Cover Crops
by Lynda Prim

Cover cropping and crop rotations are two organic gardening practices that can protect and rejuvenate soils, while fostering balanced, biological diverse garden ecosystems. Mixed plantings or intercropping of two or more plant species (polycultures) can also enhance biologically diversity. Growing monoculture in the same place year after year exhausts soil nutrients and exacerbates pest problems through eliminating habitat for natural enemies and by making it easy for insects and disease to spread from plant to plant. Polycultures tend toward higher yields, as they utilize the resources of light, water, and nutrients more efficiently than monocultures.

Cover crops such as buckwheat, oats, triticale, sorghum, and winter rye work to protect the soil from erosion, control weeds, and build organic matter. Legume cover crops, such as peas, clovers, cowpeas, fava beans, and vetch, have the additional benefit of storing nitrogen for future use by other plants.

Cover crop plants are often used in combination, such as sowing a legume such as peas or vetch with a grass such as winter rye or oats. In this case, the legume builds nitrogen while the grass provides a natural trellis for the legume. In all combinations, more diverse habitat is created for beneficial insects and soil organisms. To further enhance the attraction of beneficials, you can interplant wildflowers and other insectary plants with your cover crops. (link to gardening with beneficials article)

Following is a typical rotation of cover crops with vegetable production.

Oats and Peas -> Beans -> Winter Wheat -> Summer Squash -> Winter Rye, Austrian Winter Peas, and Hairy Vetch

This rotation provides an early spring cover crop that can be incorporated as green manure before planting summer crops. There is plenty of time to plant beans after turning in the cover crop and the beans are cultivated during the growing season for weed control. The winter wheat can be planted after bean harvest in the fall and turned in for organic matter in early spring. The wheat residue will have time to decompose in the soil before planting summer squash, which will provide some weed control through cultivation, and shading the soil. Summer squash can be harvested in time for an early fall cover crop planting of winter rye, Austrian Winter peas, and vetch.

This is just one of a wide range of possibilities. The limiting factors are climate, soil, equipment, labor, and the imagination of the gardener.

Planting and Harvesting a Green Manure Cover Crop

To sow a green manure/cover crop it is necessary to prepare the soil by tilling or raking to break up the surface and to be able to cover the seed. When the soil is prepared, broadcast the cover crop seeds as evenly as possible over the soil surface. The seeds can be covered by raking them lightly into the soil without burying them too deep. Then tamp the soil lightly with the back of the rake or with a roller to make sure the seeds are in contact with moist soil. It is helpful to pre-irrigate unless the soil already has good moisture. Keep the field or bed moist until the crop is established. For field scale, cover crop seed can be drilled or broadcast and harrowed.

A typical annual cover crop might include oats, field peas, and vetch sown in late spring. Before planting summer crops, you can mow and chop the plant material with a hand sickle, scythe, machete, or rotary mower depending on the scale of your planting, Use a digging fork or tiller to incorporate the cover crop into the soil. Incorporating cover crops in the green vegetative stage which will add organic matter and increase biological activity. Turn the soil shallowly (4 to 6 inches) so that you don't bury the living part of the soil. It is best to wait at least two weeks for the "green manure" to break down before planting. A thin layer of compost can be sprinkled into a cover crop before it it's turned into the soil to help the decomposition process and humus formation. In heavy soils, inoculating with earthworms helps speed up the decomposition.

Alternately, you can a mow the crop and add it to your compost pile and allow the root systems to break down in the soil. If you can afford to set that ground aside from vegetable or flower production, you might let the cover crop grow all season and winter-kill leaving a protective mulch on the soil for the winter. In spring you can rake the bed clean and add the plant residues to the compost pile, then plant larger seeded vegetable crops or transplants. The soil will need to be worked further to sow fine seeded crops.

Legumes and Nitrogen Fixation

Specific soil bacteria live in association with the roots of legumes. Through this mutually beneficial association, the bacteria provide the legumes with access to nitrogen in the air, while the legume roots provide nutrition to the bacteria. The captured nitrogen is stored in nodules on the roots. Whether the legumes are turned under, killed by winter temperatures, or mowed and composted, the nodules provide an essential plant nutrient for subsequent crops.

Legume Inoculants: If you have not grown a specific legume in your soil before, it may be beneficial to inoculate your seed before planting. The inoculant is a fine dry powder that contains symbiotic bacteria specific to that legume. Mix the powder with the seeds before sowing. Inoculants can be purchased online from Peaceful Valley Farm Supply.

"Which cover crop should I grow?"

No single cover crop will serve all situations. To begin answering the question of which cover crop to grow, first identify your objectives. For example, a cover crop that fixes a great deal of nitrogen may fail to prevent leaching of nutrients already present in the soil. Gardeners who use a lot of composted manure may have plenty of nitrogen already. In this case, you might choose a grass like winter rye that can take up the nitrogen and turn it into increased biomass. An important consideration in choosing a cover crop is whether the particular variety grows well in your locale. Experimenting with different varieties and consulting with farmers or extension agents in your area can be helpful. Even less than optimal cover cropping is still better than leaving bear soil exposed to the elements.

Seeds of Change Cover Crop Varieties Legumes Papago Cowpea Pisum sativum Cowpeas are the most productive heat-adapted legume. Arkansas Crowder is more productive but not as drought tolerant as Papago. Produces 3800 - 4800 lbs/acre dry matter and 80 lbs/acre of nitrogen. Annual. Legume. Plant 40-80 lbs/acre or 1-2 lbs/1000 sq. ft. Chinese Red Cowpea A great all-around annual summer legume for fixing nitrogen, increasing organic matter and, if incorporated after bloom (about 100 days),has some nemotocidal effect. Grows to 2-3 ft. Produces a pretty red flower and can contribute up to 250 lbs/acre of nitrogen. They are fairly drought resistant and will thrive in low fertility soils. Seed after danger of frost has passed and soils have warmed. Plant in rows 6-8" apart or broadcast into soil .5-1" deep, or deeper if sandy soils, sow 40-80lbs/acre or 1-2 lbs/1000 sq. ft. Hairy Vetch Vicia villosa Extremely cold-tolerant, adaptable and vigorous winter annual legume. Plant in late summer to early fall. Slow to establish, but very prolific spring growth once soil warms up. Dense, viney growth habit, 2-3 ft. high. It can attain greater height when supported by rye/oats/triticale. Contributes 80-250 lb/acre nitrogen and 3000-5000 lb/acre dry matter. Legume. Plant 30 lbs/acre or 12 oz/1000 sq. ft. Medium Red Clover Trifolium incarnatum This winter annual legume planted in late summer is an excellent choice for controlling weeds and protecting soil during the winter. In cold climates it may not survive so it must be spring planted. It produces stunning red flowers from mid to late spring and contributes 70-130 lbs/acre of nitrogen and 2000-4000 lbs/acre of dry matter. Legume. Plant 20-30 lbs/acre or .5 to .75 lbs/1000 sq. ft. Austrian Winter Pea and Speckled Field Pea Pisum sativum Austrian Winter Peas usually fall planted except in extremely cold climates where it is spring planted. Its viney growth habit provides excellent erosion control. The Speckled Field variety is spring planted as soon as the ground can be worked. It has an erect growth habit and is traditionally grown with oats. Yields 4000-5000 lbs/acre dry matter and 150 lbs/acre nitrogen. Legume. Plant 80-150 lbs/acre or 2-3.5 lbs/1000 sq. ft. Sweet Loraine Fava Beans Vicia faba This medium-sized fava bean survives winter temperatures down to 10©øF. Fall plantings grow up to 6 feet tall, and produce 4000-6000 lb/acre dry matter and 100-200 lb/acre nitrogen. Can also be planted in early spring and does best where cool, moist weather persists for 2-3 months. Legume. Plant 150 lb/acre or 2.5-3.5 lb/1000 sq. ft. Yellow Blossom Sweet Clover Melilotus officianlis This spring-sown biennial legume produces a strong, deep taproot that loosens subsoil and reduces soil hardpan problems. Blossoms attract honeybees and beneficial predatory wasps. In early summer of its second year the plant attains a height of 4 to 6 ft. Tolerates water-logged, saline, alkaline, and low-fertility soils. Adds 80-100 lbs/acre nitrogen and 2000-3500 lbs/acre dry matter. Legume. Plant 10-15 lbs/acre or .25 to .3 lbs/1000 sq. ft. Non-Legumes Black African Sorghum Sorghum bicolor This summer annual member of the grass family resembles corn, but it is more drought and heat tolerant. It is extremely productive, grows to 8 or 9 ft. and yields 5000-10,000 lbs/acre dry matter within three months of planting. It is the perfect crop to grow when large amounts of carbon-rich material are needed for your compost pile. Non-legume. Plant at 25-50 lbs/acre or .6-1.2 lbs/1000 sq. ft. Buckwheat Fagopyrum esculentum This broadleaf summer annual grows extremely rapidly, producing flowers in 5-6 weeks and growing 3-6 feet. Grows in acidic and low fertility soils. An ideal smother crop resulting in a nearly weed-free, loose seedbed. Attracts numerous beneficial insects and can make soil phosphorus, calcium and potassium more readily available. Non-legume. Plant 50-100 lbs/acre or 1.2-2.5 lbs/1000 sq. ft. Grey Walken Oats Avena sativa This fibrous-rooted, erect, annual grass grows in cool, moist conditions to 3 ft. Planted in cold climates produces 2000-4000 lbs/acre dry matter, dies during the winter, creating a thick mulch mat into which spring crops can be planted. Fall plantings in mild climates survive winter, yield 6000-8000 lbs/acre dry matter. Spring planting suitable in all climates. Excellent crop to plant with hairy vetch, peas and fava beans. The Hulless variety produces oats without thick hull. Non-legume. Plant 90-110 lbs/acre or 2 - 2.5 lbs/1000 sq. ft. Hulless Oats Avena nuda This fibrous-rooted, erect, annual grass grows quickly in cool, moist conditions to 3 ft. tall. When planted in cold climates it produces 2000-4000 lbs/acre dry matter, then dies during winter, creating a thick mulch mat into which spring crops can be planted. Fall plantings in mild climates survive the winter, yield 6000-8000 lbs/acre dry matter. Spring planting is suitable in all climates. Produces oats without a thick hull, easy process for home use, while Grey Oats are more productive. Non-legume. Spring Triticale and Winter Triticale Triticum durum X Secale cereale Triticale is a genetic cross (not a hybrid) of durum wheat and rye. The grain has similar characteristics to wheat, while the plant has the overall vigor and winter-hardiness of rye. Although this very fast growing variety is usually spring-planted, it may also be planted in fall except where winters are extremely severe. For best results, turn under when it reaches 8" to 12" tall. Plant 100-120 lbs/acre or 2-4 lbs/1000 sq. ft. Winter Rye Secale cereale This is the most cold-hardy and productive annual grass and it tolerates a wide range of soil conditions. Plant any time from early to late fall. Germinates and grows quickly to a height of 4-5 ft. Its fibrous roots markedly improve soil structure. Commonly grown with peas or vetch to provide structural support. Plant at 80-1500 lbs/acre or 2-3.5 lbs/1000 sq. ft.