Cover Crop Selection and Management in Orchards and Vineyards

Chuck Ingels, Sustainable Agriculture Research & Education Program, UC Davis
California Plant and Soil Conference Proceedings, Modesto CA.

Potential Benefits of Cover Crops

Potential Problems with Cover Crops

Addition or conservation of nitrogen

Increased costs and managements, but benefits may outweigh the cost

Reduced soil erosion

Depletion of soil moisture

Addition of organic matter to soil

Increased frost hazard

Weed suppression

Increases weed problems

Improved soil structure and water pen.

Increased pests

Improved traction

 

Increased beneficial arthropods

 

San Joaquin Valley growers have several cover cropping systems from which to choose. The choice and performance of cover crops often depends on site-specific factors, so they should first be tested in a few rows before planting large acreages. The main factors to consider when selecting a particular species or mix are costs vs. benefits, irrigation method, tillage practices, nitrogen needs, frost concerns and harvesting practices (for nut crops). Understanding the basic cover crop types and management strategies can greatly improve the chances for success.

Cover Crop Species and Blends

Monocultures of sown cover crops are frequently used, but problems may occur with the use of the same species year after year. For example, continual plantings of Cahaba white vetch may be affected by soil diseases; also, in some areas alfalfa weevil can be a serious pest of bur medic. Localized environmental niches, such as sand streaks or areas with differing soil nutrient availability, may also provide unfavorable growing conditions for a single species. Providing different species in a mix may enable one species to thrive in areas where another might be weak, increasing the chances for a healthy stand throughout the orchard or vineyard. Polycultures also attract a diversity of beneficial arthropods which may aid in pest management, although research in this area has provided mixed results.

Resident vegetation is naturally-occurring vegetation, or "weeds." While not sown, this ground cover is inexpensive to manage and has some of the benefits of sown cover crops, such as providing traction, improving soil tilth, and attracting some beneficial insects. However, resident vegetation can be highly variable among farms and even within a single orchard or vineyard. It usually contains little or no nitrogen-contributing legumes, which are essential on organic farms. Also, it can contain undesirable weeds which may compete excessively with the trees or vines. Of course, many growers use resident vegetation successfully.

Reseeding Winter Annual Cover Crops. With the increasing trend towards nontillage, many growers sow winter annual species that reseed and die in the spring and regenerate each fall--preferably with irrigation, but often with rainfall alone. Such species include clovers, medics, many vetches, oats, barley, 'Blando' brome, and 'Zorro' fescue. Species that do not effectively reseed include bell (fava) beans and field peas. In addition, many species will only reliably reseed if supplemental irrigation water is supplied in the spring. Blends of burr medic (burr clover) and subterranean, crimson, and rose clovers are common and can be quite productive when properly maintained. Where no supplemental irrigation is available (i.e., under drip irrigation), the following species that will reseed in most years: burr medic, early and mid-season varieties of subterranean clover, rose clover, 'Blando' brome (soft chess), and 'Zorro' fescue. If not replanted or if neglected, however, in time these species may simply become minor components of the ground cover. Periodic replanting every 3 to 4 years can ensure dominance by these species.

Burr medic is well adapted to California's climate and grows well only in neutral to high-pH soils; it is occasionally a major component of resident vegetation. It effectively reseeds even under fairly close mowing and, because of its high percentage of hard seed, it usually reestablishes well even when tillage is used. For these reasons, it is an excellent cover crop for raisin vineyards. Burrless varieties, such as 'Santiago,' are available commercially. Subterranean clover, or subclover, usually performs best in acid to neutral soils. It forms a dense, spreading mat which can effectively suppress weed growth, especially with periodic mowing. Early-maturing varieties, frequently used on range land, include 'Nungarin' and 'Dalkeith.' 'Blando' brome is often used in monocultural stands, although it is useful as a low-growing, mowable grass when combined with clovers and medics. 'Zorro' fescue is very expensive and is used mostly on hillside, serpentine, low fertility soils or where initial erosion control is required on cleared land.

High Biomass Mixes. Where supplemental irrigation is available, mixes which produce large amounts of biomass, or plant matter, can be used to add organic matter to the soil. The periodic addition of organic matter enhances soil microbial populations, improving soil structure and nutrient cycling. High biomass mixes contain large seeds and are usually quite easy to grow. In general, they are sown each fall and either disked or mowed in the spring. Two basic types of high biomass mixes are often available from seed companies: pure legume and legume/grass blends.

Pure legume blends, usually containing bell beans, vetch, and field peas are used to add the maximum amount of readily available nitrogen to the soil. They are usually disked in late March or April during peak flowering; this is referred to as a "green manure" cover crop. Where furrow irrigation is used, they are disked before the soil dries excessively to enable the disk to penetrate the soil. Bell beans produce vigorous, upright growth. Field peas are shallow rooted and therefore subject to drought on sandy soils; they also produce the far majority of their biomass and nitrogen in the spring. Vetches are frequently used, but may twine up vines, trees, or sprinklers if planted too close. Also, if allowed to reseed they may become a weed problem in the rows. Among the vetches, purple vetch usually produces the greatest amount of biomass by late winter/early spring, but it is also the least winter hardy. When maximum biomass production is desired before disking in March, purple vetch is may be the best vetch species to use in most areas of California, although mixing vetches is also advantageous. 'Lana' woollypod is one of the most vigorous vetches in the spring and flowers and matures earlier than purple vetch. Common vetch has extrafloral nectaries on the stipules, which provide nectar to beneficial insects. Cahaba white vetch has been shown to suppress root knot nematodes.

Various legume/grass blends are also available. The addition of grasses, such as barley, oats, or cereal rye in a mix imparts several benefits. The fibrous roots of grasses greatly enhance soil tilth and water penetration. Grasses also take up excess nitrogen from the soil, improving the growth and nitrogen-fixing ability of the legumes. Lastly, grasses provide structural support for the twining vetches and peas. Legume/grass cover crop blends can be disked, but are often simply mowed; the clippings form a moisture-conserving mulch through much of the growing season. Typical blends often consist of bell beans, one or more vetches, peas, and oats. Because bell beans and peas do not tolerate mowing, they are sometimes omitted; barley/vetch or oat/vetch blends are relatively inexpensive and are used in many orchards and vineyards.

Perennials. Perennial grasses and legumes provide a permanent cover that offers year-round traction and ease of management. Perennial clovers, such as white and strawberry, are low growing and add nitrogen, but are invasive and compete with trees and vines for water. Birdsfoot trefoil, a legume, is slow to establish but forms a low-growing, dense cover. Vigorous, summer-active perennial grasses, such as 'Berber' orchardgrass and tall fescue, devigorate trees and vines and should only be planted where vigor is excessive.

Drought tolerant and drought avoidant perennial grasses, most of which are native to California, are currently popular among many grape growers, especially in the North Coast and the San Joaquin delta area. Drought tolerant species survive dry periods by developing large root-to-shoot ratios--i.e., extensive, spreading root systems. Drought avoidant species, such as meadow barley and California brome, deal with dry periods by going dormant when water is scarce, and are most appropriate under drip irrigation. While both types can be useful, the latter species are the most appropriate in order to avoid excessive competition with vines. Under drip irrigation they will almost completely shut down growth during the summer, and thus will not compete with the vines. The seed is quite expensive initially, but can be relatively inexpensive over the life of the cover. It is important to select species that are adapted to the climate. Special care must be used in planting and establishment of these species, but once established they often effectively outcompete weeds. Low-growing mixes are available that require only one mowing per year; in any case, springtime mowing of these grasses should be avoided until after seed has matured.

Insectary Blends. Many cover crop blends are available which are sown for the purpose of attracting beneficial insects. These mixes vary greatly among seed companies, but usually include members of the carrot and sunflower families. Insectary blends are usually sown in the fall every 3 to 10 rows and treated as perennials; they are allowed to flower and reseed in order to be effective and to persist. While insectary blends do attract a diversity of predators and parasitoids, their role in pest management is unclear. Some growers have encountered problems with these blends, usually arising from poor establishment from planting too deeply (they should be only about 1/4 in. deep). Summer annual insectary species, such as buckwheat, are occasionally planted in orchards and vineyards.

Management Strategies and Considerations

Planting. Winter annual and perennial cover crops perform best when sown by mid-October, but can also be successfully grown if planted by mid-November. It should be noted that establishing small-seeded cover crops in years with little fall rains may be very difficult, especially on sandy soils. In general, lower rates can be used for early seeding, and higher rates should be used for later seeding. For example, vetch sown in late September or early October may be seeded at 30 to 40 pounds per acre, while an early November seeding would require 40-80 lbs. Good seedbed preparation is essential, especially with native grasses. Legume seeds must be inoculated with nitrogen-fixing Rhizobium bacteria to ensure nitrogen fixation. Small-seeded legumes are usually preinoculated, however large-seeded legumes (bell beans, vetch, and peas) must be inoculated by the grower, at least the first time they are sown. Use about one 8-ounce bag of inoculant per 100 pounds of seed. The most reliable method is the "wet" method, where a slurry of inoculum and adhesive are added to the seed and then allowed to dry before planting. However, the inoculant can also simply be layered (dry) with seed in the hopper. Inoculant or inoculated seed should be kept out of direct sunlight, so broadcast seed should be incorporated as soon as possible.

Mowing. Because cover crops can increase frost hazard, they are often mowed in late winter. Bell beans and peas do not perform well if mowed. Vetch should be mowed high (no lower than 8-10 in.), but must not be mowed after about late March if reseeding is desired. High biomass blends can be killed in spring if mowed close to the ground. Nontillage clover mixes should be mowed in late winter to suppress tall weeds and encourage spreading. Subclover and burr medic can usually reseed even under fairly close mowing, while crimson and rose clovers flower above the foliage and therefore must not be mowed after about late March to allow for reseeding.

Nutrition. As with trees and vines, soil fertility is critical to cover crop production. Legumes fix nitrogen, so nitrogen should not be applied shortly before or during their growth. However, legumes do require adequate sulfur (which is plentiful in most vineyards) and phosphorus for good growth. Annual grasses require nitrogen additions if grown alone, and will respond well to up to 50 pounds of nitrogen per acre. Perennial grasses may require even more than this amount. In general, grasses predominate on highly fertile sites, while legumes will usually grow best in soils with low nitrogen content. Many poor solid legume plantings (especially clovers and medics) that are overtaken by grasses and mustards may be a result of excessive soil nitrogen, so broadcast fertilization should be avoided from fall until late spring. The end result of cover cropping should be optimum crop nutrition and productivity, so tissue nutrient analyses should be conducted regularly.

In general, vetches and peas can fix far more nitrogen than clovers and medics. A green manure cover crop disked in April can add 150 pounds or more of nitrogen per planted acre, which may cause excessive vigor in grapevines, especially on highly fertile soils. Nitrogen production in orchards may be reduced due to shading and the use of wide herbicide strips. Management options in vigorous vineyards include alternate row planting, perhaps with native perennial grasses in alternating rows, and mowing instead of disking. When residues are mowed and left on the soil surface, a portion of the nitrogen will volatilize into the atmosphere. With about 80 percent of the nitrogen in leguminous cover crops contained in the above-ground portion, volatilization losses can be high--perhaps as much as half. Nontillage clovers and medics may add only about 30 to 40 pounds of nitrogen per planted acre. Perennial clovers may add substantial amounts of nitrogen, while perennial grasses use soil nitrogen and may require supplemental fertilization.

Cover Cropping in Almond and Walnut Orchards. Because almonds and walnuts are harvested picked up from the orchard floor, excessive cover crop debris at harvest could be problematic, particularly in nontilled orchards. However, most winter annual cover crops can be grown successfully if properly managed. Legume cover crops have a low carbon-to-nitrogen (C/N) ratio and therefore break down fairly rapidly after mowing, usually causing no residue problems at harvest. Grass residues have a high C/N ratio and can persist until harvest, but they are usually not a problem if mowed closely by June, followed by periodic mowing until harvest. Under drip or microsprinkler irrigation, breakdown of residues is slower, so mowing should start in late spring.

References

Bugg, R.L. 1993. Creative Cover Cropping in Perennial Farming Systems (Video). University of California, Division of Agriculture & Natural Resources. 27 min. (800) 994-8849.

ConservaSeed. California Native Perennial Cover Crops: A Technological Revolution. (Information pamphlet) (916) 775-1676.

Finch, C.U. and W.C. Sharp. 1983. Cover Crops in California Orchards & Vineyards. Natural Resources Conservation Service, Davis CA. 25 pp. (916) 757-8200.

Ingels, C. 1995. Cover cropping in vineyards: A grower profile series, Parts 1-5. American Vineyard vol. 4, nos. 5-6, 8-10. May-June, August-October, 1995. (209) 298-6675.

Ingels, C., M.V. Horn, R.L. Bugg, and P.R. Miller. 1994. Selecting the right cover crop gives multiple benefits. California Agriculture 48(5):43-48. (510) 987-0044.

McGourty, G. 1994. Cover crops for North Coast vineyards. Practical Winery & Vineyard 15(2):8-15. July/Aug, 1995. (415) 479-5819.

Miller, P.R., W.L. Graves, and W.A. Williams. 1989. Cover Crops for California Agriculture. University of California, Division of Agriculture & Natural Resources. 24 pp. (800) 994-8849.