A cover crop is a fast-growing crop that you dig or rototill back into the soil three to four weeks before the planting season begins. If the cover crop begins to go to seed, dig it in immediately—do not wait for the planting season. Decomposing in the soil, this crop replenishes the organic content, improves soil structure, adds nutrients, increases microbial activity, and helps break up compacted areas.
Planting a cover crop after the harvest season or anytime the garden will lie fallow for a couple of months keeps the soil in top condition. In addition to replenishing the soil, a cover crop protects an empty garden from erosion by wind or rain. Even weeds turned under add valuable humus and nutrients to the soil—so long as they have not developed seeds.
The best cover crops are plants that naturally fix nitrogen in their roots as they grow. Plowing these plants into the soil boosts nitrogen content as it improves the soil structure. Legume plants are some of the best nitrogen fixers.
For the soil to benefit most from their nitrogen-fixing capacity, plow under legume cover crops before they form flowers. Clover is one of the most useful spring and summer cover crops. Plant red, white, and yellow clover in spring or summer. Plant ladino clover in early spring or early fall. Crimson clover is a spring or fall cover crop. Plant soybeans in spring or summer, hairy vetch in late summer or fall, and alfalfa in spring.
Among the more commonly used non-legume cover crops is annual ryegrass, which you plant in spring. Plant buckwheat, oats, rape, and Sudan grass in spring or summer. In late summer or fall, plant annual winter rye and allow it to overwinter, then till it under four or five weeks before spring planting.