weeds of North America for Android. Android App. XID Services: Pullman, Washington.
Whitson, T.D. 2006. Weeds of the West. Western Society of Weed Science in cooperation with the Western United States Land Grant Universities Cooperative Extension Services: Laramie, Wyoming.
Color photographs of many weed species can be found at the Weed Science Society of America (WSSA) website http://wssa.net/wssa/weed/weed-identification/. Additional photographs and distribution information can be found at the U.S.D.A. Agricultural Research Service's PLANTS database, http://plants.usda.gov/.
Weed Seeds in Feed and Forages
You can introduce new weeds onto your farm by purchasing weedy feed grains and forages. Since seeds of most weed species readily pass through the guts of livestock alive, weed seeds can spread with manure throughout the farm in a single season. Note that even a few dozen seeds per ton may be enough to start a serious weed infestation. For example, velvetleaf-infested corn from the Midwest probably caused the rapid spread of this weed across the dairy farms of New York and New England during the 1970s and 1980s.
Discussing potential weed problems with the farmer from whom you are buying feed is probably the best insurance against accidental introduction of a new weed species in feed. The alternative is to inspect the feed for weed contamination. If you are buying grain, attempt to check samples from the bottom of the bin. Small weed seeds tend to sift downward as the bin is filled so the bottom material will likely show them. Carry along a screen and sift off the grain so you can examine the material that is left. If you can identify the seeds of the weeds you already have on the farm, you will recognize any that are unfamiliar. Some Extension agents can also help identify weed seeds. With forages, the easiest way to check for weeds is to walk the field before it is chopped or hayed. Identifying weeds in green chop is usually hopeless. Buying a few sample bales from different parts of the stack and pulling them apart to look for seed stalks, however, can help you avoid buying half a barnful of contaminated hay. The same principle applies to bedding straw, since it will eventually end up in the field as well. We have frequently observed Canada thistle seeds in straw, and we are aware of an infestation of mayweed chamomile that was initiated by infested straw used as mulch.
Weed Seeds in Compost and Manure
Composting can effectively kill weed seeds if it is done properly. This is not easy, however, and if you make your own compost it is safer to use seed-free material rather than relying on the composting process to protect you. The problem with adding weedy compost or manure to your fields is not that you will immediately be overwhelmed with weeds: Usually the weed seed density of field soil is higher than that of manure or compost. Rather, the problem is that you may introduce a new pernicious weed species that will cause management problems for years to come. By the same principle, however, if you only use materials generated on the farm to make compost (including the feed that made the manure), you probably have nothing to fear from weed seeds in that compost.
Many materials used for making compost are commonly contaminated with weed seeds. Late cut hay will certainly contain weed seeds. Straw can be examined for fruiting stalks of weeds, but it is often clean. Leaves from street trees usually have very few weed seeds but may contain traces of toxic materials like motor oil and rubber dust. Cotton gin waste is usually heavily contaminated with weed seeds, and compost made from gin waste is frequently contaminated as well (Norsworthy et al. 2009). You should consider all manure from off the farm to be contaminated unless you have tested it. Horse manure and manure from livestock that have access to weedy pastures or pastures along roadsides are most likely to contain a high density and diversity of weed seeds.
You can test manure and compost for weed seeds by mixing several quarts taken from various parts of the pile with potting mix in a 1:1 ratio and spreading it in flats. Keep the flats warm during the day and cool, but not cold, at night. For example, run the test inside on a windowsill in winter, outside in the summer, and in a cold frame or unheated greenhouse during the spring or fall. Water the flats regularly and observe any weed seedlings that emerge over the following two to three weeks. This test will usually show if weed seeds are present, but it may not accurately predict their density or which species are present, since some seeds may be dormant.
To kill weed seeds during the composting process, the pile should reach at least 140°F for at least two weeks. Some of the more resistant species may not be killed even by this treatment. For a small compost pile, achieving a high sustained temperature will prove difficult. Thoroughly mixing the pile several times is necessary to ensure all the materials are aerated and attain the required temperature for a sustained period. Mixing with a front-end loader is less likely to provide the sustained high temperatures needed to kill weed seeds than using a compost turner. Whenever you use compost that is potentially contaminated, watch your weed flora carefully for the next year and vigorously attack any new species that appear.
Weed Seeds On and In New Livestock
Weed seeds commonly travel on the fur and soil clinging to the feet of livestock. Clean off any newly purchased animals and dispose of the waste in a way that will avoid introducing weeds into the field. Most species of weed seeds pass largely unharmed through the guts of horses and all ruminants. Isolate newly purchased animals for a few days indoors or in a small exercise yard until seeds have passed before releasing them onto pastures. Dispose of any manure collected during those few days. Monitor the exercise yard and promptly eliminate any weed species you don’t recognize.
Weed Seeds on Machinery
Tillage and cultivation implements commonly transport weed seeds and storage organs of perennial weeds between fields. The problem arises primarily in two situations: 1) when you are trying to contain a problem weed to one part of the farm and 2) when you are doing custom operations on another farm or having them done on your farm. The more soil that clings to an implement, the more likely you are to transport weed seeds. Plows and disks can transport a lot of soil. For example, we once cleaned 46 pounds of soil off of a 6-bottom plow after it had traveled 1.5 miles back from the field. Cultivators usually carry less soil, but the hooked arrangement of shovels is effective at transporting perennial roots and rhizomes. Rotary tillers are also good at transporting perennials because shoots wrap around the tine shaft, and they drag roots and rhizomes along with them. Tractor tires can also carry a substantial amount of soil.
Some idea of the potential of weed seed transport in soil can be judged from the density of seeds in the soil. Weed seed densities often reach several hundred seeds per square foot of soil. A density of 100 seeds per square foot in a plow layer 8 inches deep translates to about one seed per half pound of soil. Thus, the risk of transporting seeds in soil is relatively low until the weed becomes abundant in the field. The chance of picking up seeds on equipment increases, however, if conservation tillage leaves the seeds concentrated near the soil surface. Also, species are often more abundant on headlands, which tend to be tilled or cultivated last—just before the implement leaves the field. Finally, seeds of weeds along field edges can easily get stuck in the soil clinging to tractor tires. Thus, watching for unwanted species on headlands and field edges when doing custom work can help prevent weed spread.
If transport of an unwanted species seems likely, wash your implement either before or after transporting it, depending on which is appropriate. A power washer is more effective than a hose for cleaning caked soil out of crevices in implements. The best place to wash is in the field before you leave, but that is rarely practical. A thick, healthy lawn is a good place to clean implements since few agricultural weeds can establish in a frequently mowed lawn. Avoid washing on weedy areas where you park implements, since establishing a population there will lead to seeds subsequently getting carried into the fields on tires. Also, avoid washing the soil into road ditches. Weed populations often establish and spread along ditches and embankments first before subsequently spreading into fields.
Combine harvesters are probably even more effective than tillage implements for transporting weed seeds between fields. They have a variety of locations where seeds can lodge and then later become dislodged. One study found that more weed seeds accumulated in the central divider assembly of a corn head than elsewhere, and this can be easily cleaned with a vacuum. Forage harvesters have been shown to move seeds long distances within fields when chopping corn (Heijting et al. 2009), and this indicates their potential for dispersing seeds between fields as well.
Weed Seeds in Irrigation and Flood Water
Most common agricultural weeds will survive for several months completely immersed in fresh water, and many can survive immersion for more than a year (Comes et al. 1978). Moreover, flower and fruit parts cling to the seeds of many species, trapping air and providing buoyancy. Consequently, seeds can be carried long distances by flood waters and deposited in low lying fields when the flood recedes. Therefore, watching for and eliminating new species that show up after floods can prevent weed problems later.
Many weed species are also dispersed in surface irrigation water (Kelley and Bruns 1975). For example, 164 different weed species were found in water from the Columbia River and irrigation canals supplied from the river. The density and diversity of weed seeds in irrigation water can be reduced by managing vegetation along canal banks and drainage ways feeding into the canals, for example, by grazing or tillage (Kelley and Bruns 1975). Ultimately, however, the most effective way to prevent new weed species from entering a field in irrigation water is to screen the water before it is distributed. Since the density of most species in irrigation water is low, growers should not assume that just because a field has been irrigated for many years, all potential species have already arrived. Moreover, water courses of all types can act as effective routes for the dispersal of invasive species new to the region, as is demonstrated by the close correlation of giant hogweed sites in the United Kingdom with stream courses.
Awareness of Emerging Weed Problems
Knowledge and vigilance are the keys to successfully preventing outbreaks of new weed species and managing existing problems. To repel new invaders, you need to know that they are new to your farm. Moreover, to use the species-specific strategies outlined in later chapters of this book, you need to know which species you are trying to control. Although distinguishing between a few closely related species can be challenging, for the most part weed identification is not difficult. Learning to identify the weeds on your farm plus a few likely potential invaders will pay dividends for you and for future generations.
Chapter 3 Summary
- The core principle of ecological weed management is the integration of multiple tactics that attack weed populations at multiple points in their life cycles.
- Sound ecological weed management begins with a diverse crop rotation that allows implementation of the preceding principle.
- Rotation between crops with different planting dates, growth periods and harvest times tends to disrupt weed reproduction. Rotation between different types of crops allows for the use of diverse tactics during the overall crop rotation.
- Enhancing crop competition is a critical component of weed management and becomes more important as weed pressure increases. Increasing crop density always improves competitiveness of the crop, and narrowing row spacing often improves crop competitiveness as well. For summer crops, north-south oriented rows are most competitive, whereas for cool season crops east-west oriented rows are most competitive. Use of crop cultivars that are good competitors can facilitate weed management. Use of transplants increases the competitiveness of small seeded vegetable crops. Sometimes, additional crop competitiveness can be achieved by growing two crops together, particularly when one matures more quickly than the other.
- Nutrient amendments like green manures and composts with high organic matter content tend to favor long season crops like corn relative to weeds, whereas amendments that rapidly release nutrients like poultry manure and Chilean nitrate will tend to favor weeds. Banding rapidly released amendments next to the crop row or side dressing them after crop establishment will help direct nutrients to the crop rather than to the weeds.
- Cover crops can contribute to the diversity of crop rotations and provide opportunities to suppress weed growth and seed production during periods between cash crops, in addition to their many benefits for improving soil quality. To achieve good weed suppression, attention should be given to planting cover crops to ensure a dense, uniform stand.
- Cover crop residue can provide weed suppression for no-till crop production. When cover crops flower, they can be killed with a roller-crimper and the next cash crop can be planted directly into the residue. If the cover crop has been sufficiently productive, the residue will suppress the emergence of most annual weeds. This procedure allows the soil conservation benefits of no-till planting for some crops within the crop rotation.
- Organic mulch materials can also be brought to a field or bed and spread to suppress emergence of annual weeds. Weed species vary in their susceptibility to suppression by mulch, with smaller seeded species more susceptible than larger seeded species.
- A variety of synthetic mulch materials are available to provide a physical barrier to prevent weed emergence. These vary greatly in durability and difficulty of disposal after harvest.
- When transitioning an old hay field to organic vegetable production, reducing perennial weeds and the weed seed bank prior to planting the first vegetable crop is often less costly than expending labor to manage extreme weed pressure. When transitioning a field from conventional to organic field crops, perennial forage crops or soybeans are likely to have fewer weed problems and lower yield loss than corn.
- In warm, sunny climates, covering the soil with clear polyethylene tarps for several weeks can heat-kill most weed seeds in the top few inches of soil (solarization), thereby creating a weed-free seedbed for high value crops.
- Natural product herbicides like acetic acid and essential oils can effectively burn down weeds if used at high concentrations. They are, however, expensive and will damage crops, so their use is restricted to localized directed sprays or pre-emergence applications (e.g., for creating a stale seedbed).
- Livestock can provide substantial weed control. Goats and sheep can clear brushy pastures. Pigs can root out storage organs of difficult-to-control perennial weeds. Chickens can clean up weeds and weed seeds after harvest. Geese can remove grassy weeds from cotton, fruit and perennial vegetable crops. Maintenance of food safety requires thoughtful timing in the use of livestock for weed management.
- Strategies focused on preventing weed reproduction can help reduce weed problems and the expense of weed management in future years. Some approaches to prevention include hand rogueing, promptly cleaning fields after harvest, capturing or destroying weed seeds during combine harvesting, and maintaining clean field margins.
- Simple steps can block the arrival of new weed species onto the farm and thereby prevent future problems. These steps include careful inspection of forage, cover crop and grain seed, feed grain, and purchased hay and straw; knowledge of weed problems on farms supplying your forage, compost or manure; precautions to prevent arrival of weed species on or in the guts of newly purchased livestock; cleaning machinery that has been on other farms before using it in your fields; and filtration of surface irrigation water. Awareness of new weeds moving into your region and an understanding of weed problems on other farms in the neighborhood can provide early warning of problem weeds that may show up on your farm.
Chapter 3 References
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