What are cover crops?

Cover crops are plant species planted either alone or in mixes that are used in between cash crop or vegetable crop plantings to benefit the soil and succeeding crops.

Cover crops have long been used to reduce soil erosion and add organic matter to improve the soil in farming operations. With the development of no-till cropping systems, cover crops are recognized for their ability to provide moisture-conserving residues (organic matter left on the ground as a mulch) as well as nitrogen for the succeeding crop. Recent concern for water quality has provided additional reasons to use cover crops. Cover crops improve water infiltration and take up and hold nutrients, especially nitrogen that was not used by the previous crop, which reduces fertilizer run-off. 
Cover crops suppress weeds and reduce the need for herbicides by competing with them for space and nutrients and by providing a mulch to cover the soil surface. Some covers, such as radish and canola, also release chemicals that suppress weed seed germination and may reduce populations of soil-borne plant pathogens. (1)

Cover crops planted in late summer are a cost effective way to build better gardening soil. Cover crops are also referred to as green manure crops. They can be grains, grasses, or legumes that grow during fall and winter which is then plowed, spaded or tilled under in the spring. While growing, cover crops help reduce soil compaction and prevent all forms of erosion. Their roots penetrate and help loosen hard soils, allowing better water and air infiltration. Growing covers use available nitrogen and other minerals that winter rains or runoff generally wash away. Inoculated legume cover crops fix atmospheric nitrogen in nodules in the soil for use by the next crop.
field crop
When the cover crops are worked into the soil, they return organic matter to the soil, building better soil health and providing needed nutrients for the next vegetable crop. All garden soil needs organic matter to maintain the soil health. Soil health comes from the soil structure, bacteria, fungi, earthworms and other forms of life. Organic matter is quickly used by earthworms and other soil organisms, so a continuous, year-round supply is needed.

Cover crops should grow quickly, cover the area to shade out weeds, and be easy to work into the soil in the spring. Multiple species mixtures will provide greater benefit as each species will attract different biologicals and aid the soil with its own unique characteristics. Combinations of legume, grass, and grain varieties such as winter peas, vetch, millet, clover, oats, and radish can greatly improve soil health and provide a healthier crop the following year. (2)
Cover crops are most commonly planted from August through early October. Certain cover crop varieties will winter kill, leaving an excellent mulch that is easy to plant into or incorporate in the spring. Other cover crops will survive winter temperatures and provide green growth in the spring. This living mulch should be mowed prior to flowering. If incorporated into the soil, it should be allowed to rest for 3 or 4 weeks prior to planting.

Walnut Creek Seeds, as an Ohio dealer, has focused on several cover crop species which bring specific benefits to the gardener, farmer and rancher.

Look for publications from your local university about the use of cover crops.