- It is estimated that 50-75% of Oregon vineyards cultivate at least every other alley in order to 1) reduce water stress 2) increase vineyard canopy temperatures and 3) increase nutrient availability. Because many vineyards are situated on steep hillsides, frequent tillage could result in increased soil erosion, decreased soil quality and potential pollution of watersheds. Seven cover crop treatments were established at two commercial vineyards in the northern Willamette Valley in the fall of 2003 and monitored for establishment and their impact on grapevines in 2004 and 2005. Treatments were as follows: 1) winter annuals (oats, rye and vetch), 2) clover mix (subclovers, clovers and medic), 3) native grass mix (Willamette Valley upland prairie species), 4) native meadow mix (forbs plus grasses), 5) perennial grass mix (sheep fescue, dwarf perennial rye and hard fescue), 6) resident vegetation, and 7) a clean cultivated control. Each treatment was replicated four times at each of the vineyards in a randomized complete block design. Treatments were applied to four adjacent alleys flanking 8 or 10 vines in three vine rows with one clean cultivated boarder dividing blocks. Cover crop establishment was measured by destructively removing biomass during the growing season. Weeds were sorted from cover crops, and both were dried, weighed and measured for nitrogen (N) content. Over the course of the growing season, soil water was measured in the vine row and alleys with time domain reflectometry, and midday vine leaf water potential was measured with a pressure bomb. Shoot lengths were measured twice during the season. Vine leaf blades were collected at bloom and veraison for nutrient analysis (N,P,K, S, Ca, Mg, Mn, Cu, B, Zn and Fe). Root samples were taken at bloom and post harvest in the vine row and alley in three treatments (winter annuals, perennial grass mix and clean cultivated) and analyzed for colonization by arbuscular mycorrhizal fungi (AMF). At harvest, fruit yield was measured and fruit quality assessed by measuring soluble solids (BRIX), titratable acidity, pH and N content. Shoot prunings were collected and measured after vine dormancy. We expected to see 1) a higher amount of water in the soil, less vine water stress and more vigorous vine growth in the clean cultivated treatment compared to the others and 2) either an increase in vigor or concentration of N in vine tissues in response to the clover mix treatment.
Biomass production and coverage of the soil by cover crops, as well as responses to treatments in the soil and vine often varied between sites. In general, cover crop treatments, including the clean cultivated control, had little effect on soil water content, vine water status, or vine vegetative growth. There was, however, a clear N affect from the clover mix treatment on vines, even without mechanical incorporation of cover crop residues. Vine leaf N and juice YANC both increased, and yield per vine and cluster weights both decreased in the clover mix treatment. However, the yield reductions were more pronounced in year two and only at one site. The increase in juice N was possibly an indirect effect of the lower yield, concentrating N in the remaining fruit. Results from this two year study indicate no apparent advantage to keeping the alleyways of established vineyards weed free with cultivation.