Manage Weeds On Your Farm

Part II. Major Agricultural Weeds of the U.S.

SARE Outreach
Charles L. Mohler, John R. Teasdale, Antonio DiTommaso | 2021 | 416 pages

How the Species Chapters Were Developed

The species treated here include the most common and difficult to control agricultural weeds in the United States and southern Canada. The species were chosen largely through a systematic analysis of Bridges and Bauman (1992), which lists the 10 most abundant weeds in each of the major crops grown in each of the 50 states. Space limitations and lack of information prevents us from covering all of the less common, though not necessarily less troublesome, species encountered by farmers. The common names used in this book are those of the Weed Science Society of America’s Composite List of Weeds (http://wssa.net/wssa/weed/composite-list-of-weeds). However, we also provide a list of other common names by which the species may be known in various parts of North America. Scientific names follow the USDA Plants Database (https://plants.usda.gov). Taxonomic controversies, such as whether waterhemp is one species or two, and the proper family in which to place common milkweed, have been bypassed by following the USDA Plants Database.

Information on non-herbicidal methods for managing most individual weed species is sparse. Published sources were consulted when these could be located, but many of the management recommendations for particular species were developed directly from the ecological behavior of the species. Thus, many recommendations for individual species have not been field tested or have been tested only in a preliminary manner. They should be considered informed suggestions as to how to cope with particular problems. In any case, you should always try a new management method on a small area before applying it to the whole farm. We, the authors, will appreciate your contacting us directly with accounts of successes and failures using the procedures we suggest.

The ecological information compiled for each species came from a variety of sources. Where literature reviews of a species were available, we relied heavily on these but consulted the primary sources when the information supplied in the review was ambiguous or contradictory. For many of the species, no general review was available, and the entire account was constructed from primary sources. For many species, information on one or two aspects of the species' ecology could not be located. In particular, information on plant response to disturbance and soil conditions (drought, fertility and physical conditions) was often sparse or anecdotal. We give what information could be found, but in some cases, the information is not the aspect of the species that one might most want to know. In chapters that cover two or three related species, sometimes information on a topic is only available for some but not all of the species, and then the lack of information is particularly noticeable.

References have been reduced to a few further readings. However, the taxonomy chapters with full documentation are available at https://weedecology.css.cornell.edu/. In addition to these formal sources, however, we relied substantially on our 100+ years of personal observations on these weed species as well as on observations of many farmers and colleagues.

Some categories of information have inherent problems of interpretation. The tolerance to drought and shade has been studied for few weed species. Usually, only the general impression of those who have worked with the species is available. When quantitative data could be located, summarizing these in terms that are useful for a grower has sometimes been difficult. 

Most seed survival studies have placed seeds deep in the soil and left them undisturbed. Such studies provide an upper limit on how long the seeds last under optimal conditions for survival. Typically, seeds die off at a constant rate, and the depletion of the seed bank is faster near the surface and faster when the soil is disturbed by tillage (see Chapter 2). Where data on depletion of seeds in disturbed soil were available, we have converted them to percentage loss per year. This provides a more direct way to think about how management of the seed bank will affect weed pressure. For some species, however, survival data vary substantially between experiments.

Usually, seed production (described for each species in the “Reproduction” section) has been measured on plants that emerged at the optimal time of year and grew with little or no competition. Under extreme competitive stress from crops and/or other weeds, most weed species produce very few seeds per plant. Most agricultural situations are intermediate between these extremes. Early flushes of weeds are often controlled, and the crop provides some competition. Thus, most weeds in an agricultural field often produce only 0.1–5% of the published maximum seed output. We have tried to specify when data are derived from plants growing with minimum competition and when data are derived from plants growing in typical cropping environments. 

We give weights and measures in American units. The one exception is that we consistently give seed weights only in milligrams (abbreviated “mg”). The smallest commonly used American unit of weight is the ounce, but the seeds of most weed species are in the range of 0.00001 to 0.0001 ounces. Such numbers are difficult to read and even more difficult to comprehend. As explained in Chapter 2, however, seed weight is a very important property of a weed species, and the difference between 0.00001 ounces (0.28 mg) and 0.0001 ounces (2.8 mg) has substantial implications for management. Most varieties of lettuce seeds weigh close to 1 mg, so if the mg is an unfamiliar unit, just think of seed weights as multiples or fractions of a lettuce seed.

How to Find Ecological Information and Develop a Management Plan for Species not Covered in this Book

Many species that have limited geographical range or are problems only in a few crops or in particular circumstances could not be covered in this book. Nevertheless, some of those species pose substantial problems to particular growers. The basic concept of this book is that understanding the ecological characteristics of a weed provides insight into how to manage the species. Consequently, when faced with managing a weed not covered here, begin by gathering critical information about its ecology. 

Begin by identifying the weed species. Each taxonomic level (family, genus and species) potentially provides additional information about the weed. For example, species in the mallow and morningglory families usually have hard seeds that do not germinate until the seed coat softens. The genus potentially provides additional information—for example, the genus Amaranthus (pigweeds, amaranths and waterhemps) are heat loving C4 plants. But the species within a genus often differ in ways that affect management, and having a positive identification to species will be helpful. Many resources are available for helping identify weeds (Box 3.2). Your local Cooperative Extension office or crop consultant can also help. 

Ecological information about individual weed species is not widely available. That is one reason we have compiled so much of it in this book! A search of internet sites may locate a Cooperative Extension fact sheet or scientific papers on the species that contain useful information. Information on some weeds can also be found in the series "Intriguing World of Weeds," originally published in the journal Weed Technology but available also at the WSSA website https://wssa.net/wssa/weed/intriguing-world-of-weeds/. Some weeds found in the northern United States and Canada have been thoroughly described in the series "Biology of Canadian Weeds," published in the Canadian Journal of Plant Science. These have been collected into five volumes by the Agricultural Institute of Canada (Mulligan 1979, 1984, Cavers 1995, 2000, 2005). They are likely available in the library at your state land grant university or at a major agricultural college but may be difficult to locate otherwise. Two large compilations of information on particular weeds also contain some ecological information (Holm et al. 1977, 1997). Again, these are likely to be available only in the library of a land grant university or major agricultural college.

Fortunately, you can obtain much useful information by observation. Record your observations in a notebook or in a computer document so that all the information is organized in one place and does not get lost. Careful observation over time will require some effort and persistence. Note, however, that the weed problem likely did not develop in a single year, and resolving the problem will likely require several years. So, investing a little time over the course of a growing season to systematically observe the weed will likely save effort in the long run. The quality and quantity of the information you obtain and your ability to think about it are likely to improve if you share the work and results with a friend. Children can also be recruited to collect the information as a hands-on educational activity. Information you are likely to find useful and methods for obtaining it follow.

  • What sort of weed is it? Is it a summer annual, winter annual, stationary perennial or creeping perennial? Does it grow upright, sprawl across the ground or twine up crops? Do the plants begin life as a low growing rosette that later develops a flower stalk, or does it begin vertical growth immediately? How tall does it grow in your crops? These basic attributes affect a wide range of management considerations.
  • How big are the seeds? Collect some seeds and compare them with the seeds of species discussed in this book. Seed size affects depth of emergence, ability to emerge through mulch, ability of seedlings to grow despite shade from a crop, and likely modes of dispersal (for example, plants that produce many small seeds often move with soil on tires, machinery and livestock).
  • What time of year does it emerge most abundantly in your fields? What other times of year does it emerge in lesser numbers? This will tell you how rotating between crops with different planting times may affect the weed. It will also indicate the best times of year for using a tilled fallow to weed the soil.
  • From what depth do seedlings emerge? You can discover this by carefully excavating seedlings shortly after they emerge. On most grassy weeds, the seed remains attached to the seedling for several weeks after germination, and you can measure the distance from the seed to the base of the shoot. On broadleaf weeds, measure the distance from the primary root to the green part of the shoot. Do not wait too long to make these observations, since most weeds will produce secondary roots near the base of the shoot. Measure 50–100 seedlings and count how many fall into various depth categories: 0–0.5 inch, 0.5–1 inch, etc. Knowing the depth of emergence will tell you what percentage of the seedlings you can hope to uproot with a tine weeder or rotary hoe. If a substantial proportion of seedlings emerges from below this depth, you will need to aim for physical damage and burial of the seedlings.
  • For creeping perennials, where are the storage roots or rhizomes located? Carefully dig down through a patch of the weed and observe where the thickened horizontal roots or rhizomes are located. Are they within the plow layer? Do many lie below the plow layer? This information will tell you whether you can effectively break up the storage roots or rhizomes, or whether you will have to focus on repeatedly killing shoots to exhaust the plant's storage reserves. As you are digging up the plant, see how long the new growth on the storage roots or rhizomes is and use this to guess how fast the plant spreads without the aid of tillage. Test the strength of the storage roots or rhizomes. Can they be worked to the surface to dry during hot weather, or do they fragment too easily for this?
  • When does the weed produce seeds? Does it begin producing seeds the same time of year regardless of when it emerges (then flowering is tied to day length), or does it begin producing seeds at a specific interval after the plants emerge? Knowing when seeds are produced will help you plan rotations that suppress the weed and will help in thinking about how to prevent seed production. For some species, the time of seed maturity is not obvious. Generally, however, if seeds fall out when you shake the plant over a white cloth or into a white bucket, then the seeds are mature. Knowing when mature seeds are shed from the plant can suggest whether they can be collected and removed from the field during combining.
  • Does the species die as the seeds are maturing, or does it continue to flower and set seed for many weeks after the first seeds are dropped? Mark a few plants that are just beginning to flower so you can find them again later. Then visit them every two weeks and note whether they are flowering or shedding seeds. This information will help you plan ways to limit seed production.
  • Are seeds dormant when they are shed from the parent plant? Spread some freshly collected seeds on a stack of two or three moist paper towels on a plate. Cover with another moist towel. Put them in a clear plastic bag and set them in a light, warm, location where the temperature will fluctuate between day and night. A north facing windowsill is often a good choice. Observe whether few or most germinate within a week or two. If few germinate, then the seeds are mostly dormant when shed. This information will help you plan crop rotations and fallows to control the weed.
  • How does the species respond to fertility? Take some relatively poor, exhausted soil of the same soil type as yours (perhaps from a neighbor's farm). Knowing how the chemical analysis of this soil compares with yours will be helpful, but it is not essential. Add various amounts of your usual fertility sources and place the resulting mixes into medium sized flowerpots. To ensure that your fertilization rates are within a reasonable range, compute the area of the pot in acres (a small number!). Then calculate how much compost, manure or other amendment would correspond to a typical application. Try mixes that have 0 (no addition), 1x and 5x your typical application rate. You will want about three pots with each mix. Bury the pots to within an inch or two of the rim in a place where they will not get disturbed but where you are sure to observe them periodically. The edge of a garden may work well. Sow a few seeds of your weed into each pot, or, if it is a creeping perennial, plant two sections of the rhizome or storage root in each pot. After plants emerge, thin down to one or a few plants, depending on the potential size of the species. Also pull out competing weeds. Grow the plants until they flower. Do the plants in highly fertilized soil flower earlier? How does plant size compare in the three treatments? This information will help you discover whether excess or imbalanced fertilization is aggravating your weed problem. Note that the high (5x rate) may correspond most closely with your fields since organic amendments often build up organic matter and stored nutrients.
  • What management operations likely allowed your weed to thrive? Think closely about those situations where your weed is most problematic and those where it is least. What crops are most favorable for it? What cropping practices are associated with that crop that may favor your weed? Think about the degree of soil disturbance, the fertility source, the growth form and competitiveness of crops, the length of the growing season before harvest, and the timing of when all of these occur. An understanding of what cropping practices contribute most to the growth of your weed will provide a good clue as to what practices may control it.

You will likely have several inspirations about how to manage the weed by the time you have collected the information above. To add to these and help integrate them into a comprehensive control strategy, read the accounts of similar species provided in this book. Reading about species in the same genus or family that have similar growth habits will be most informative. Note how the ecology of your target weed is similar and different from each of the weeds you are reading about. Where the characters are similar, see how those aspects of the weed's ecology are translated into management recommendations. Lastly, compile your management tactics into an overall plan of attack on the weed. While composing your plan, recognize that the weed problem is a result of your overall farm management, and that its solution will likely require some departure from business as usual. The solution may be relatively easy, but more likely, you will have to balance the costs of tolerating the weed against costs of changing machinery, the range of crops grown, sources of fertility etc. Although your problem weed is unlikely to disappear entirely, we are confident that you can create an overall management strategy that will allow you to keep the weed in check and improve the profitability of your farm operation.

References

Bridges, D.C. and P.A. Baumann. 1992. Weeds causing losses in the United States. In Crop Losses Due to Weeds in the United States. D. C. Bridges (ed.). pp. 75–147.

Cavers, P.B. ed. 1995. The Biology of Canadian Weeds. Contributions 62–83. Agricultural Institute of Canada: Ottawa.

Cavers, P.B. ed. 2000. The Biology of Canadian Weeds. Contributions 84–102. Agricultural Institute of Canada: Ottawa.

Cavers, P.B. ed. 2005. The Biology of Canadian Weeds. Contributions 103–129. Agricultural Institute of Canada: Ottawa.

Holm, L.G., D.L. Plucknett, J.V. Pancho and J.P. Herberger. 1977. World's Worst Weeds: Distribution and Biology. East-West Center, University Press of Hawaii: Honolulu.

Holm, L., J. Doll, E. Holm, J. Pancho and J. Herberger. 1997. World Weeds: Natural Histories and Distribution. John Wiley & Sons: New York.

Mulligan, G.A. ed. 1979. The Biology of Canadian Weeds. Contributions 1–32. Publication 1693. Agriculture Canada: Ottawa.

Mulligan, G.A. ed. 1984. The Biology of Canadian Weeds. Contributions 33-61. Publication 1765. Agriculture Canada: Ottawa.