Types of Cover Crops
Many types of plants can be used as cover crops. Legumes and grasses (including cereals) are the most extensively used, but there is increasing interest in brassicas (such as rape, mustard, and forage radish) and continued interest in others, such as buckwheat. Some of the most important cover crops are discussed below.
Leguminous crops are often very good cover crops. Summer annual legumes, usually grown only during the summer, include soybeans, peas, and beans. Winter annual legumes that are normally planted in the fall and counted on to overwinter include Austrian winter field peas, crimson clover, hairy vetch, and subterranean clover. Some, like crimson clover and field peas, can overwinter only in regions with mild frost. Berseem clover will overwinter only in hardiness zones 8 and above. Hairy vetch is able to withstand fairly severe winter weather. Biennials and perennials include red clover, white clover, sweet clover, and alfalfa. Crops usually used as winter annuals can sometimes be grown as summer annuals in cold, short-season regions. Also, summer annuals that are easily damaged by frost, such as cowpeas, can be grown as a winter annual in the deep southern United States.
One of the main reasons for selecting legumes as cover crops is their ability to fix nitrogen from the atmosphere and add it to the soil. Legumes that produce a substantial amount of growth, such as hairy vetch and crimson clover, may supply over 100 pounds of nitrogen per acre to the next crop. Legumes such as field peas, bigflower vetch, and red clover usually supply only 30 to 80 pounds of available nitrogen. Legumes also provide other benefits, including attracting beneficial insects, helping control erosion, and adding organic matter to soils.
If you grow a legume as a cover crop, don’t forget to inoculate seeds with the correct nitrogenfixing bacteria. Different types of rhizobial bacteria are specific to certain crops. There are different strains for alfalfa, clovers, soybeans, beans, peas, vetch, and cowpeas. Unless you’ve recently grown a legume from the same general group you are currently planting, inoculate the seeds with the appropriate commercial rhizobial inoculant before planting. The addition of water or milk to the seed-inoculant mix helps the bacteria stick to the seeds. Plant right away, so the bacteria don’t dry out. Inoculants are readily available only if they are commonly used in your region. It’s best to check with your seed supplier a few months before you need the inoculant, so it can be specially ordered if necessary. Keep in mind that the “garden inoculant” sold in many garden stores may not contain the specific bacteria you need; so be sure to find the right one for the crop you are growing and keep it refrigerated until used.
Winter Annual Legumes
Crimson clover is considered one of the best cover crops for the southeastern United States. Where adapted, it grows in the fall and winter and matures more rapidly than most other legumes. It also contributes a relatively large amount of nitrogen to the following crop. Because it is not very winter-hardy, crimson clover is not usually a good choice for the regions where significant frost occurs. In northern regions, crimson clover can be grown as a summer annual, but that prevents an economic crop from growing during that field season. Varieties like Chief, Dixie, and Kentucky Select are somewhat winter-hardy if established early enough before winter. Crimson clover does not grow well on high-pH (calcareous) or poorly drained soils.
Field peas are grown in colder climates as a summer annual and as a winter annual over large sections of the South and California. They have taken the place of fallow in some dryland, small-grain production systems. Also called Austrian winter peas and Canadian field peas, they tend to establish quickly and grow rapidly in cool moist climates, producing a significant amount of residue—2 1/2 tons or more of dry matter. They fix plentiful amounts of nitrogen, from 100 to 150 or more pounds per acre.
Hairy vetch is winter-hardy enough to grow well in areas that experience hard freezing. Where adapted, hairy vetch produces a large amount of vegetation and fixes a significant amount of nitrogen, contributing 100 pounds of nitrogen per acre or more to the next crop. Hairy vetch residues decompose rapidly and release nitrogen more quickly than most other cover crops. This can be an advantage when a rapidly growing, highnitrogen-demand crop follows hairy vetch. Hairy vetch will do better on sandy soils than many other green manures, but it needs good soil potassium levels to be most productive.
Subterranean clover is a warm-climate winter annual that, in many situations, can complete its life cycle before a summer crop is planted. When used this way, it doesn’t need to be suppressed or killed and does not compete with the summer crop. If left undisturbed, it will naturally reseed itself from the pods that mature below ground. Because it grows low to the ground and does not tolerate much shading, it is not a good choice to interplant with summer annual row crops.
Summer Annual Legumes
Berseem clover is an annual crop that is grown as a summer annual in colder climates. It establishes easily and rapidly and develops a dense cover, making it a good choice for weed suppression. It’s also drought tolerant and regrows rapidly when mowed or grazed. It can be grown in the mild climates during the winter. Some newer varieties have done very well in California, with Multicut outyielding Bigbee.
Cowpeas are native to Central Africa and do well in hot climates. The cowpea is, however, severely damaged by even a mild frost. It is deep rooted and is able to do well under droughty conditions. It usually does better on low-fertility soils than crimson clover.
Soybeans, usually grown as an economic crop for their oiland protein-rich seeds, also can serve as a summer cover crop if allowed to grow until flowering. They require a fertile soil for best growth. As with cowpeas, soybeans are easily damaged by frost. If grown to maturity and harvested for seed, they do not add much in the way of lasting residues or nitrogen.
Velvet bean (mucuna) is widely adopted in tropical climates. It is an annual climbing vine that grows aggressively to several feet high and suppresses weeds well (figure 10.2). In a velvet bean–corn sequence, the cover crop provides a thick mulch layer and reseeds itself after the corn crop. The beans themselves are sometimes used for a coffee substitute and can also be eaten after long boiling. A study in West Africa showed that velvet bean can provide nitrogen benefits for two successive corn crops.
Similar tropical cover crops include Canavalia, Crotalaria, Tephrosia, all of which can be used as mulches after maturing.
Biennial and Perennial Legumes
Alfalfa is a good choice for well-drained soils that are near neutral in pH and high in fertility. The good soil conditions required for the best growth of alfalfa make it a poor choice for problem situations. Where adapted, it is usually grown in a rotation for a number of years (see chapter 11). Alfalfa is commonly interseeded with small grains, such as oats, wheat, and barley, and it grows after the grain is harvested. The alfalfa variety Nitro can be used as an annual cover crop because it is not very winter-hardy and usually winterkills under northern conditions. Nitro continues to fix nitrogen later into the fall than winter-hardy varieties. However, it does not reliably winterkill every year, and the small amounts of extra fall growth and nitrogen fixation may not be worth the extra cost of the seed compared with perennial varieties.
Crown vetch is adapted only to well-drained soils, but it can be grown under lower fertility conditions than alfalfa. It has been used successfully for roadbank stabilization and is able to provide permanent groundcover. Crown vetch has been tried as an interseeded “living mulch,” with only limited success at providing nitrogen to corn. However, it is relatively easy to suppress crown vetch with herbicides to reduce its competition with corn. Crown vetch establishes very slowly, so it should be used only for perennial cover.
Red clover is vigorous, shade tolerant, winterhardy, and can be established relatively easily. It is commonly interseeded with small grains. Because it starts growing slowly, the competition between it and the small grain is not usually great. Red clover also successfully interseeds with corn in the Northeast.
Sweet clover (yellow blossom) is a reasonably winter-hardy, vigorous-growing crop with an ability to get its roots into compacted subsoils. It is able to withstand high temperatures and droughty conditions better than many other cover crops. Sweet clover requires a soil pH near neutrality and a high calcium level; it does poorly in wet, clayey soils. As long as the pH is high, sweet clover is able to grow well on low-fertility soils. It is sometimes grown for a full year or more, since it flowers and completes its life cycle in the second year. When used as a green manure crop, it is incorporated into the soil before full bloom.
White clover does not produce as much growth as many of the other legumes and is also less tolerant of droughty situations. (New Zealand types of white clover are more drought tolerant than the more commonly used Dutch white clover.) However, because it does not grow very tall and is able to tolerate shading better than many other legumes, it may be useful in orchard-floor covers or as a living mulch. It is also a common component of intensively managed pastures.
Commonly used grass cover crops include the annual cereals (rye, wheat, barley, oats), annual or perennial forage grasses such as ryegrass, and warm-season grasses such as sorghum–sudan grass. Nonlegume cover crops, which are mainly grass species, are very useful for scavenging nutrients—especially N—left over from a previous crop. They tend to have extensive root systems, and some establish rapidly and can greatly reduce erosion. In addition, they can produce large amounts of residue and, therefore, can help add organic matter to the soil. They also can help suppress weed germination and growth.
A problem common to all the grasses is that if you grow the crop to maturity for the maximum amount of residue, you reduce the amount of available nitrogen for the next crop. This is because of the high C:N ratio, or low percentage of nitrogen, in grasses near maturity. The problem can be avoided by killing the grass early or by adding extra nitrogen in the form of fertilizer or manure. Another way to help with this problem is to supply extra nitrogen by seeding a legume-grass mix.
Winter rye, also called cereal or grain rye, is very winter-hardy and easy to establish. Its ability to germinate quickly, together with its winter-hardiness, means that it can be planted later in the fall than most other species, even in cold climates. Decomposing residue of winter rye has been shown to have an allelopathic effect, which means that it can chemically suppress germination of weed seeds. It grows quickly in the fall and also grows readily in the spring (figure 10.3). It is often the cover crop of choice as a catch crop and also works well with a roll-crimp mulch system—in which the cover crop is suppressed by rolling and crimping at the same time and crops are seeded or transplanted through the mulch (see figure 16.7).
Oats are not winter-hardy. Summer or fall seedings will winterkill under most cold-climate conditions. This provides a naturally killed mulch the following spring and may help with weed suppression. As a mixture with one of the clovers, oats provide some quick cover in the fall. Oat stems help trap snow and conserve moisture, even after the plants have been killed by frost. Black oat is very popular with farmers in South America, where it is mulched for no-till row crops.
Annual ryegrass (not related to winter rye) grows well in the fall if established early enough. It develops an extensive root system and therefore provides very effective erosion control while adding significant quantities of organic matter. It may winterkill in cold climates. Some caution is needed with annual ryegrass; because it is difficult to kill, it may become a problem weed in some situations.
Sudan grass and sorghum-sudan hybrids are fastgrowing summer annuals that produce a lot of growth in a short time. Because of their vigorous nature, they are good at suppressing weeds. If they are interseeded with a low-growing crop, such as strawberries or many vegetables, you may need to delay seeding so the main crop will not be severely shaded. They have been reported to suppress plant-parasitic nematodes and possibly other organisms, as they produce highly toxic substances during decomposition in soil. Sudan grass is especially helpful for loosening compacted soil. It can also be used as a livestock forage and so can do double duty in a cropping system with one or more grazings and still provide many benefits of a cover crop.
Buckwheat is a summer annual that is easily killed by frost. It will grow better than many other cover crops on low-fertility soils. It also grows rapidly and completes its life cycle quickly, taking around six weeks from planting into a warm soil until the early flowering stage. Buckwheat can grow more than 2 feet tall in the month following planting. It competes well with weeds because it grows so fast and, therefore, is used to suppress weeds following an early spring vegetable crop. It has also been reported to suppress important root pathogens, including Thielaviopsis and Rhizoctonia species. It is possible to grow more than one crop of buckwheat per year in many regions. Its seeds do not disperse widely, but it can reseed itself and become a weed. Mow or till it before seeds develop to prevent reseeding.
Brassicas used as cover crops include mustard, rapeseed, and forage radish. They are increasingly used as winter or rotational cover crops in vegetable and specialty crop production, such as potatoes and tree fruits. Canola grows well under the moist and cool conditions of late fall, when other kinds of plants are going dormant for winter. Rape is killed by harsh winter conditions but is grown as a winter crop in the middle and southern sections of the U.S. Forage radish has gained a lot of interest because of its fast growth in late summer and fall, which allows significant uptake of nutrients. It develops a large taproot—1–2 inches in diameter and a foot or more deep—that can break through compacted layers, allowing deeper rooting by the next crop (figure 10.4). Forage radish will winterkill and decompose by spring, but it leaves the soil in friable condition and improves rainfall infiltration and storage. It also eases root penetration and development by the following crop.
Canola and other brassica crops may function as biofumigants, suppressing soil pests, especially root pathogens and plant-parasitic nematodes. Row crop farmers are increasingly interested in these properties. Don’t expect brassicas to eliminate your pest problems, however. They are a good tool and an excellent rotation crop, but pest management results are inconsistent. More research is needed to further clarify the variables affecting the release and toxicity of the chemical compounds involved. Because members of this family do not develop mycorrhizal fungi associations, they will not promote mycorrhizae in the following crop.