A cover crop is a general term for any plant grown to prevent erosion, to improve soil structure, and to maintain or build soil fertility. Sometimes, you'll see the term "green manure" used in gardening literature. These are crops used primarily to add nutrients to the soil and are tilled into soil when they're still green. The distinction between the terms isn't terribly important as long as you choose the best plant for the purpose you have in mind. In warm-winter climates, green manures overwinter; in other areas, they are tilled under in summer or fall.
In September, most gardeners can plant overwintering cover crops. If you live where winters are mild, such as USDA Hardiness Zones 8 and south, you can plant and till under a green manure crop before planting a winter cover crop. Wherever you live, sow seeds of your winter cover crop at least 30 days before the last frost date in your region. In this article, I'll discuss cover crops you can plant in the fall and either till under in the late fall or overwinter and till under in spring.
Cover-Crop Benefits. Most gardeners know about the benefits of adding organic matter to the soil. Many, as I do, build the soil with annual applications of compost, manure, leaves, and grass clippings. However, in some areas, those forms of organic matter may be difficult to find or transport. Cover crops not only add large amounts of organic matter, but they provide other benefits as well. Let's look at some of them.
Adding Organic Matter. A primary reason to grow a cover crop is to increase the amount of organic matter in the soil. Additional organic matter improves the soil's structure, increases its water retention and drainage, and improves its aeration. It also provides necessary food for earthworms and microorganisms that increase biological activity in the soil. Increased biological activity in turn helps keep the soil healthy by enhancing decomposition; well-nourished beneficial microorganisms also compete better against disease-causing organisms.
Erosion Control. Traditionally, an important reason to use a cover crop was to "cover" the soil during the winter. Hardy crops such as winter rye are particularly important in preventing erosion and topsoil loss, especially in areas with high winds and inconsistent snow cover.
Loosening Compacted Soil. Certain cover crops, such as bell beans and oilseed radishes, have aggressive taproots, sometimes reaching 3 feet deep, that help break up compacted soils. The taproots also "mine" nutrients such as calcium from deeper soil, and when the plant dies, the nutrients are released in the root zone for the next crop.
Nutrient Balancing. Legume cover crops such as hairy vetch and crimson clover-through a symbiotic relationship with the rhizobia bacteria on their roots-convert atmospheric nitrogen into a form they can use to grow. When the cover crop is tilled under, the nitrogen is released for the next crop. In contrast, cover crops such as phacelia absorb and accumulate excess levels of nitrogen and other nutrients already in the soil that may otherwise leach out and pollute ground water or nearby streams.
Weed Control. Some cover crops are good weed blockers. Broad-leaved cover crops such as buckwheat and sunn hemp shade and smother weeds with their vigorous growth, and others like winter rye grain release s that prevent weeds from germinating.
Attracting Insects. Cover crops such as crimson clover and mustard, if allowed to flower, attract bees and beneficial insects that help with pollination and insect control in the garden.
Despite their many advantages, cover crops aren't always the ideal choice. Here are some considerations that may dissuade you from growing a cover crop.
Tilling Under. To gain the most benefits, gardeners should incorporate overwintering cover crops into the soil at or before full bloom in spring. Some crops, such as hairy vetch, put on so much top growth that you must cut or mow them before you turn them under. These may be difficult to handle unless you have a powerful mower, tractor, or tiller. With that equipment, the tops can be cut and tilled under, used as mulch around plants, or composted. Gardeners without such machines find it easier to grow cover crops that aren't winter-hardy in their areas or those that produce less top growth and are easy to turn under by hand in spring. Cover crops that die over the winter, including oats and mustard in the Northeast, still protect soil somewhat with their root systems.
Delayed Planting. Cover crops must be turned under in spring at least two weeks before planting. That requirement may delay your usual planting time if you have a wet soil or a wet spring, or if you're growing cover crops that mature later. Furthermore, you may have to till a cover crop in early, losing some benefits of nitrogen fixing and organic matter. If the tilled-in crop hasn't decomposed by planting time, your plants may have a temporary nitrogen deficiency as microbes use soil nitrogen to break down the organic matter from the cover crop.
Stealing Water. In dry areas, as in the Southwest, cover crops consume vital moisture from the soil, especially when they're getting established. In such situations, choose a crop such as barley that needs minimal water to germinate, grows well in dry soils, and is planted before annual rains in the area.
Weeds. If you allow some cover crops (annual ryegrass is one) to go to seed, they can become weeds. To derive maximum benefits from the cover crop and to prevent reseeding, till it under at or before flowering.
For most home gardeners, the annual cover crops are the best choices. Sow annuals in fall, and they either die in winter or naturally complete their life cycle by the next spring. (Perennial cover crops such as alfalfa (Medicago sativa) and red clover (Trifolium pratense) require more maintenance and are best used as permanent crops in orchards and vineyards.)
You can plant cover crops two ways, depending on your other plantings: Sow seeds after you harvest earlier crops (lettuce, beans, or tomatoes, for example), or plant low-growing cover crops between rows of any fall-planted crop, such as broccoli or spinach.
Prepare the Soil. Don't skimp on soil preparation. Prepare the soil as you would for any other annual crop. Till the area and make sure the pH is between 6 and 7 (a soil test may be necessary). Broadcast seed at the recommended rates. On drier, sandy soil, plant three times as deep as the seed's width; on heavier soil, plant twice as deep as the width.
Plant. Sow seed at least 30 days before your first expected frost date. For cover crops that are only marginally hardy in your area, push back the sowing date to 60 days before the first frost. The more established a cover crop is before winter, the more likely it will overwinter well. Water the beds, if necessary, to help get the crop established. If you plant a legume crop, inoculate the seeds with the nitrogen-fixing bacteria prior to planting (available from cover-crop seed suppliers).
Choosing a Cover Crop. To decide which cover crop to grow, consider your region, the soil's needs, and your tools. To help novice gardeners, some seed companies sell mixes of grasses and legumes for and mild-winter areas; choices include Cold Zone Soil Builder Mix, which contains hairy vetch and cereal rye, and Soil Builder Mix, with bell beans, winter peas, vetches, and oats. Planting such mixes is the easiest way to sow cover crops.
Hardy legumes increase soil nitrogen and organic matter. After a slow fall start, they grow rapidly in March and April and may not mature until May in some regions. Mow these cover crops in spring at or before flowering, then till them under.
Field pea (Pisum arvense and P. sativus). Grows 6 inches to 5 feet high; hardy to 10 to 20° F. 'Austrian Winter' pea is low growing, late maturing. 'Magnus' grows to 5 feet. Sow 2 to 4 pounds per 1,000 square feet.
Berseem clover (Trifolium alexandrinum). Grows 1 to 2 feet high; hardy to 20° F. Will regrow after cutting. Produces high amounts of nitrogen. Sow 2 pounds per 1,000 square feet.
Crimson clover (T. incarnatum). Grows 18 inches high; hardy to 10° F. Matures late and fixes less nitrogen than other clovers. Attracts bees. Sow 1/2 to 2 pounds per 1,000 square feet.
Dutch white clover (T. repens). Grows 6 to 8 inches high; hardy to -20° F. Perennial and shade tolerant so may become weedy. Sow 1/2 to 1 pounds per 1,000 square feet.
Hairy vetch (Vicia villosa). Grows to 2 feet high; hardy to -15° F. Hardiest annual legume. Tolerates poor soil, matures late. Sow 1 to 2 pounds per 1,000 square feet.
Fava beans (V. faba). Grows 3 to 8 feet high; hardy to 15° F. Bell bean is a shorter (3 foot) relative. Sow 2 to 5 pounds per 1,000 square feet.
Tropical legumes grow quickly in fall to increase soil nitrogen and add abundant organic matter, but need warm growing conditions. Plant in late summer or early fall in the Southeast and Southwest before winter cover crops. Best grown as summer annuals in the North.
Sunn hemp (Crotolaria juncea). Grows 5 to 6 feet high; hardy to 28° F. Needs same growing conditions as corn. Cut or mow before stems become woody. Can also reduce nematodes. Sow 1 to 2 pounds per 1,000 square feet.
Sesbania (S. macrocarpa). Grows 6 to 8 feet high; hardy to 32° F. Grow like sunn hemp, but more tolerant of flooding, drought, salinity, low fertility. Sow 1 pound per 1,000 square feet.
Cowpea (Vigna sinensis) . Grows 1 to 2 feet high; hardy to 32° F. Tolerates poor and acidic soils. Prefers humidity and tolerates drought. Sow 2 to 3 pounds per 1,000 square feet.
Grasses grow quickly, tolerate cold, increase organic matter, and improve compacted soils' structure. They also control erosion but don't increase nitrogen. Mow these annual grass cover crops in spring before seeds set, or till under.
Oats (Avena sativa). Grows 2 to 3 feet tall; hardy to 10 to 20° F. Produces least organic matter of grasses, but tolerant of wet soils. Sow 2 pounds per 1,000 square feet.
Barley (Hordeum vulgare). Grows 2 to 3 feet tall; hardy 0 to 10° F. Fast maturing and tolerant of dry and saline soils; intolerant of acidic soil. Sow 2 to 3 pounds per 1,000 square feet.
Annual ryegrass (Lolium multiflorum). Grows 2 to 3 feet high; hardy to -20° F. Fast growing and tolerates flooding. Absorbs excess nitrogen from soil. Can become weedy. Sow 1/2 to 2 pounds per 1,000 square feet.
Winter rye (Secale cereale). Grows 4 to 5 feet tall; hardy to -30° F. Best grass for cold winter climates; tolerant of low fertility, acidic soils. Sow 2 to 3 pounds per 1,000 square feet.
Other annual cover crops such as buckwheat, mustard, and phacelia aren't ordinarily considered fall crops. They increase organic matter but not nitrogen.
Buckwheat (Fagopyrum esculentum). Grows 1 to 3 feet tall; hardy to 32° F. Fast growing warm season crop. Grow in summer in North, Fall in South. Sow 3 pounds per 1,000 square feet.
Brown or black mustard (Brassica species). Grows 1 to 3 feet high; hardy to 0° F. Strong taproot mines minerals, but can become a pest. Attracts bees. Sow 1 pound per 1,000 square feet.
Phacelia (P. tanacetifolia). Grows 2 to 3 feet tall; hardy to 20° F. Fast-growing succulent plant. Absorbs excess nitrogen and calcium in soils. Attracts bees. Sow 1/4 pound per 1,000 square feet.
Oilseed radish (Raphanus sativus). Grows 2 to 3 feet tall; hardy to 20° F. Grows fast with a strong taproot. Kills nematodes when tilled into soil, but may harbor Brassica family diseases. Sow 2 pounds per 1,000 square feet.
Photography by National Gardening.com