Experiment, and Watch for New Developments in Organic Weed Management

eOrganic author:

Mark Schonbeck, Virginia Association of Biological Farming


When dealing with weeds organically, don’t be afraid to try something new! You may read about a new cultivation implement, NOP-allowed herbicide or bioherbicide, cover crop, or crop rotation strategy in a farm magazine that sounds like it might be worth trying on a tough weed problem on your farm. You might think of a novel way to use, combine, modify, or fabricate weed control implements to better match your cropping system or weed complex. A certain cover crop or vegetable variety might seem to compete unusually well against the weeds. Close observations of weed–crop interactions might point the way to an entirely new strategy to give the crop the advantage. Try out any of these ideas by conducting a simple on-farm experiment. Farmer innovation and on-farm trials are the time-honored way that humankind has made advances in farming and food production.

The Farmer as Innovator and Researcher

Ever since the dawn of agriculture, farmers have tried new ways to grow crops and protect them from pests, diseases, and weeds. This is how traditional agricultural systems and crop varieties have evolved over the millennia. Long before land grant universities and agribusiness corporations became our primary agricultural research centers, farmers did their own experiments.

During the rise of modern industrial agriculture in the 20th Century, the role of farmer as researcher diminished as specialists developed more technical, quantitative, and statistical approaches to the science of growing food and fiber. In the past 20 years, however, farmers have regained recognition and confidence as researchers and innovators. Nowhere has this been truer than in organic and sustainable agriculture. The federal Sustainable Agriculture Research and Education (SARE) program, the Organic Farming Research Foundation, and other government and private granting agencies offer competitive grant programs to support farmers to conduct field trials of new practices, farming systems, or crop varieties.

Agricultural experiment stations and research institutes may be better equipped than most farms to conduct highly complex field trials and make precise measurements like response curves of crops and weeds to fertilizer or herbicide rates. However, what better place to assess the practicality of a new tool, practice, crop, or production system than on working farms? Furthermore, farmers often come up with innovative tools and solutions based on a thorough knowledge of their soils, crops, weeds, and production systems, and they are most qualified to discern what practices are most practical and economically feasible.

How to do Experiments on Your Farm

In order to get the most out of on-farm experimentation, there are a few simple rules and procedures to follow that can minimize the risks of trying something new, and help distinguish actual effects of the experimental tool or method from random variation in crop yields or weed populations. It is important to remember that the outcome of every experiment is influenced by chance, as well as the experimental treatments themselves. Conducting an experiment more than once or at more than one site can help determine whether the new tool or method actually enhances weed control.

First, start small. Try an experimental practice on a small area. This reduces the financial risks related to yield losses in the event that the new practice or tool does not work well. Starting small also keeps initial costs down if the new system entails purchasing inputs such as cover crop seeds or mulching materials, or increased labor. If the experimental practice shows promise, try it on a larger scale the following year.

Second, always include a suitable control with which to compare your new or experimental practice. Do a side-by-side comparison of the new tool, tactic or system with your normal practice. Be sure to conduct the experiment on as uniform a field or plot as practical to minimize bias arising from pre-existing differences in soil conditions or weed populations in the areas receiving the experimental and control treatments. Side-by-side comparison on a uniform piece of ground makes it easier to estimate the potential of the experimental practice or system to improve weed control.

Third, consider conducting a replicated trial. Remember that any apparent difference between a single bed with the new treatment and an adjacent “control” bed could be due to random variation rather than any actual effect of the experimental practice, or it could be a combination of both. For example, one bed might have higher nutrient levels than the other because domestic or wild animals left droppings, or some wood ashes were dumped. One bed may have a larger weed seed bank because a large weed was allowed to set seed a couple of years earlier. Such differences can be impossible to detect visually after tillage, crop production, and weather have erased the evidence.

The best way to discern “real” treatment effects from these random variations is to do the comparisons several times in a replicated trial. Lay out three or more pairs of experimental plots. These might be small sections of a single bed (e.g., 5 ft by 20 ft per plot), with one of each pair of plots receiving the new practice or tactic, and the other the “control” or standard practice. To avoid bias, toss a coin for each pair of plots to determine which gets the experimental treatment.

If you are trying out a new cultivation implement or combination of implements, it may be more practical to cultivate one or two tractor widths through the entire length of the field with the experimental tool(s), and cultivate adjacent passes with your normal setup. Compare weed control in the two systems at various points over the length of the field. Whereas this is not true replication, it may give a better indication of treatment efficacy than a single pair of small plots, as it is not so likely that one swath will be consistently more fertile or more weedy than the adjacent swath over the entire length of the field.

If you want to compare more than two treatments (e.g., different cover crops or several ways to set up cultivation tool bars), do so in grouped sets or blocks of plots or beds, with one plot in each block assigned at random to each treatment.

Keeping track of several pairs or sets of plots throughout a growing season can be difficult for busy farmers. Two other ways to replicate a field trial are to conduct the comparison on several farms (a great way to cooperate with other farmers in an effort to improve weed management or production practices), or to do the comparison in two or more successive seasons.

Evaluate the experimental treatment(s) quantitatively by making one or two simple measurements on each plot. Record marketable crop yields, and evaluate weeds by clipping and weighing all the aboveground weed growth in a randomly selected square yard (or other area) within each plot. Crop stand counts and weed population counts can also be useful measures. When all the quantitative data have been collected, draw a simple dot graph showing crop yields, weed weights, and other relevant measurements for each treatment. This can help you discern whether the experimental treatment actually had an effect. For example, if the weeds in a square yard averaged 1.5 lb in the treatment and 3 lb in the control, all four replicates for the former fell between 1.2 and 2.0 lb, and replicates for the latter ranged from 2.4 to 3.5 lb, this would suggest that the treatment actually suppressed weed growth. The two clusters of dots at different levels will clearly illustrate the outcome. However, if the same averages were based on more widely "scattered" data (e.g., 0.1 to 5.5 lb for treatment, and 0.6 to 9.4 lb for control), it is more likely that the difference resulted from random variation. Statistics is a mathematical procedure for evaluating the degree of likelihood that an apparent difference between treatments is an actual difference. For more complex experiments, it is helpful to work with an agricultural scientist who is familiar with basic statistical analysis.

Qualitative observations can provide valuable supplemental information, and may point the way to future experiments. Examples might include "noticed more ladybird beetles in experimental treatment" or "tomatoes in control treatment had less blossom end rot." These could indicate secondary effects of treatments that you will want to take into account in assessing the outcome.

In evaluating a new method or tool, try to assess its costs and benefits in both economic and resource conservation terms. Just because some product or procedure measurably reduces weed growth or enhances yield, this does not necessarily mean it should be adopted. Be sure that dollar costs (e.g., expensive seed for a new experimental cover crop) or adverse effects on soil (e.g., an aggressive cultivation tool) do not outweigh the benefits suggested by field trial results.

For more information on conducting on-farm trials of new practices, experimental design, and statistics, see the additional resources listed at the end of this article.

What’s New and Upcoming in Organic Weed Management

Farmers, agricultural engineers, and entrepreneurs continue to develop new cultivation and other weed control tools, and combinations of tools and tactics. New improved techniques for thermal weeding, soil solarization, nighttime cultivation, or other tactics may become available. Another innovation still in the research phase is the "punch planter." Unlike a planting shoe and press wheel, which leaves a continuous strip of packed soil conducive to germination of small seeded weeds, this device punches individual holes for each crop seed and leaves a loose soil surface, which is less likely to promote within-row weed germination (Rasmussen, 2003).

The numbers of cover crops, mulching materials, and ways to manage them for optimum weed management continue to increase. Researchers are comparing weed tolerance and competitiveness against weeds by different cultivars of cash crops, and have begun efforts to breed increased weed tolerance or weed suppressiveness into some crops. For example, researchers at the Michael Fields Agricultural Institute are evaluating corn varieties and breeding lines to identify those that combine weed suppressiveness with satisfactory yields and harvest characteristics (Goldstein et al., 2008).

While relatively few organic herbicide and bioherbicide products have become commercially available thus far, some additional new products may reach the market in coming years. At least two bioherbicides are now labeled for in-crop use as banded or directed sprays between rows. None can be sprayed directly over the crop. A number of factors need to be evaluated before using these products, including application strategies to reduce crop injury and maximize weed control, methods to improve or maintain efficacy while reducing cost, and whether they are allowed under USDA Organic Standards.

In addition, ongoing research in crop rotations, conservation biological control (weed seed predation), allelopathy, and crop–weed–soil–microbe interactions may eventually point the way to improved practices or cropping systems that suppress weeds. While this research has not yet yielded sufficiently consistent data to support field application at a commercial scale, it may point the way to new tactics that farmers can use to give crops a greater edge over the weeds. Some examples include:

  • Applying mycorrhizal fungi or encouraging their proliferation to give mycorrhizal host crops (grains, legumes, tomato family, cucurbits, onion family) a competitive advantage over non-mycorrhizal weeds including pigweeds, nutsedges, lambsquarters, purslanes, smartweeds, docks, and wild mustards (Jordan et al., 2000).
  • Designing crop rotations that maximize allelopathic effects against a target weed; for example, sorghum, sudangrass, sorghum-sudangrass hybrids, and sweet potato all show strong inhibitory effects on nutsedges (Cheema et al., 2004; Harrison and Peterson, 1991; Rice, 1995).
  • Designing crop rotations that optimize growing conditions for certain vegetables by modifying soil microbial communities or weed populations; for example fall plantings of forage and daikon radish cover crops have strongly suppressed winter weeds yet favored early spring spinach in two years of trials in Virginia (Schonbeck, 2007).
  • Growing and incorporating brassica or other cover crops to promote weed seed decay or kill germinating weed seedlings before they can get established ("biofumigation") (Haramoto and Gallandt, 2004; Kumar et al., 2008)

Whereas none of these has yet been validated sufficiently for Extension agents to recommend their widespread adoption, all are compatible with good organic production practices, and some may enhance weed management in certain soils, regions, crop rotations, or production systems. Although these effects are not usually dramatic, Liebman and Gallandt (1997) point out that ecological weed management often consists of multiple tactics, each having a small effect but all together yielding satisfactory weed control. Again, try these practices on a small scale, observe results, and keep watching the “new science” columns in your favorite farming magazines or newspapers.

References and Citations

  • Cheema, Z. A., A. Khaliq, and S. Saeed. 2004. Weed control in maize (Zea mays L.) through sorghum allelopathy. Journal of Sustainable Agriculture 23(4): 73–85. (Available online at: http://dx.doi.org/10.1300/J064v23n04_07) (verified 23 March 2010).
  • Goldstein, W., A. Wood, and B. Barber. 2008. Methods to breed field corn that competes better with weeds on organic farms. Organic Farming Research Foundation Information Bulletin 16: 18–19. (Available online at: http://ofrf.org/sites/ofrf.org/files/docs/pdf/ib16.pdf) (verified 11 March 2010).
  • Haramoto, E. R., and E. R. Gallandt. 2004. Brassica cover cropping for weed management: A review. Renewable Agriculture and Food Systems 19: 187–198. (Available online at: http://dx.doi.org/10.1079/RAF200490) (verified 23 March 2010).
  • Harrison, H. F., Jr., and J. K. Peterson. 1991. Evidence that sweet potato (Ipomoea batatas) is allelopathic to yellow nutsedge (Cyperus esculentus). Weed Science 39: 308–312. (Available online at: http://www.jstor.org/stable/4044934) (verified 23 March 2010).
  • Jordan, N. R., J. Zhang, and S. Huerd. 2000. Arbuscular-mycorrhizal fungi: Potential roles in weed management. Weed Research 40: 397–400. (Available online at: http://dx.doi.org/10.1046/j.1365-3180.2000.00207.x) (verified 23 March 2010).
  • Kumar, V., D. C. Brainard, and R. R. Bellinder. 2008. Suppression of Powell amaranth (Amaranthus powellii), shepherd’s-purse (Capsella bursa-pastoris) and corn chamomile (Anthemis arvensis) by buckwheat residues: Role of nitrogen and fungal pathogens. Weed Science 56: 271–280. (Available online at: http://dx.doi.org/10.1614/WS-07-106.1) (verified 23 March 2010).
  • Liebman, M., and E. R. Gallandt. 1997. Many little hammers: Ecological approaches for management of crop–weed interactions. p. 291–343 In L. E. Jackson (ed.) Ecology in agriculture. Academic Press, San Diego, CA.
  • Rasmussen, J. 2003. Punch planting, flame weeding and stale seedbed for weed control in row crops. Weed Research 43: 393–403. (Available online at: http://dx.doi.org/10.1046/j.0043-1737.2003.00357.x) (verified 23 March 2010).
  • Rice, E. L. 1995. Biological control of weeds and plant diseases: Advances in applied allelopathy. University of Oklahoma Press, Norman, OK, 439 pp.
  • Schonbeck, M. 2007. Evaluation of frost-killed cover crops for organic spring vegetable production: A supplemental report on experiments conducted July 2006 through June 2007. Submitted to the Organic Farming Research Foundation in October 2007.

Additional Resources


Published July 31, 2009

This is an eOrganic article and was reviewed for compliance with National Organic Program regulations by members of the eOrganic community. Always check with your organic certification agency before adopting new practices or using new materials. For more information, refer to eOrganic's articles on organic certification.