Mark Schonbeck, Virginia Association for Biological Farming
Ecological weed management begins with careful planning of the cropping system to minimize weed problems, and seeks to utilize biological and ecological processes in the field and throughout the farm ecosystem to give crops the advantage over weeds. In addition, mechanical and other control measures are usually needed to protect organic crops from the adverse effects of weeds. This is particularly true in vegetables and other annual crops, for which production practices keep natural plant succession at its earliest stages, thereby eliciting the emergence of pioneer plants that can become agricultural weeds.
While tillage and cultivation can degrade soil quality and increase the risk of erosion losses, many other organic weed management tools (Table 1) are more soil-friendly. For example, a diversified rotation of vigorous cash crops and cover crops can enhance soil organic matter, tilth, and fertility, provided that a sufficient quantity and diversity of residues are returned to the soil to feed the soil life. Grazing livestock after a production crop to remove weeds or interdict weed seed set can add fertility in the form of manure, though intensive grazing can also compact the soil. In the interest of food safety, care must be taken to avoid direct contact of fresh manure with vegetables and other food crop. Mowing and flame weeding (if properly done to avoid excessive heating of the soil itself) are much easier on soil structure than cultivation, and can be just as effective in certain stages of weed and crop development. Mowing or rolling a cover crop to form an in situ mulch can enhance the soil benefits of the cover crop, compared to tilling it in, and can effectively suppress many annual weeds. Other organic mulches, such as straw and chipped brush, add organic matter, whereas synthetic clear or colored plastic films and weed barrier fabrics do not. All mulches are very effective in preventing soil erosion.
|The Grower’s Mind (planning, observation, and ingenuity)||X||X|
|Vigorous Cash Crops||X|
|Opaque Synthetic Mulches (black plastic, etc.)||X||X|
|Conservation Biological Control (conserve weed consumers present on farm)||X||X|
|Tillage and Cultivation Tools and Implements||X|
|Mowers and other Cutting Tools||X|
|Rollers and Roll-crimpers (for converting mature cover crops into in-situ mulch)||X||X|
|Minor and experimental tools:|
|OMRI certified organic herbicides||X|
|Bioherbicides (specific pathogens of weeds)||X|
|Management of soil microflora||X||X|
|Specific crop–weed allelopathic interactions||X||X|
|Classical biological controls for specific weeds (usually against invasive exotic weeds in rangeland and natural ecosystems)||X|
|Clear plastic mulch (soil solarization)||X||X|
Ecological weed management consists of many-component strategies tailored to each region, cropping system, and farm. Matt Liebman and Eric Gallandt (1997) describe the process as using “many little hammers”, including “indirect controls”, such as crop variety, planting date, and nutrient management, rather than relying only on the “direct controls” or “large hammers” of cultivation and herbicides. In their words, “the use of a combination of methods can lead to (i) acceptable control through the additive, synergistic, or cumulative action of tactics that may not be effective when used alone, (ii) reduced risk of crop failure or serious loss by spreading the burden of protection across several methods, and (iii) minimal exposure to any one tactic, and consequently reduced rates at which pests adapt and become resistant.” (Liebman & Gallandt, 1997, p. 326)
The following list outlines twelve key steps toward successful organic weed management that are discussed in greater detail in a series of related articles that can be found on this website. Note that these steps do not comprise a precise linear sequence of instructions; rather they offer a conceptual framework within which each farmer can develop a site-specific strategy. This process requires systems thinking, which views the field as a complex system of interacting components—such ascrops, weeds, soil, insects, and microorganisms—that form a web of relationships, not a linear sequence of cause-and-effect. Similarly, the following steps are employed together in a synergistic manner, and thus differ from the sequence of instructions for assembling a car or a farm implement. For example, Step 6 (cover crops) can be seen as a part of Step 2 (minimize niches for weeds), and Step 1 (know the weeds) provides vital information for other steps, particularly steps 3 (keep the weeds guessing), 4 (design for effective weed control), and 7 (manage the weed seed bank). Biological processes (Step 9) include indigenous biocontrols that help reduce the weed seed bank (Step 7) as well as the competitive and allelopathic effects of cover crops (Step 6). Step 11 (observe weeds and adapt practices) is an ongoing feedback loop that informs and fine-tunes all the other steps. Utilizing this or another suitable framework, the organic grower selects and assembles a set of “many little hammers” that, working together, keep the farm’s weeds from becoming major weed problems.
1. Know the Weeds Obtain correct identification of the major weeds present on the farm. Monitor fields regularly throughout the season. Keep records on what weeds emerge at different seasons, and on efficacy of any preventive and control measures taken. Learn each weed’s life cycle, growth habit, seasonal pattern of development and flowering, modes of reproduction and dispersal, seed dormancy and germination, and how the weed affects crop production. Find the weed’s weak points—possibly the stages in its life cycle that are most vulnerable to control tactics—and stresses to which the weed is sensitive; these can be exploited in designing a management strategy.
"Know the weeds" is listed first because it informs most of the succeeding steps. However, gaining a thorough knowledge of the farm’s weed flora is an ongoing process over many seasons (perhaps the lifetime of the farmer!) that drives the year-to-year refinement of the farm’s weed management system.
2. Design the Cropping System to Minimize Niches for Weed Growth In planning the crop rotation, avoid creating open niches in time or space. Plan tight rotations that follow one crop harvest promptly with the next planting. Open niches in space between crop rows can be reduced by using a narrower row spacing, intercropping, relay cropping, overseeding cover crops into established vegetables, or no-till management of cover crops prior to transplanting vegetables.
3. Keep the Weeds Guessing with Crop Rotations Plan and implement diversified crop rotations that vary timing, depth, frequency, and methods of tillage; timing and methods of planting, cultivation, and harvest; as well as crop plant family. Alternate warm- and cool-season vegetables. Rotate vegetable fields into perennial cover for two or three years to interrupt life cycles of annual weeds adapted to frequent tillage. Schedule tillage and cultivation operations when they will do the most damage to the major weed species.
4. Design the Cropping System and Select Tools for Effective Weed Control Develop control strategies to address anticipated weed pressures in each of the farm’s major crops. Choose the best cultivation implements and other tools for cost-effective preplant, between-row, and within-row weed removal. Plan bed layout, as well as row- and plant spacing, to facilitate precision cultivation. Choose irrigation methods and other cultural practices that are compatible with planned weed control operations.
5. Grow Vigorous, Weed-competitive Crops A healthy, fast-growing crop that can outcompete weeds is the best way to prevent weed problems. Choose locally-adapted crop varieties that grow tall or form lots of foliage that can shade out weeds. Maintain healthy, living soil. Provide optimum growing conditions—planting date and spacing, moisture, soil tilth and aeration, fertility, and pest and disease management. Deliver water and fertilizer within-row to feed the crop and not the weeds. Note that either insufficient or excessive levels of major nutrients (nitrogen, phosphorus, and potassium) can give certain weeds a competitive advantage over the crops.
6. Put the Weeds Out of Work—Grow Cover Crops! Cover crops do the same job as weeds, only better. They rapidly occupy open niches, protect and restore the soil, provide beneficial habitat, add organic matter, and hold and recycle soil nutrients. They suppress weeds through direct competition and sometimes through allelopathy—the release of plant-growth-inhibiting substances into the soil. Whenever a bed or field becomes vacant, plant a cover crop immediately so that it can begin the vital restorative work that nature accomplishes with pioneer plants or weeds. Good cover cropping plays a major role in Step 2 (minimzing open niches), and can put the weeds out of a job.
7. Manage the Weed Seed Bank—Minimize “Deposits” and Maximize “Withdrawals” Prevent formation and release of viable weed seeds, and proliferation of rhizomes and other propagules of perennial weeds. Avoid importing new weeds with manure, mulch hay, and other materials from off-farm sources. Utilize stale seedbed, cultivated fallow, or targeted tillage practices to draw down seed banks of the major weeds present. Encourage weed seed mortality and weed seed consumption by ground beetles and other organisms (see Step 9 below).
8. Knock Out Weeds at Critical Times Plant vegetables into a clean seedbed, hit early-season weeds while the are small, and keep crops clean through their critical weed free period (through the first third or half of the life cycle of most vegetables). Prevent seed set by “escapes” and late season weeds. When practical, interrupt vegetative propagation by invasive perennial weeds through timely removal of top growth.
9. Utilize Biological Control Processes to Further Reduce Weed Pressure Rotate livestock, poultry, or weeder geese through fields to graze weeds and interrupt seed set. To ensure food safety and comply with USDA Organic Standards, time such grazing so that fresh droppings are not deposited any less than 120 days prior to harvest of the next crop. Encourage weed seed predation and decay by maintaining high soil biological activity and providing habitat (mulch, cover crops, hedgerows) for belowground and aboveground weed seed consumers (conservation biological control). Enhance overall soil biological activity to tip the competitive balance in favor of crops, and possibly to shorten the "half life" of the weed seed bank.
Classical biological controls (introduced natural enemies) are commercially available for a few invasive exotic weeds.
10. Bring Existing Weeds Under Control Before Planting Weed-sensitive Crops Weed control in perennial horticultural crops like asparagus, small fruit, and some cut flowers can be quite difficult, especially when perennial weeds dominate the weed flora. Bring existing weed pressures under good control through repeated tillage and intensive cover cropping before planting any perennial vegetable, fruit, or ornamental crops. Choose fields with the best weed control or lowest weed pressure for weed-sensitive annual vegetables with a long critical weed free period, such as carrot, onion ,and parsnip. Be sure weeds, especially perennial weeds, are under good control before attempting no-till management of cover crops prior to cash crop planting.
11. Keep Observing the Weeds and Adapt Practices Accordingly Note and record any changes in weed species composition, emergence and growth pattern, or weed pressure, and modify practices as needed. For example, an increase in certain annual “weeds of cultivation” may indicate a need to reduce tillage or diversify the crop rotation. An increase in invasive perennials may require tilling deeper or more aggressively for a time. Watch out for the arrival of new weed species that could pose problems.
Expect weed populations and flora to shift over time. Every farm decision and field operation can elicit changes in the weed community, as can weather variations, to say nothing of long term climate changes. “Reading” the weeds each year becomes an information feedback loop, guiding weed management practices for the following season.
12. Experiment Try out new tactics and strategies to deal with major weed challenges. Farmers continually develop innovative strategies based on new tools that they fashion themselves or that researchers develop, new uses for old tools, and new combinations of preventive and control tactics. In the words of University of Vermont Extension Specialist Vern Grubinger (1997):
Experiment to fine-tune your weed management tactics.
- Start on a small scale with tools and techniques that are new to your farm.
- Identify your most problematic weeds and compare different combinations or rotations, cover crops, and cultivation tools to see how effective they are in providing control.
- Keep an eye out for new tools, or new ways to use old tools.
- Leave a control row or section untreated, so you can see the effectiveness of your tactics.
~ Grubinger, 1997.
Part of experimenting is to watch for new developments. Researchers and farmers continue to explore and expand the horizons of possibility in ecological and organic weed management. Check farming magazines and publications for practical applications of their work, from new cultivation tools to new strategies for particularly stubborn weed problems. Some cutting edge areas of research may take longer to yield practical results, yet bear watching and possible integration into a farm’s weed management strategy. These range from natural herbicides, bioherbicides, and classical biological controls, to specific weed–crop allelopathic interactions and manipulation of weed–crop–soil–microbe relationships to give the crop a competitive edge over certain weeds.
While none of these endeavors is likely to yield a “big hammer” to replace herbicides or steel, they can contribute additional “little hammers” to enhance efficacy and reduce the amount of tillage and cultivation needed.
An effective organic weed management system cannot be spelled out precisely because ecological weed management is inherently site specific and responsive to changes in the farm ecosystem. There is effectively no “organic weed control cookbook” to replace the precise herbicide protocols that have been developed for conventional production of row crops and some vegetables. No scientist can come up with a better weed management strategy for a particular farm than the strategy a skillful organic farmer can develop by applying ecological weed management principles to the particular suite of crops, weeds, soil conditions, and available resources on her or his farm.
This outline is not the only valid roadmap available. There is nothing set-in-stone about the number of steps or the order in which they appear here. Other outlines for ecological weed management have been developed for particular regions and cropping systems; these may be more directly applicable to your farm or situation. See the References and Additional Resources below for some specific examples.
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.