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Winter cover crop mix. Almost no weeds in this area. Includes Austrian winter peas, cereal rye, hairy vetch, crimson clover. |
In addition to the usual benefits of cover crops – increasing organic matter, feeding microorganisms, suppressing nematodes, adding nutrients, preventing erosion, etc. — allelopathic cover crops can be selected to help combat weeds.
Allelopathic plants release chemicals that can inhibit the growth of other plants and reduce seed germination, including weeds. Some common allelopathic cover crops include:
Cereal Rye (Secale cereale) – One of the most widely known cool season allelopathic cover crops. It releases allelochemicals, which can suppress weed germination and growth.
Hairy Vetch (Vicia villosa) – Also a cool season crop often used in combination with cereal rye, hairy vetch has some allelopathic properties that can suppress certain weeds.
Buckwheat (Fagopyrum esculentum) – Known for its quick growth and ability to suppress weeds during warmer weather through both shading and allelopathy.
Sorghum-Sudangrass (Sorghum bicolor x Sorghum bicolor var. sudanese) – This hybrid tropical crop is highly allelopathic and is often used to suppress weeds and even some pests during warm seasons. This is a large plant, but it can be chopped and dropped several times during its growth to keep it more manageable. It is even reported to discourage nutsedge.
Sunflower (Helianthus annuus) – Releases allelopathic compounds which inhibit the growth of nearby plants, making it useful for weed control during late spring and early summer.
Mustard (Brassica spp.) – Many mustard species produce glucosinolates that can suppress weeds and soil-borne pests throughout fall and milder winters into early spring.
Most cover crop mixes contain several of these varieties with different mixes for summer and winter planting.
Allelopathic plants can be used strategically in crop rotations or mixed plantings to reduce weed pressure and improve soil health, but care should be taken as their allelopathic properties may also affect desired food crops if not managed properly. For instance, plants with tiny seeds, like lettuce and carrots, tend do be more susceptible to allelopathic chemicals so you wouldn’t want to plant them in areas where weed-suppressing cover crops are growing or have just been grown. Other plants with larger seeds are more resistant, such as beans and squash, or may be somewhat allelopathic themselves. All allelopathic chemicals are not the same and specific plants may not be susceptible to a particular chemical.
This is an area which needs more research and there is not much information available about specific plants and their allelopathic characteristics or tolerances. Depending on your goal, allopathic plants can be helpful or harmful.
Plant alleopathic cover crops and companion plants where you want to combat weeds. When you are planting a subsequent crop in that area, try one or more of these techniques:
- Terminate the cover crop at least two weeks or more before planting new crops. It’s a good idea to add mulch so the soil does not dry out. When planting the next crop, the mulch can be moved aside where you’re planting.
- Instead of chopping and dropping or turning under, pull up the allelopathic cover crop, roots and all, and deposit it in some other area to help deter weeds. The roots and decaying matter from terminated crops can continue to exude allelopathic chemicals.
- Apply several inches of new soil or compost before planting seeds to encourage germination in soil that does not contain detrimental chemicals.
Utilizing a crop rotation guide using information from whatever sources are available and your own observations will help manage the interactions between succeeding plantings in a way to maximize the desired results for both edible crops and cover crops. The four-year crop rotation schedule developed and used at the VegHeadz garden is available in the left sidebar.
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