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Cover Crops for Raised Beds

Cover crops are an important part of sustainable agricultural management. Typically utilized in large-scale agriculture, cover crops help increase soil fertility, suppress weeds, and reduce soil erosion between the planting and harvesting of higher-value food crops.

For example, in one year feed corn may be planted in mid-spring and harvested from dry stalks in early fall. Although corn roots and some stalks will remain in the field, the soil is exposed and subject to erosion during winter, due to precipitation and runoff. Planting a cover crop that thrives in cooler weather (such as winter rye) will help stabilize the soil – growing roots anchor soil better, and the increased biomass from the stems and leaves above the soil slows rainfall. This allows moisture to gently work it’s way through, rather than pummeling the soil with compaction-inducing raindrops that further smooth the surface and aggravate runoff.

After growing through the fall and then going dormant in the winter, winter rye (and several other cover crops) resume active, vigorous growth in early spring, suppressing weeds as it already has an established root system from fall planting. The cover crop is then tilled into the soil, and the decomposing plant material quickly adds carbon and nutrients for the next planted food crop, such as soybeans. For gardeners and small-scale farmers accustomed to maintaining a compost pile and spreading it on beds before planting, this is a quick and economical way to increase fertility and improve soil structure.

Translating large-scale agriculture practices for the backyard or small farm setting can present a challenge. While tilling is not recommended for raised beds and may be impractical for small spaces, even choosing the correct cover crop and coordinating the planting time can be difficult to determine.

Is a cover crop even necessary? For gardeners and farmers that have the time and resources to prepare their beds (adding compost and spreading mulch) for winter, cover crops may not have much of an added benefit. However, if quality compost and mulch are not available or are in limited quantities, planting a cover crop will help improve the soil. The trickiest thing about planting a cover crop in the fall is planting it while there is still enough time left in the growing season. Typically, gardeners do not think about feeding or protecting their soil until after the last harvest, which is typically around the time of the first frost (and when many plants begin to die or go into winter dormancy). Although many cover crops are hardy in cool weather, they will not grow during the short and colder days of winter. In Chicagoland, the ideal time to plant most cover crops is in September – when there’s 1-2 months left in the growing season. 

What to do when the cover crop resumes vigorous growth in the spring? Biennial and perennial cover crops such as crown vetch, white clover, sweet clover, and red clover will have difficult-to-remove, established root systems by the time you are ready to start spring planting. Unless tilled into the soil and left to decompose before planting, these will continue to grow throughout the season and compete with your garden plants for nutrients, space, and water. For growing in raised beds and smaller, no-till spaces, winter-killed annuals such as buckwheat, field peas, kodiak mustard, and crimson clover are a better choice. Planted densely around Labor Day, they will die around November and cover the soil in a thick mat, protecting it from erosion while the decaying plants add carbon and nutrients to the soil through the decomposition process. In the spring, what remains of the decomposed cover crops can stay in place (with seeds and seedlings planted through it), or it can be brushed aside when prepping the bed for planting.

Buckwheat growing in a raised bed in early November.