Question: What is the purpose of using a cover crop? Some cover crops are planted at different times of the year. How do I choose the right one for the time of year and when do I plant it?
What is a cover crop?
Cover crops are grown to provide a temporary vegetative cover for the soil between periods of normal crop production. Planting cover crops is a common practice that dates back to the Roman Empire. It has been used extensively in the US by Native Americans, and other farmers until synthetic fertilizers gained widespread use in the 1950s. Using cover crops is once again becoming popular with commercial growers and home gardeners as a component of sustainable agriculture. 
Also known as green manure or living mulch, cover crops can be an important part of crop rotation used to maintain and improve the soil health. Rather than being grown for harvest, cover crop plants are grown specifically to enrich the soil, support healthy plant growth for future crops, manage weeds, and control diseases and insects. Cover crops can increase the productivity of vegetable and fruit crops, as well as providing high quality fodder for grazing livestock.
Cover crops are divided into three main categories: grasses (wheat, rye, oats, buckwheat, or barley), legumes (peas, beans, or clovers) and broadleaf nonlegumes (brassicas, forage radishes, turnips, mustards). Legumes are typically grown to to capture or fix nitrogen from the air and make it available to plants. Grasses and nonlegumes are generally planted to trap the nutrients that are left behind after a cash crop is harvested or to cover, protect and enrich the soil.
Benefits of cover crops
Cover crops provide significant benefits to farmers and home gardeners. They:
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- Improve soil quality, by improving biological, chemical and physical soil properties.
- Protect the soil from erosion and runoff. They keep valuable nutrients in the soil and prevent pesticides and other chemicals from degrading local waterways.
- Reduce soil compaction. Many cover crops have long roots that help break up deep compacted soils and grass cover crop roots reduce surface compaction. This allows crops to access water and nutrients more easily.
- Add organic matter and nutrients to improve soil structure and stability. The capacity to retain moisture and nutrients for plant growth is also increased.
- Add or conserve nitrogen. Legume cover crops produce or fix nitrogen, increasing the amount available to support plant growth. Non-legume cover crops scavenge or trap nitrogen that would otherwise move out of the root zone. If green cover crops are tilled into the soil and decompose, the trapped nitrogen also becomes available to plants over time.
- Provide food and carbon sources for microorganisms that enrich the soil.
- Reduce the need for chemical inputs to produce healthy crops.
- Suppress weeds by growing faster to outcompete the weeds or by acting as a surface mulch. Some cover crops produce allelochemicals which inhibit seed germination and plant growth.
- Manage pests. Cover crops provide habitat for beneficial insects and kill insects that are detrimental to crops. Rapeseed and other brassica cover crops release chemical compounds that may be toxic to nematodes, fungi, and insects.
- Suppress disease. When tilled into the soil as green manure, cover crops can promote healthy plant growth. No till cover crops can provide a physical barrier that reduces soil splash or soil borne pathogens. The increase in beneficial microorganisms in the soil also seem to protect against disease.
- Provide a diverse habitat and food for pollinators and other wildlife which benefits plants and the surrounding ecosystem. Buckwheat, rapeseed, lupines, legumes and many clover cover crops are very attractive to pollinators.
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Choose the Right Cover Crop
Success in cover cropping requires selection of a species or mix that provides the specific benefits described above and is compatible with crops grown for harvest.
Each species of cover crop has its own characteristics and is suited to provide some, but not all, of the benefits described above. It is important to choose the cover crop best suited for the purpose.
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- What crops will you be growing? This will help determine what type of cover crops you choose and when they will be planted.
- What are your management goals: improving soil quality, suppressing weeds, preventing soil erosion, disease and pest management? Knowing your goals will help you select the right cover crops.
- How will you integrate cover crops into crop production? Cover crops can be a strip crop in between rows, in beds or in blocks or a field taken out of production for a season or year or more.
- How will you manage the cover crop after it is established? Will you remove the cover crop after it dies, leave it in place when planting the next crop or mow it and till it under?
- Do the benefits of cover cropping outweigh the disadvantages? Cover crops are generally low maintenance, additional nutrients are rarely needed, and chemical inputs can be reduced or eliminated. However, it often takes years before the full benefits of cover cropping is achieved.
Planting at the Right Time
Some cover crops are planted in the fall, remain in place in the winter, are killed or die in the spring, then removed or left on the ground or tilled into the soil. However, depending on their specific characteristics (winter or frost hardiness, rate of growth, and other growth habits) and goals of cover cropping, they can also be planted in the spring or summer.
- Fall-seeded cover crops include annual rye grass and hardy small grains (rye, oats, wheat, spelt and triticale) and a few legume cover crops sown for winter soil protection and nitrogen scavenging.
- Spring-seeded cover crops include spring oat, barley, wheat, triticale, rye. They provide early-season vegetative cover and some weed suppression. Sometimes legumes are mixed with oats, which outcompete weeds until the legumes become established.
- Early summer-seeded cover crops include sorghum-sudan grass, millet, buckwheat and forage soybean. They are fast-growing crops used to suppress weeds and add organic matter.
- Late summer-seeded cover crops include rye or annual ryegrass, oats, forage radish and mustard. They are usually sown after an early harvest of a vegetable crop and a month or two before frost.
Interested in learning more? The USDA cover crop manual provides general information on choosing cover crops for Virginia, and this cover crop decision tool will aid in the decision making. In addition, University of Maryland Extension has a helpful planting guide on cover crops for vegetable gardens. As the Cornell Extension states, the keys to success involve a 1) fast start (before the weeds emerge), 2) no gaps (no gaps in the cover crop stand) and 3) kill on time (before seeds are produced).
With this information, the resources below, and some advance planning, you are well on your way to discovering the best cover crop for your particular home garden. Happy gardening!
References
“Building Healthy Soil”, Diane Relf, Extension Specialist, Environmental Horticulture,Virginia Cooperative Extension, Publication 426-711, 1 June 2017
“Cover Crops”, Cleve Campbell, The Garden Shed, Piedmont Master Gardeners, Vol. 1, No. 9, Sept. 2015
“Cover Crops”, Jon Traunfeld, Home and Garden Information Center, University of Maryland Extension, Updated 11 Aug. 2021
“Cover Crops – Keeping Soil in Place While Providing Other Benefits”, NRCS, U S Department of Agriculture
“Cover Crops and Green Manures”, New England Vegetable Management Guide, College of Natural Sciences, University of Massachusetts Amherst
“Cover Crop for Vegetable Growers”, Cornell Cooperative Extension, Cornell Vegetable Growers Program, Carol MacNeil
“Cover Crops and Green Manures in Home Gardens”, Jill McKenzie, University of Minnesota Extension, 2018
“Virginia Cover Crops Fact Sheet Series No. 1: Beneficial Uses of Cover Crops,” Cathy Flemming & Wade Thomason, Publication CSES-120NP, Virginia Cooperative Extension, 2020
“Virginia Cover Crops Fact Sheet Series No. 2: Cover Crop Performance Evaluation in Field and Controlled Studies,” Cathy Flemming & Wade Thomason, Virginia Cooperative Extension, Publication CSES-121NP, 2020
“Virginia NRCS Cover Crop Planning Manual, Virginia Technical Note, Agronomy #10”, Natural Resources Conservation Service, United States Department of Agriculture, Oct. 2015
