The Role of Crop Rotation in Weed Management
Ideally, weed management in an organic cropping system involves the integration of a broad range of cultural practices. Although cultivation after planting is usually a key component, a variety of other factors make important contributions to weed control on organic farms (table 3.6). All of these practices occur within the context of a sequence of crops that are planted on a field—the crop rotation. The identities of the crops are critical for disease and insect management, but for weed management the identity of the crop species is less critical than the type of soil preparation and cultural weed control practices used with each crop. In general, crop sequences that take advantage of multiple opportunities to suppress and remove weeds from the field will improve weed management on the farm (61).
Few studies have examined the role of crop rotation in weed suppression, and most have looked at crop rotation in conventional agricultural systems with herbicides. Such studies primarily indicate the impact of varying the type of herbicide used rather than other factors associated with crop rotation. The few rotation studies without herbicides have generally found that more diverse systems had a lower density of problem weeds but a greater diversity of weed species (60, 62). This is reasonable, since the variation in cultural practices during the rotation will tend to disrupt the life cycle of each particular weed species but create niches for a greater variety of species.
Although the sparse literature offers only general insights into the design of weed suppressive crop rotations, the ecology of weeds offers clues to the sort of crop sequences that are most likely to minimize weed problems. The usefulness of the principles outlined below has been documented by practicing farmers in many cases.
Include clean fallow periods in the rotation to deplete perennial roots and rhizomes and to flush out and destroy annual weeds
Most perennial weed species will resprout after their roots and rhizomes have been cut into small pieces by tillage implements (40). Thus, many new shoots are produced, but each shoot is weakened due to less belowground food storage. If these plants are again cultivated into the soil, they are further weakened. A few such cycles once every two to three years can greatly suppress or eliminate most perennial weeds whose roots are within the plow layer and help keep deeper-rooted perennials in check. Similarly, tillage tends to stimulate the germination of seeds of most weed species (73), and subsequent cultivation will kill the seedlings so they will not compete with future crops. This is an important approach to reducing the density of weed seeds stored in the soil. In northern Pennsylvania, Eric and Anne Nordell have nearly eliminated weeds from their vegetable farm by alternating between a year with a cash crop and a year with weed-suppressing cover crops and a cultivated summer fallow (39). Over the years, they have greatly shortened the fallow periods as weeds have become less problematic.
Follow weed-prone crops with crops in which weeds can easily be prevented from going to seed
Weed control is inherently more difficult in some crops than others. For example, unless mulches are used, winter squash and pumpkins tend to become weedy because cultivation and hand weeding are essentially impossible after the vines have run out of the row. Moreover, weeds have usually set seed by the time these full-season crops have matured. Following such crops with a rapid succession of short-season crops like spinach and lettuce that are harvested before weeds can set seed will kill off many of the seeds produced in the vine crop. This reduces weed problems in subsequent crops. Similarly, small grains cannot be effectively harrowed after the stems begin to elongate, and, especially in spring-sown grain, weeds often go to seed before harvest. The long-term consequences of this seed production can be reduced by following the small grain with an easily cultivated, highly competitive crop like soybean or potato.
Weed control is often more difficult in direct-seeded vegetables than transplanted vegetables because the direct-seeded crops have a prolonged early period when the crop competes poorly and cultivation is difficult. Direct-seeded species like carrot and spinach that have small seeds are more of a problem in this regard than are large-seeded crops like snap beans and sweet corn. If weeds proliferate in the direct-seeded crop, it is usually advisable to follow it with an easily cultivated species that is transplanted or has a large seed.
Plant crops in which weed seed production can be prevented before crops that are poor competitors
Weed control is often difficult in crops like onion and carrot because they are slow growing and cast relatively little shade. Although some weeds establish from the long-term seed bank in the soil, many of the weeds encountered in a given year establish from seeds shed in the previous year or two. Consequently, growing a crop in which weed seed production can be prevented before planting a poor competitor can reduce the amount of precision cultivation and hand weeding required for successful production of the poor competitor. Cropping strategies in which management prevents weed seed production include successive plantings of short-season crops, short-cycle cover crops alternating with clean fallow periods, crops grown with weed-suppressing mulch, and highly competitive crops that are intensively cultivated (for example, potato). The crop types that prevent seed production will vary depending on the practices of the farmer. For example, mulched vine crops may be used as a weed “cleaning” crop because the combination of mulch and a dense canopy effectively suppresses weeds, whereas without mulch, vine crops often contribute to the weed seed bank because they become difficult to cultivate or hand weed late in the season.
If weeds become a serious barrier to the production of noncompetitive crops, growing poor competitors in a special crop rotation in which they alternate only with cleaning crops may prove worthwhile. When weed populations have declined substantially, the rotation can be broadened to include occasional crops in which seed production is more difficult to prevent.
Rotate between crops that are planted in different seasons
Weed species have characteristic times of the year during which they emerge. Common ragweed emerges most readily in early spring and is often a problem in spring-sown organic small grains like barley and oat. In contrast, henbit and shepherd’s purse are typically fall-germinating species and are likely to be found in winter wheat and spelt. Rotations that include both spring-planted and fall-planted crops tend to suppress both sorts of species. The spring-germinating weeds tend to be competitively suppressed by fall-sown grain because it is already well established and growing vigorously by the time they germinate. Conversely, fall-germinating species tend to be destroyed before they can set seed when soil is tilled for a spring crop.
Similarly, competitive spring-planted crops tend to suppress midsummer germinating species like purslane, whereas summer tillage for midseason planted vegetables will kill most spring-germinating weed species before they can set seeds.
Work cover crops into the rotation between cash crops at times when the soil would otherwise be bare
Weeds establish most easily when the ground is bare. Plant canopies suppress seed germination of many weed species by reducing the amount of light and the relative amount of red-wavelength light reaching the soil surface (5). In addition, cover crops compete with any weeds that do emerge. Thus, for example, planting winter rye or mustard cover crops following plow-down of pasture reduced spring weed cover from 52 percent to 9 percent with rye and to 4 percent with mustard (70).
Many studies have shown that grain and legume crops are more competitive against weeds when planted at high density and uniformity (74, 121), and the same principle applies when these species are used as cover crops. Increasing seeding rates by 50 to 100 percent relative to recommended rates for grain or forage production usually produces noticeably improved weed control, particularly if the cover crop is broadcast. Very high-density sowings of competitive cover crops like buckwheat, soybean, and grain rye can completely smother even many perennial weeds. Experimenting on small areas to find the right balance between seed cost of cover crops and weed control is often worthwhile.
Cover crops interseeded into standing cash crops during the last cultivation can help suppress late-emerging weeds but may also compete with the crop (9). Interseeding of cover crops is discussed more fully in chapter 7.
Avoid cover crop species and cover crop management that promote weeds
Many cover crops can behave as weeds if allowed to go to seed due to a delay in mowing or incorporation. Buckwheat and winter grains are particularly prone to cause problems if allowed to seed, because they are fast-growing, competitive species.
Hairy vetch should be avoided on any farm that grows winter grain. Some portion of any hairy vetch seed lot will be “hard” seeds (seeds that are dormant due to a seed coat that does not allow absorption of water). Even if the hairy vetch cover crop is not allowed to go to seed, some of the hard seeds from the original sowing will germinate in subsequent winter grain crops and can severely reduce both yield and grain quality.
Long-season cover crops like red clover and sweet clover are useful for supplying nitrogen and improving soil structure, but they are relatively uncompetitive early in their development and can become weed infested. Because they remain in the ground most of a year or more, annual weeds have time to set seeds, and perennials have plenty of time to increase by growth of roots and rhizomes (underground stems). Such cover crops thus work best when sown with or interseeded into a grain “nurse” crop that can suppress weeds while the legume cover crop develops. Even with a nurse crop, however, weeds can be a problem in the following crop, and many growers choose to avoid red clover and sweet clover before most vegetable crops.
Even hairy vetch or a winter grain like rye may allow seed production by winter annuals like chickweed and shepherd’s purse if the cover crop is not incorporated promptly in the spring. The problem is greatest when the cover crop stand is light or spotty. The problem can be avoided, however, by incorporating the cover crop at the first sign of capsule formation on the weeds.
Rotate between annual crops and perennial sod crops
Although weed seed populations decline more rapidly when the soil is tilled, substantial decreases in populations of most annual weeds occur when sod crops are left in the ground for a few years (79). This occurs by natural die-off of seeds in the soil and because annuals that germinate in a perennial sod are competitively suppressed by the already well-established perennial legumes and grasses. The few annuals that do establish are usually prevented from setting seeds by repeated mowing or grazing. One study found that 83 percent of farmers surveyed in Saskatchewan and Manitoba noticed decreased weed problems following sod crops, and most indicated that the effect lasted more than one year (27). In addition to reducing annual weeds, many of these farmers indicated a reduction in Canada thistle, probably because mowing several times each year depleted food storage in the thistle’s roots.
Precautions are required, however, when alternating sod crops with annual crops. Perennial grass weeds like quackgrass should be well controlled prior to planting the sod, or they will likely increase during the sod phase of the rotation. Also, annual weeds may set many seeds during establishment of the sod crop, thereby negating the expected decline in the weed seed bank. Weed seed production can be minimized by using a grain nurse crop that competes with the annual weeds, early harvest of the nurse crop for forage or straw before annuals go to seed, and subsequent mowing of the sod during the establishment year. Another strategy to reduce weed seed production during sod establishment is to plant the sod crop in late summer or early fall.
Although the impact of crop rotation on weeds will not be seen as quickly as the impact of tillage or cultivation, the cumulative affect of a well-planned rotation strategy can, over several years, greatly decrease weed density. Rotation planning is a key way organic growers can substitute brain power for labor and purchased inputs. This principle applies not just to weeds, but also to diseases, insects, soil nutrients, and soil health, as discussed in previous sections of this chapter.
For further reading, see References 5, 9, 27, 39, 40, 60, 61, 62, 70, 73, 74, 79, 96 and 121.