Adoption of permanent cover crops and no-till systems is considered integral to achieving California state air quality standards regulating airborne particulate matter, as indicated in Senate Bill 656. Imposition of such management techniques in a vineyard could create competition with vines for limited water resources. We evaluated the effects of permanent, untilled and mowed cover crops that were either tilled or mowed on vineyard floor composition, weed populations, vine water relations and growth, and fruit composition over three years within a mature commercial Merlot vineyard subjected to deficit irrigation in Lodi, San Joaquin Valley, California. The vineyard floor in this experiment supported resident vegetation that was tilled (standard grower practice), an oat cover crop, or a legume/oat cover crop. The two planted cover crops were either tilled or mowed (i.e. no-till). Biomass of cover crops, weeds and legumes varied by year and treatment, but consistent effects among treatments were not observed. Weed species composition and cover segregated with the presence or absence of tillage rather than cover crop type, and weed species composition of the resident vegetation was distinct from those in the cover crop treatments. Some treatments like ‘Oats/Legumes + NoTill’, ‘Oats + NoTill and ‘Oats/Legumes + TillResident Vegetation + Till’ reduced soil water content (Θ_v) in at least one of shallow three soil layers spanning 0–30cm, 30–60cm and 60–100cm in 2008 and ‘Oats/Legumes + NoTill’ in the two upper layers in 2009, but these differences had no consistent influence on plant water status. Distinctions in Θ_v between years were attributed partly to the cessation of rainfall two months earlier in 2008 than 2009, despite similar total annual quantities. Significant reductions in Θ_v imposed by the ‘Oats/Legumes+NoTill’these treatments did not reduce vine vegetative growth in one year, but these effects did not manifest in yield and fruit composition. Values for the Ravaz Index for two of the three years indicate that the vineyard was overcropped for all treatments, but maximizing production in this region is a common practice. Weak competitive effects of the cover crops for water were likely associated with the use of a well-established mature vineyard, and demonstrated that these management strategies could be employed to improve air quality to meet California air quality regulations with limited few effects on the vine water status and productions.

Permanent cover crops are commonly used in vineyard floor management because of their beneficial effects to soil and vine health, but studies evaluating their competitive effects on vines have been conducted primarily in nonirrigated vineyards. Future air quality regulations could mandate the use of no-till floor management practices in California’s Central Valley. We evaluated the combined effects of cover crop type (oats alone or oats grown with legumes) and tillage on soil nutrient availability, vine nutrition, growth, and yield characteristics of Vitis vinifera cv. Merlot grown under regulated deficit irrigation in a commercial vineyard from 2008 to 2010. Five treatments were used: Resident Vegetation (RV) + Till, Oats + Till, Oats/Legumes +Till, Oats + NoTill, and Oats/Legumes + NoTill. No differences in soil nutrient availability were found among the treatments. Of the numerous nutritional constituents analyzed on leaf petioles and blades, only NO₃-N_petiole was affected by floor management. At nearly all growth stages among all years, NO₃-N_petiole of tilled treatments was twice the no-till treatments. At harvest, yield, mean cluster weight, cluster number per vine, and aboveground cover crop biomass differed among treatments in 2009 and/or 2010 but not in the first year (2008); however, responses were not consistent among treatments within each respective year. Importantly, yields were similar from all four cover crop treatments compared to the typical management (RV + Till), suggesting that use of cover crops and/or no-till practices may be implemented in an irrigated vineyard with little immediate effect on grape productivity in mature vineyards.