Manage Weeds Using Cover Crops
February 10, 2014 Restriction of synthetic herbicide use in organic agricultural systems increases the complexity of weed management, leading farmers to cite weeds as one of the greatest barriers to successful organic production. To meet this challenge, most organic growers employ a wide range of tools in an integrated weed management system.
My 2011 survey of Midwest organic farmers found that the top four most commonly adopted weed management practices were crop rotation (86%) between-row cultivation (78%), primary tillage (76%), and cover cropping (66%).
A vigorous cover crop can prevent weeds from germinating and growing. Photo credit: James DeDecker.
Define Cover Crops
Cover crops are crops grown between harvest and planting of commodity or feed crops, usually not for harvest, but for the production of biomass and the various agroecological benefits this additional biomass can provide.
While cover crops are most often thought of as a means of preventing erosion and improving soil health, they can also be applied as an effective weed management tool. Cover crops can suppress weeds in four primary ways:
- By diversifying and filling gaps in a crop rotation;
- By competing with weeds for light, nutrients and moisture;
- By releasing chemicals that inhibit the germination and growth of weeds (allelopathy); and
- By attracting beneficial organisms that feed on weeds and weed seed.
The following will expound on how each of these suppressive mechanisms functions and can be applied.
Diversifying And Filling Gaps
Diverse crop rotations suppress weeds by forcing them to grow in different crops with varying life cycles, growth habits, spatial arrangements, and management requirements. Rotating dissimilar crop species also allows the application of a wide range of control measures and exposes weeds to more natural mortality factors.
For example, adding a year or two of a perennial legume crop like clover between annual vegetable crops can reduce the germination of annual weeds normally triggered by soil disturbance and also permits regular mowing that can suppress problem perennial weeds like Canada thistle.
It’s also important to remember that many weeds are pioneer species that tend to establish quickly and thrive in disturbed habitats. While mechanical control methods like cultivation can knock back weeds for a period of time, disturbing the soil essentially resets the succes sional clock, bringing new weed seed to the soil surface and initiating more weed germination and growth.
While it may be possible to till enough to keep weeds down, tillage degrades soil health and equipment is costly to operate. As an alternative, cover crops can be grown in short windows before and after cash crops to fill gaps in a rotation, minimizing exposure of bare soil and limiting the ability of most weeds to become established.
Research at Michigan State University’s Kellogg Biological Station found that oilseed radish grown following snap beans reduced weed biomass 98.5% compared to plots left bare after beans were harvested.
Choosing The Best Option
Many cover crop options are available, and each farming system is different. Successful use of cover crops for weed management depends on strategic implementation.
Before planting a cover crop, consider:
- Is a particular species/variety likely to fit your objective of weed suppression via a particular mechanism?
- Is the seed readily available at an acceptable cost?
- Is the cover crop suited to your field conditions, soil, cropping system, traffic patterns, etc?
- Is there a high likelihood of good establishment given your equipment and labor availability, timing, etc.?
- Do you have a viable strategy for suppression and residue management?
- Is the cover crop likely to cause any additional problems due to weediness, insects, etc.?
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Fierce Competition With Weeds
A vigorously growing cover crop, whether alone or interplanted with a cash crop, can prevent weeds from germinating and/or growing to the point that they produce seed or compromise cash crop yields.
However, successfully applying this strategy requires knowledge of weed ecology, careful selection of cover crop species, and favorable conditions for cover crop establishment and growth.
Unfortunately, the competitive impact of cover crops is not restricted to weed species. Research indicates cover crops capable of out-competing weeds will likely also suppress an interplanted cash crop. For example, a 1998 study by Norweign weed scientist Lars Olav Brandsaeter and others found that a white clover living mulch suppressed both weed and cabbage growth. In the case of living cover crops, cash crop suppression is most often the result of competition for water.
Cover crop residues can also negatively impact the establishment of a cash crop. This is usually related to physical interference with seed placement in the soil, prevention of soil warming, tie-up of nitrogen, or the release of allelopathic compounds.
Chemicals That Inhibit Weeds
Some cover crops produce compounds that inhibit the germination and growth of other plants, including weeds and crops. This property is known as allelopathy. Known allelopathic cover crop species include cereal rye, hairy vetch, crimson clover, buckwheat, alfalfa, barley, rapeseed, oat, and sorghum. Allelopathic compounds are present in the roots and above-ground portions of these plants in varying concentrations, and can act against weeds while the cover crop is alive and growing, or when cover crop biomass is incorporated into the soil.
In some cases, the phytotoxicity of allelopathic compounds is increased when they are modified by soil microbes in the process of decomposition. Small-seeded weed and crop species seem to be most affected by allelopathic compounds due to their greater surface-to-volume ratio and orientation nearer the soil surface where allelopathic crop residues are usually concentrated.
Unfortunately, allelopathic effects can be difficult to isolate and apply in the field. This is because the suppressive effect created by these cover species is a combination of allelopathy and the sort of run-of-the-mill plant competition described earlier. Also, the production, activity, and decomposition of allelopathic compounds can be highly variable based on environmental factors like soil fertility, moisture, pH, and temperature. In general, the allelopathic effects of cover crop residues are quite short lived and hard to predict.
Encourage Beneficials
The final mechanism through which cover crops can suppress weeds is predation. Weeds are an important source of food for many organisms. While most weed predation is focused on seeds, various organisms prey upon weeds at every stage in their life cycle. Common weed predators include birds, insects, rodents, earthworms, and microbes.
Including cover crops in your system increases and diversifies the habitat available to weed predators and encourages their presence. This approach of supporting populations of control agents that already exist in a field is known as conservation biological control. Conservation biocontrol of weed predators can function in a couple of ways. Planting and/or maintaining between-field cover in the form of herbaceous strips, fence rows, and hedge rows provides habitat for weed predators year-round and increases their abundance.
Including cover crops within a field provides protection for weed predators, encouraging them to stay in the field and eat more weed seed. A 2003 study comparing spring wheat alone to wheat underseeded with red clover found a two-fold increase in giant foxtail seed predation rates in the system including a red clover cover crop. A similar effect could likely be achieved in vegetable systems, such as buckwheat seeded into early season broccoli or kale. There is also strong evidence that reducing tillage to leave weed seeds on the soil surface contributes to increased seed predation.
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