In September, most gardeners can plant overwintering cover crops. If you live where winters are mild, such as USDA Hardiness Zones 8 and warmer, you have time to plant and till under a green manure crop before planting a winter cover crop. Green manure crops are plants that are tilled under in summer, while cover crops are meant to cover the soil all winter and are tilled under in late winter or spring.Benefits of Cover Crops
Most gardeners know about the benefits of adding organic matter to the soil. Many of us build the soil with annual applications of compost, manure, leaves, and grass clippings. However, in some areas, these forms of organic matter may be difficult to find or transport, making cover crops an especially good way to add organic matter. Plus, cover crops allow you to avoid importing soil amendments. By growing your own organic matter, you eliminate the fossil fuels needed to transport imported amendments, helping to make your yard a closed loop of energy inputs.
Cover crops provide many benefits:
They add organic matter. A primary reason to grow a cover crop is to increase the amount of organic matter in the soil. Adding organic matter improves the soil's structure, increases its water retention and drainage, and improves 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.
They control erosion. Traditionally, cover crops were used to "cover" the soil during the winter. Hardy crops, such as winter rye, are particularly good at preventing erosion and topsoil loss, especially in areas with high winds and inconsistent snow cover.
They loosen 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.
Buckwheat is a quick-growing cover crop, and its white flowers are favorites of bees. The plant not only adds organic matter to the soil, but also shades and smothers weeds.
They balance nutrients. 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 plants 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 nitrogen already in the soil that may otherwise leach out and pollute ground water or nearby streams.
They help control weeds. 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, release chemicals that prevent weed seeds from germinating.
They attract beneficial insects. When cover crops, such as crimson clover and mustard, are allowed to flower, they attract bees and beneficial insects that help with pollination and insect control in the garden.Getting Started
For most home gardeners, annual cover crops are the best choices. Sow annuals in fall, and they either die from the winter cold or naturally complete their life cycle by the next spring. Perennial cover crops, such as alfalfa (Medicago sativa) and red clover (Trifolium pretense), require more maintenance and are best used as ground covers in orchards and vineyards.
Annual ryegrass is a good cover crop to plant between rows of crops. In cold weather areas it may naturally die back in winter, making it easy to till under in spring.
You can plant cover crops two ways, depending on your other plantings: Sow seeds after you harvest 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. Broadcast seed at the recommended rates. On drier, sandy soil, plant seeds three times as deep as the seed's width; on heavier soil, plant twice as deep as the width.
Plant. Sow seeds at least 30 days before your first expected fall 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. Water garden beds, if necessary, to help get the crop established. Prior to planting, inoculate the seeds of legume cover crops with nitrogen-fixing bacteria (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. Planting mixes is the easiest way to sow cover crops. Here are some examples of cover crops to grow in warm- and cold-winter areas.
Four Kinds of Cover Crops
Crimson clover is a leguminous cover crop that helps build the soil. It can become invasive if it isn't tilled under before flowering.
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 and 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. If allowed to go to seed, can become weedy.
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.
The tropical legumes below 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. These are 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. Grows 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 the structure of compacted soils. 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.
While cover crops, such as this red clover, are good for the soil, they also are tasty, too. Remember, this is the edible landscaping newsletter!
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.More Great Stories on Fall Soil Building