After pulling out the last of this season's tomatoes and cold-season crops, consider planting cover crops. Fall is the perfect time to start thinking about building the soil and replenishing soil nutrients for the next growing season. 

Cover crops are plants that are specifically grown for soil protection and enrichment. They also reduce the use of fertilizers (reducing cost), prevent soil erosion, minimize weeds, improve soil structure, protect water quality, and as they break down, add organic matter. 

"Each year as home gardeners intensively grow vegetables in their gardens, this depletes soil nutrients. Planting and incorporating cover crops is one tool for getting them back," says University of Illinois Extension horticulture educator Gemini Bhalsod.

Cover crops have been grown for hundreds of years to improve soil quality, but their use was not widespread until the 1980s. "Now it is not just organic and conventional farmers that know the benefits of cover cropping, home gardeners are catching on too, and you can even plant them in your raised beds," Bhalsod explains. 

Legumes and grasses are the two most common cover crop groups. Legumes fix nitrogen from the atmosphere into soils and grasses uptake and consolidate nutrients in the soil that crops did not use during the growing season. The nutrients are then reintroduced when the plant is turned in. Root biomass also contributes organic matter; loosens compact soils, improving drainage; and increases water-holding capacity of sandy soils.

Which cover crops are best for gardens? "Most home gardeners do well with planting a legume and grass mix," she says. Winter rye and winter wheat both germinate and grow in cool weather, and winter rye is very cold tolerant. Early to mid-fall is a good time to plant these. Legumes like hairy vetch, clover, and winter peas need to be planted in late summer to early fall. Consider combining at least one plant from both groups. Popular mixes include winter rye and peas, or clover, vetch, and winter rye.

Many cover crops are killed by the winter cold, but if they are given enough time to grow, they will still protect soil through the winter. "Oats, radish, and peas won't survive the cold winter, but the dead plants act as mulch and can be easily incorporated in the spring," Bhalsod adds. Consider planting these in the summer, after a spring harvest, or in between summer crops to give them enough time to get established.

Winter-hardy crops will go dormant in the winter and resume growth in the spring. "It's important that plants like winter rye, vetch, and clover are mowed down in the spring to stop growth and prevent seed heads from forming," Bhalsod says. Timing on this is important – if cover crops reseed themselves, it can lead to unwanted weeds. 

Most cover crops can be planted right after harvesting vegetables. Plant at least four weeks before consistent cold weather to allow plants to establish before the winter. Large-seeded crops can be planted in close rows, but small seeds can be broadcast. Be sure to cover seeds with a small layer of soil and water well.

In the spring, Bhalsod recommends mowing the cover crop before it sets seeds and then till it under.

"Plant residue needs to be tilled in two to four weeks before planting vegetables, so allow enough time," she says. "Breaking down plant material requires nutrients, so it is important not to plant vegetables until the residue has had a chance to break down."