COVER CROPS FOR EROSION CONTROL IN NAPA COUNTY VINEYARDS. Phillip R. Blake*

Transcription

1 COVER CROPS FOR EROSION CONTROL IN NAPA COUNTY VINEYARDS Phillip R. Blake* Introduction Premium wine grapes and the Napa Valley have long association to many wine affectionados the world over. of Napa County grapes has grown over the years, so has plantable acreage. Upland areas have been cleared and vineyards at an increasing rate due among other things available valley lands and high purchase costs. been a synonymous As the reputation the demand for developed for to dwindling Today, I'd like to briefly sununarize the extent of sheet and rill soil erosion problems in Napa County, and look at some of the cover crop plants and floor management techniques being used to control soil loss and benefit hillside vineyard management. Erosion Study A 1985 Soil Loss Study conducted by Angela White of the Napa County Resource Conservation District (1), found that of the 29,000 total planted vineyard acres in the county, just over 7,000 were developed on lands of 5% or greater slope. Because the study was initiated to inventory soil conservation concerns, the 5% slope break was chosen to represent vineyards of significdnt erosion potential. This potential is calculated using the Universal Soil Loss Equation (U.S.L.E.). The method, simply put, estimates yearly soil detachment rates using rainfall, soil erodibility, slope, vegetative cover and terrain factors. The equation measures detachment only and does not quantify "on" or "off the field" sedimentation rates. In the study, estimated soil loss rates varied widely between extremes of terrain and soil type, but overall averaged 14 tons per acre. For visualization purposes, this rate is approximately equivalent to 1/10 of one inch of soil. The average "T" factor or allowable U.S.L.E. loss factor assigned to soils in Napa County hillsides varies between 2 and 5 tons per acre, per year, so a comparison of the two values demonstrates the potential magnitude of topsoil loss. The most dramatic examples of hillside vineyard soil loss, concentrated flow, gulley and landslide erosion cannot be predicted with the U.S.L.E. and can only be reduced in some cases by cover crops, not controlled. Nonetheless, an important point is in order here as the average hillside vineyardist does not generally recognize erosion as a problem unless it creates an eyesore or effects the farming operation. Sheet and rill erosion represent a more gradual, long-term loss of soil that is effectively controlled by a good cover crop program. According to a river basin study report (2) completed by the Soil Conservation Service in 1985, no-till cover crops were the most cost effective per acre erosion control practice used in north coast county vineyards. *District Conservationist, U.S.D.A. Soil Conservation Service, Napa County. 174

2 Typical Floor Management Floor management systems in Napa County are about as varied as those used in central valley vineyards. On the hillsides, particularly where terraces are involved, soil in the vine row is rarely cultivated for weed control. In most management schemes, various combinations of translocative and pre-emergent herbicides are applied and middles are either chopped and disked or left mown and uncultivated. Our study indicated that approximately 600 acres were under some form of no-till cover crop management in 1985, so clearly middle cultivation is still the most widely used practice. With my limited time, I'd like to briefly describe typical cover crop plants and some management pros and cons we have found in our work with hillside vineyardists. Cover Crop Plants Generally, there are three categories of cover crops: 1. Quick establishing grasses seeded as temporary cover in recently developed vineyards. 2. Introduced grasses and legumes for no-till management. 3. Resident cover plants managed for non-tillage. In the "quick cover" category of plants, we find annual ryegrass (Lolium multif lorum) and the cereal grains, most notably barley (Hordeum vulgare) and oats (Avena sativa) as the plants of choice. The reasons for selection are the relative speed of establishing strong root systems, supplementation of soil organic matter, and low cost as compared to other introduced cover plants. Unfortunately, the benefit of rapid, robust root development also means high volumes of top residue to contend with when rainfall is average or above. Because erosion potentials are so high in the first 1 to 2 years of vineyard development, most managers are willing to accept the management difficulties. On the hillsides, these plants are rarely persistent in great numbers down the road and till~ge or management for other types of cover proceed after high erosion potential has subsided. There are a number of i~troduced grasses and legumes seeded for cover, and it is this category of plants that the Soil Conservation Service has conducted most studies since These plants include "Blando" brome, (Bromus mollis), Rose clover, (Trifolium hirtum), "Zorro" annual fescue, (Vulpia myuros), Berber orchardgrass, (Dactylis glomerata), annual bluegrass (Poa annua), and O'Connor"s legume; (Trifolium fragiferum). These plants have been tested both under tillage and non-tillage mowing programs. Because of poor fall re-establishment and unreliabl.e early season erosion protection, all of these plants are best suited for non tillage. Because of limited time, I'd like to concentrate on our findings with "Zorro" annual fescue, the plant showing most promise in this category. "Zorro" has shown perhaps the most favor with growers due to its relatively low stature and residue, ability to persist under a variety of soil conditions, 175

3 and ease of control in vine row strips with herbicides. This plant is an early maturing annual grass and its earliness presents an added benefit of weed species shift under a properly-timed mow program. In average moisture years, an initial mowing in February or March can keep vigorous weeds from forming a suppression canopy. We have found that mower blades set 4 to 6 inches high seem to achieve this effect the best. A final mowing in late spring, when most "Zorro" plants have recently set mature seed appears to encourage specie shift away from other more aggressive winter annual weeds that are completing a seed cycle slightly behind the fescue. On the negative side of the species shift, most mowing studies have shown a definite trend toward a prevalence of thistle family weeds. Use of Roundup applied as a spot treatment after the final mowing is commonly used by growers to combat this problem. This treatment seems to be particularly effective on these weeds and other connnon broadleaf s that persist in the decaying sod. The drawback to wider use of "Zorro" has typically been the cost of seed. It is connnonly in demand for wildfire re-vegetation and other specialty erosion control plantings, so price stability has fluctuated from year to year. Another problem we have noted is delayed first year establishment. Because of fall grape harvest activity, cover crops are often sown later in the fall than we like to see. When Zorro is seeded late, its growth typically lags well behind competing winter weeds to the point that the purity 1of the stand can be greatly reduced. Once established, regrowth in successive years improves dramatically, probably due in part to mulch from the previous season's growth. This brings up an important footnote regarding the erosion control effectiveness of permanent cover vs. tillage systems management. As we all know, unless a vineyard is under total year-round chemical eradication of floor cover, winter weeds eventually grow in the middles. These weeds are generally controlled by tillage as the frost season begins in early spring. In years of early, low intensity fall rains and reasonably dry spring weather, weed cover can provide erosion control protection comparable to a no-till system. Because weather is not consistent and forgiving, and because tillage systems typically do not provide selection of weeds with characteristics we desire, mown cover seems to present a pref erred year-to-year edge. Managing resident plants for cover is another option for growers who are interested in converting to non-tillage. Some of the plants previously mentioned as introduced cover candidates are, of course, on the labels of several herbicides under the "Weeds Controlled" heading. In many cases, plants such as soft chess, annual bluegrass, annual fescue, and rose clover are naturally plentiful enough in a vineyard to be managed through mowings and selective use of herbicides. Dean Donaldson, Director of U.C. Cooperative Extension in Napa County, is investigating in part, the effect of herbicides and floor management techniques on native cover species shift and composition in north coast vineyards. 176

4 Perhaps the most appealing aspect of managing resident plants for cover is establishment cost. Depending on seed selection, optional use of fertilizer, and seedbed preparation methods,planting costs have ranged from $30.00 to more than $ per acre. This factor alone discourages some growers. Expense has been offset to some degree by federal cost share monies available from the U.S.D.A./A.S.C.S. Agricultural Conservation Program. Favorable factors other than erosion control play into a decision to adopt non-tillage cover crop management. Reduction of tillage-induced surface rocks, potentially lower floor management costs, and a firmer winter floor for equipment operation are leading reasons growers adopt this practice. Rock removal or set aside represents a yearly cost in some vineyards and the wear and tear rocks cause tillage implements also of course costs money. In some cases, major rock removal is necessary to prepare a floor for mowing, but usually this is a one-time expense. Floor management cost reduction is most dramatic where one or two mowings are used to manage cover, chemical strip treatment practices are equal from tillage to non-tillage, and where several return diskings were common under the tillage program. Depending on management practices employed, reductions of 30% to 50% are common. This point alone may justify costs of initial introduced cover establishment. A number of unanswered questions remain regarding the effect no-till cover may have on the vineyard environment and wine quality. Concerns, which include increased frost damage potential, pests and diseases, management of new weed populations, potential yield reduction and summer fire hazard all need to be addressed in assisting grower decision making. Dean Donaldson's ongoing No-Till Task Force project, independent grower studies and Soil Conservation Service field trials in progress will hopefully shed more light on these as the data becomes available. The variations if soils, management preferences and differing microclimates in north coast wine vineyards will always present the need for individual experimentation. Grower patience and the willingness to accept a "weedy" appearance in the vineyard are perhaps two of the most critical factors of a successful no-till experience. The vagaries of nature and the restlessness created by the sight of a parked disk in the equipment yard have contributed to the demise of more than one cover crop program. The encouraging word for soil conservation is that interest in permanent cover crops is on the. increase with hillside vineyardists and even in less erosive valley lands, the trend is toward decreased tillage. I would like to close with the observation that many times as agricultural researchers and advisors, we have a tendency to focus our interest within the confines of the vineyard or field we work with. Soil erosion, like many other agricultural issues, can carry on-farm problems beyond the fence line as a public cost or environmental concern. The recommendations that come out of our work with cover crop floor management will increasingly need to convey a balance between the vineyard environment and the larger downstream environment. 177

5 References Cited 1. White, Angela S Napa County Resource Conservation District Hillside Vineyard Inventory. NCRCD Publication. 2. U.S.D.A. Soil Conservation Service - River Basin Planning Staff 1985 Hillside Vineyards Unit Redwood Empire Target Area. U.S.D.A. Publication. 178