Consider cover crops your most important crops, because the requirements for abundant food crops — building soil fertility, improving soil texture, suppressing weeds, and inhibiting disease and crop-damaging insects — can be best met by the abundant use of cover crops, season after season.
5 Benefits of Cover Crops
Soil Fertility. A vast array of soil organisms decompose once-living plants into nutrients easily taken up by plant roots, and add to your soil’s humus content (the final residues of organic matter in your soil, which assist nutrient uptake, improve texture and hold moisture). I grow organic matter in place using cover crops because, in many ways, a living cover crop is even better than adding manure and compost for fertility.
The area of most intense biological activity — ultimately the definition of soil fertility — is the rhizosphere, the zone immediately around plant roots. Plants release nutrients through their roots to feed their buddies in the soil — beneficial microbes and mycorrhizal fungi — that increase access to water and convert soil nutrients into forms more readily utilized by plants. If the intense bioactivity in the rhizosphere is the key to fertility, imagine the contribution of closely planted cover crops with vastly more root mass than more widely spaced food crops.
Soil Texture. Mycorrhizal fungi (beneficial fungi that grow in association with plant roots) produce glomalin, a substance which glues microscopic clay and organic matter particles into aggregate clumps, stabilizing the soil and making it nice and crumbly. This crumbly texture is more porous to oxygen and water. Bacteria encouraged by cover crops produce polysaccharides, which also act as soil glues.
Grass and grain cover crops with fine, dense root masses loosen soil texture as they decompose. Others, such as sweet clovers and sorghum-sudangrass hybrids, grow deep, aggressive taproots that break up soil compaction.
Erosion Prevention. A cover crop’s tight canopy protects the soil from the drying and scouring effects of wind and the forceful impact of heavy rain. The loosened soil structure achieved by cover cropping allows rapid absorption of rain and prevents runoff.
Soil Moisture. Organic matter added by cover crops acts like a sponge in the soil, absorbing rainwater and holding it for gradual release to plant roots. Thus, gardens that have been home to regular use of cover crops become more resistant to drought.
Protection From Weeds, Diseases and Insect Damage. Garden beds frequently planted with cover crops will have fewer problems with weeds. Cover crops suppress weeds, out-competing them for water and nutrients and shading them under a tight canopy, sometimes releasing chemical compounds that inhibit germination of weed seeds (a phenomenon called allelopathy). Plus, the roots of cover crops release nutrients that feed beneficial microbes in the soil. These microbes then suppress pathogens that cause root diseases. Some cover crop plants, such as rape, rye and sorghum-sudangrass hybrids, inhibit root-knot nematodes, which can be disastrous to beets, carrots and other root crops in some regions.
You can cut cover crops and use them as mulches to boost populations of beneficial ground-dwelling species, such as rove beetles and spiders. Other cover crops can provide a wonderful habitat for the pollinators that help keep your food garden thriving. Cover crops that flower, for example, provide important food sources for honeybees and butterflies.
Cover Crop Species
Depending on your climate, gardening goals and the time of year, you’ll have a variety of cover crop options. Cover crop species vary widely in their tolerance of cold, heat, soil moisture extremes and soil types. It’s often good practice to plant a mix of different species.
The most important division among species is between legumes and non-legumes. Legumes, such as cowpeas, vetches and clovers, serve as hosts to rhizobia, bacterial alchemists that live in plant roots and convert nitrogen from the atmosphere into nitrogen compounds that help plants grow. Some of this “fixed” nitrogen remains attached to legume roots in nodules — like beads on a string — and becomes available to other plants after the legume dies.
Non-legumes include all other cover crops: grasses such as small grains, various millets, annual and perennial rye, and sorghum-sudangrass hybrids; crucifers such as mustards, rape and forage radish; and buckwheat.
For detailed information on the major cover crop species, see Managing Cover Crops Profitably by the Sustainable Agriculture Network. Even though this book is geared toward farmers, it includes great information for the home gardener. (Clicking on the link in this paragraph will take you to a page on the Sustainable Agriculture Research and Education program’s website where you can download a free copy of the most recent edition of this book.)
Cover Cropping Strategies
The most important strategy of all is: Do it now! When I complete a food crop harvest in fall, that same day I plant an overwinter cover crop. If I harvest a spring crop such as lettuce from a bed that I won’t be planting again until fall, I sow a fast-growing interim cover crop that does well in summer heat, such as buckwheat or cowpeas. The best time to plant a cover crop is anytime a bed is not covered by a food crop or mulch.
The easiest way to incorporate more cover cropping is the half and half strategy: I dedicate every other garden bed to cover crops for an entire year. I may grow several fast-growing covers such as cowpeas and buckwheat in succession, or a cover such as sweet clover, which takes a full year to yield all its benefits. In the following year, the beds previously in cover crops now grow food crops, and vice versa. This strategy allows you to grow mulches in place. Using a scythe or sickle, you can cut cover crops that produce a lot of biomass — hairy vetch, rye, and sorghum-sudangrass hybrids — and lay them out as mulches in an adjacent bed that’s planted with food crops. Most cover crops cut when in their vegetative growth stage will regrow to produce even more mulch. If you can’t give half of your garden space to cover crops, how about one bed out of three? Or four? Or even 10?
Some cold-hardy cover crops grow well through fall frosts, then die if the ground freezes solid — this is known as winterkilling, and you can use to your advantage. When I remove the top growth from asparagus beds in late September, I plant a mix of winter peas and oats. They create a knee-high tangle of green by the time the ground freezes, and they both reliably winterkill here in northern Virginia, leaving a thick, weed-suppressing mulch come spring. Plus, the leguminous peas fix nitrogen in the soil for the heavy-feeding asparagus.
It’s possible to grow both a food crop and a cover crop in the same bed at the same time. Under tall-growing crops with a small footprint, such as tomatoes or pole beans, plant an undersown cover crop, such as low-growing Dutch white clover or perennial ryegrass. If such a cover has already been established, leave it in place as a living mulch and open up holes for large transplants such as tomatoes or broccoli, or open up rows for beans.
Shade-tolerant species — annual ryegrass, cereal rye, hairy vetch, some clovers — can be sown as overseeded cover crops into existing crops such as corn or brassicas up to several weeks before harvest. The cover crops will grow slowly under the existing canopy, then come on strong when the food crop has been removed.
Try frost-seeding a cover crop into overwintered grains. In late winter, broadcast the seeds — small round ones such as clovers work best — into the grain beds. Winter freezes and thaws will work the seeds into the soil, where they will germinate in spring rains. The grain could mature into a food crop for feeding your pigs or chickens while serving as a nurse crop to establish the clover, which will grow rapidly after you harvest the grain.
Some species work as reseeding cover crops. Subclovers (cool-season legumes) will die back in winter, but the seeds they leave behind will remain dormant through much of the next growing season, then sprout in the fall to establish a new cover.
Permanent cover crops are appropriate in orchards, vineyards and border areas never planted with food crops. Keeping these areas in mixed flowering species — perennials such as clovers, or annuals that reseed themselves such as crucifers — protects the soil, supports pollinators and encourages insect diversity. Encouraging lots of different kinds of insects is the key to preventing crop damage, as the bug-munching insects will help you control the crop-munching insects. In high-traffic areas, covers that can take a good deal of wear are in order, such as annual ryegrass and white clovers.
Planting Cover Crops
Plant cover crops with as much care as your food crops. Make a furrow for larger or more vigorous seeds, such as cowpeas or sorghum-sudangrass, sow thickly, and then cover with soil. For smaller seeds, such as clovers, crucifers and small grains, scatter, rake in, and tamp the bed with the back of a garden rake to ensure good soil contact. To speed germination, apply a light mulch and water occasionally. Seeds of vigorous covers, such as annual ryegrass, oats and hairy vetch, will germinate if left on the surface, especially if broadcast just before a soaking rain.
You can plant a food crop as soon as the cover crop is killed unless there could be a temporary problem of allelopathy or nitrogen tie-up (keep reading for more information). In such cases, wait about three weeks or so before planting.
Killing a Cover Crop
It’s better to avoid deep mechanical tillage, which disrupts soil life and breaks down soil structure. Tall, heavy stands of cover crops, such as rye and hairy vetch, are a nightmare to till in with a power tiller in any case. So what’s the best way to kill a cover crop so decomposer organisms can break it down to feed your soil?
Remember that a cover crop in the vegetative stage (i.e., not flowering) usually regrows after being cut. Most cover crops in the reproductive stage (i.e., flowering), however, will die if cut. A complete no-till strategy that works for most covers is to cut the cover just above soil line after it has flowered, and transplant crops such as tomatoes, peppers or broccoli through the severed tops, leaving the cut tops and cover crop stubs as a mulch. For small areas, use a hand sickle for the cutting. (We’ve found a used butcher knife also works well. — MOTHER) Or, use a heavy field hoe to chop cover crop plants just below the soil surface to kill them without disturbing the soil deeper down. Another option is to loosen the soil with a broadfork and pull the cover plants out by the roots, again laying them on a bed as mulch. As your soil becomes more friable, the broadfork may not be necessary.
My favorite way to till in a cover crop is to place a hardworking flock of chickens on the bed. Allow them to roam inside a chicken tractor or temporary fencing. For a great example of how well this method works, check out these before-and-after photos of a garden planted with a lush mix of cover crops and the same garden two weeks later, after the chickens went to work on the area.
Thirsty rye may deplete available soil moisture enough to inhibit the following crop in a dry season. A few crops — small grains such as rye and sorghum-sudangrass hybrids — may temporarily release chemicals that will inhibit the growth and germination of other plants. Though these allelopathic effects help with weed control, they can suppress germination and growth of food crops that follow, especially small-seeded ones, such as lettuce, spinach and onions. Just wait a few weeks after killing a cover crop to plant your food crop and you won’t run into problems.
Cover crops susceptible to diseases, herbaceous insects or nematodes could serve as vectors if grown beside or followed by food crops with the same susceptibilities. If you allow cover crops to mature their seeds before being killed, they can later volunteer as weeds in following crops. However, the more diverse our gardening practices and the more complex our rotations, the less likely we will encounter such problems. And the more we learn about cover crop species, the more we discover management solutions to problems that could occur. For example, if rye has been allowed to mature, the high carbon content of its residues, both roots and top mass, can temporarily tie up nitrogen in the soil and inhibit growth of the following crop. But if you grow the rye in a mix with a compatible legume, nitrogen fixed by the legume will minimize nitrogen tie-up.
Begin experimenting with cover crop species in your garden today, and you’ll soon see that the enormous advantages far outweigh any problems that may crop up. Ultimately, cover crops will lead to healthier soil and bigger, better harvests.
For more on using cover crops in your garden, see Cover Crops: Options, Tips and Advantages for the Home Garden and Cover Cropping Your Garden.
Harvey Ussery uses cover crops galore in his Virginia garden. His website, The Modern Homestead, includes much more information about his homesteading and gardening endeavors. He is the author of The Small-Scale Poultry Flock (2011).