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Cover Crops
(Exerpts from Bugg, R.L.& M. Van Horn, 1998, Proceedings of the Viticulture Seminar, Australian Society of Viticulture and Oenology, Adelaide.)
Cover crops are important tools for ecological vineyard management and are seeing increasing use to promote soil life, prevent soil erosion, add nitrogen, improve soil structure, manage soil moisture, enhance trafficability during wet weather, suppress resident vegetation and promote beneficial arthropods. Improperly selected or managed cover crops may reduce vine growth and yield, or grape quality. The exacerbation of frost problems is also a concern. There is a rich array of cover-cropping options. In vineyards, a wide range of species may be used, including several annual and perennial plants that can be managed to self-regenerate. Water availability and quality are major issues in several major viticultural areas of California. Many viticulturalists in North Coast counties manage winter-annual resident vegetation as a de facto cover crop intended to protect the vineyard soil from erosion during the most intense winter rains.
California native perennial grasses
California native perennial grasses used in vineyards include several bunchgrasses and a few sod-forming species. Several introduced and true native species of perennial grasses have been tried in various parts of California, principally from the Lodi-Woodbridge area north. These present both challenges and opportunities to winegrape viticulturists. In general, these grasses are suitable in vineyards that are dry-farmed or irrigated by sprinkler or drip systems, and may be used with some difficulty in those that are irrigated by flood or furrow. Tillage at any time and prolonged immersion during summer will kill most kinds of California native perennial grasses.
Difficulties that may arise include:
· The relatively high price of seed relative to most other cover crops.
· Poor seedling vigor makes establishment difficult where high seed densities of vigorous winter-annual species occur.
· Possible increases in pocket gophers, necessitating special vigilance and an intense trapping program.
· Possible excessive devigoration of vines through competition for soil moisture or nitrogen, necessitating careful matching of rootstock vigour, native perennial grass phenology, seeding pattern, and irrigation system.
Some advantages include the following:
· One-time establishment cost for native perennial grasses may be amortized over a ten-year period.
· Some native perennial grasses are summer dormant, drought tolerant and have low stature.
· Fibrous roots of perennial grasses absorb nitrate, bind soil, add soil organic matter (humus).
· Native perennial grasses may reduce weeds.
· Pollen of grasses is food for some lacewings and predatory mites.
· Some native perennial grasses work well with winter annual legumes.
· Perennial grasses provide excellent trafficable surfaces that allow vehicles entry for critical fungicide
applications during wet springs.
Growers should keep in mind that mild water stress between bloom and veraison may have a positive effect on grape quality. Different species of grasses may have widely differing patterns of seasonal growth and soil moisture use. Some species show low biomass production or warm-season dormancy, either of which may reduce soil moisture demand, and hence competition with vines.
Establishment and Management
In general, seasonal dormancies are broken and vegetative growth of established California native perennial grasses begins before or with the first autumn rains, and persists at least into mid-March. During this period, infrequent mowing will cause no long-term damage to the stands. Once seed heads form in late April, mowing should be done with greater caution or not at all, until seed has matured. Seed maturation dates vary among different grass species. After seed maturation, vegetative growth slows or stops for many species of bunch grass, and foliage may disintegrate, as happens for various forms of pine bluegrass. In general, close mowing may be undertaken by early to mid-July, with no damage to the grass stands. Both early and late mowing may assist in the reduction of the (Lactuca serriola)and yellow starthistle(Centaurea sostitialis). Frost management in high-risk areas may necessitate mowing at times other than those optimal for native perennial grasses.
Erosion Prevention
Ecological soil management implies protection of topsoil against erosion and intervention to prevent problems downstream. In areas with high potential to form gullies, it is important to include turf-forming as well as bunch grasses. Creeping red fescue and creeping wildrye(Leymus triticoides)are used in this sort of setting. Erosion prevention necessitates restoration or retention of roadside and riparian vegetation. This presents a challenge in light of the haborage of Pierce´s disease(Xylella fastidiosa)and its leafhopper vectors in riparian vegetation and warm-season perennial grasses.
Cover crops affect infiltration
Under some conditions, improved infiltration caused by cover cropping may lessen the need for irrigation. However, all cover crops require water for growth, and their net effect on the balance sheet may vary with soil type, plant materials, and management technique. Early-maturing winter annual cover crops may be of special value. Use of mown cover crop residue as mulch can help retain soil moisture and encourage earthworms.
Cereals may lead to improved infiltration on heavy soil, as was shown by W.A. Williams of the U.C. Davis Department of Agronomy and Range Science. Williams found that Barley (Hordeum vulgare cv ´atlas 40´), cereal rye (Seacale cereale cv´Svalof Fourex´), annual ryegrass (Lolium multiflorum), and soft chess (Bromus mollis cv´Blando´) significantly improved infiltration rate in a loamy, well-drained soil, but that black mustard (Brassica nigra) did not. Infiltration rate of irrigation water was inversely related to the nitrogen concentrations of nonleguminous green manures at the time of incorporation by discing. Williams attributed this pattern to the more persistent low-nitrogen residues maintaining root channels through which water can pass. Work by Folorunso et al. confirmed that cover cropping can lead to soil structural improvements, leading to reduced penetration resistance and increased water infiltration. Cover cropping may affect soil structure by supporting the growth of soil microbes. Some of these microorganisms produce polysaccharides that in turn promote the formation of water-stable aggregates, particles of soil that are resistant to erosion by water. This phenomenon leads to improved soil structure and increased rates of water penetration. In a study of various effects of loam soil the following was found. The higher amounts of carbon available to microbes in the permanent cover and mowed barley treatments appeard to translate into improved soil structure and water infiltration. This occurred through short-term increase inheavy fraction carbohydrates, but was not reflected in statistically higher concentrations of total organic carbon or microbial biomass carbon. The loam soil in this study had low organic matter, little structure and was very prone to crusting. Thus, it was more susceptible than some other soils to rapid improvement through additions of organic matter. Also, in other soil types, clay, humified organic matter, or iron oxides may be more important in the aggregation process.
(Exerpts from Bugg, R.L.& M. Van Horn, 1998, Proceedings of the Viticulture Seminar, Australian Society of Viticulture and Oenology, Adelaide.)
For more information, visit the following website:
http://www.sarep.ucdavis.edu
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