Cover crops or “green manures” are not harvested, but instead contribute to soil improvement where they are planted. A cover crop is a crop that is planted to protect the soil from erosion, suppress weeds, maintain soil moisture, increase organic matter in the soil, and recycle soil nutrients. Most gardens benefit from the use of cover crops when planted, instead of leaving the garden fallow or unplanted. Cover cropping is a helpful tool to use with crop rotation to plant for pest and disease management.
Cover crops are divided into two groups: legumes and non-legumes. Legumes have the ability to “fix” atmospheric nitrogen gas into “soil nitrogen”, making it into an available form of nitrogen for plants. Purchasing legume seeds that have already been pre-inoculated with a rhizobacteria coating aids in better germination. Nitrogen fixation will be low without the use of rhizobacterium. Fixed nitrogen allows the gardener to rely less on added synthetic fertilizer. Although cover crops will not supply all of the nitrogen for the following year, they will improve soil structure and nutrient levels for long-term sustainable growth over time.
Non-legumes are planted primarily to provide biomass. This includes carbon-based plant materials such as stems, roots, and leaves. Biomass improves the structure and water holding capacity of the soil while feeding beneficial soil microbes. Non-legume cover crops include grain crops such as oats, rye, and buckwheat. Several brassica crops like mustard, turnip, and daikon radish are also valued non-leguminous cover crops.
For legume and non-legume groups, there are both warm-and cool-season crops species to choose from. Cool-season legumes include Australian winter peas, hairy vetch, crimson clover, and red clover. Cool-season non-legumes include barley, oats, rye, winter wheat, and the brassicas. Warm-season legumes include cowpeas and soybeans. Warm-season non-legumes include buckwheat and sorghum-sundangrass.
A mixed planting of legume and non-leguminous cover crops is preferred for most situations. This mixture has the best benefit of nitrogen fixation and biomass production. A popular fall/winter cover crop combination in crimson clover (legume) and cereal rye (non-legume).
A popular summer combination is cowpeas (legume) and buckwheat (non-legume) Many seed catalogs sell premixed cover crop combinations, but you can also mix your own.
Before sowing the cover crop, turn over the garden with a rotary tiller. Clear the area of weeds and plants parts that were not previously removed from the site. Level the soil with a garden rake. Sow seeds by hand for small areas, broadcasting as evenly as possible. Broadcast back and forth over the area several times to distribute the seed evenly. Lightly cover seeds by raking to ensure good seed to soil contact. Larger areas may require seeding equipment. In some years, irrigation may be necessary to ensure a good stand.
For maximum benefit, a cover crop should be terminated (killed) while in the flowering stage.
At this point, the crop will return the greatest amount of biomass and nutrients to the soil. If cover crops are not terminated before seed formation, then their seeds may become weeds in a later crop. Mowing or weed eating in small areas are options for terminating cover crops.
Once terminated, the cover crop can either be left on the soil surface to decompose as a mulch, known as “no-till”, or it can be tilled into the soil where it will decompose below the soil surface. If the no-till option is used, the cover crop will need to be mowed or crimped down prior to planting of the vegetable crop.
Cover crops that are tilled into the soil will usually need to be mowed or otherwise chopped prior to tilling. Time cover crop seeding so that the cover crop is terminated and tilled under about three to four weeks prior to planting the vegetable crop. This will provide adequate time for the cover crop residue to breakdown before planting the vegetable crop.
Soil microbes convert the decomposing cover crop into organic matter for addition to the soil profile. Regular use of cover crops over a period of years will slowly raise the organic matter level in the soil and increase the activity of soil organisms such as earthworms and fungi in the soil. As these organisms decompose the organic materials, they help improve soil structure and tillage, making the soil a more favorable place for root development. Organic matter is constantly decomposing and cannot be built up permanently in the soil. Soil building is a continual process in the garden.
For more information on cover crops, please visit the Home and Garden Information Center website at hgic.clemson.edu. Tune in on Tuesday nights to watch “Making it Grow” at 7 p.m. on SCETV or mig.org.
E-mail Outen at callenb@clemson.edu.
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