Cover Crops as a Tool for Vineyard Floor Management to Improve Soil Quality of Rocky Vineyard Soils

Objective 1: Demonstrate the viability (e.g., establishment, stand density, growth) of various cover crop species for winter vineyard floor management in very rocky soils.

Experimental site and design. The variety demonstration will be hosted at a commercial vineyard site located in Milton-Freewater (45° 57' 9" N, 118° 26' 50" W) in an unplanted section with the representative Freewater very cobbly loam soil. A variety of species detailed in Table 1 below will be trialed in small, non-replicated plots, including selections from Olmstead (2006) that have been vetted for climatic suitability, a species of interest to local industry (e.g., Eruca vesicaria), and new species (e.g., Onobrychis viciifolia, Medicago falcata) recommended by regional experts at GO Seed (https://goseed.com/, Salem, OR). Plots will be 1 m² with 0.5 m between plots. Plots will be broadcast sown at the label rate or that listed in Olmstead (2006) and then lightly incorporated using a metal rake. Because the plots are intended to be unirrigated, the exact sowing date will be timed with adequate predicted rainfall. Plots will be sown initially in fall 2023 or spring 2024 (Year 1) and resown in Years 2 and 3 for annual species or those perennial species that do not sufficiently reseed themselves.

Table 1. Cover crop species to be used in the demonstration trial.

 Data collection. Temperature and humidity data will be collected at 15-minute intervals at the experimental site throughout the entire project using an ExactLog logger (iButtonLink Technology, Whitewater, WI). Critical phenology dates will be recorded for each plot to determine days to sprouting, approximate flowering time, seed maturity, quiescence, and natural termination (when applicable). During the establishment period (~October - May), monthly measurements of stand density will be made by randomly placing a 0.25 m² quadrat and counting the number of plants. At the same time, density and species of native/weed vegetation will be recorded. Pending the timing of a spring field day, the plots may be destructively harvested to determine biomass production. Photos will be taken of all plots at every data collection date to include in education and outreach materials.

Data presentation. Data will not be statistically analyzed due to lack of plot replication, but the data from all years of the demonstration trial will be presented for the education and outreach objectives of the project. Emphasis will be placed on phenology of cover crops, stand density and survival, qualitative management considerations, and visual evaluation through photographs and field tours. The SARE Technical Bulletin How to Conduct Research on Your Farm or Ranch provides a basic framework and rationale for the on-farm demonstration trial proposed in this project.

Objective 2: Measure impact of a winter cover crop on vine phenology, vegetative growth, fruit composition, and soil quality in vineyards with very rocky soils.

Experimental site and design. The winter cover crop trail will be hosted in a commercial vineyard block of Vitis vinifera cv. Syrah located in Milton-Freewater (45° 57' 34" N, 118° 26' 1" W) with Freewater very cobbly loam soil. The experiment will be arranged as a randomized completed block design in with five blocks consisting of four vine rows. Within each block, a control treatment (clean cultivated) and a cover crop treatment will be randomized to the left or right side and then placed to ensure the two experiment rows within a block are flanked by the proper treatment on either side. The cover crop treatment will be seeded each year with annual ryegrass (Lolium multiflorum) in anticipation of September or October rains using a 421-S Multi-Purpose Chest-Mounted Spreader (Solo Incorporated, Newport News, VA) and lightly incorporated. The cover crop treatment will be mowed prior to budbreak to prevent frost damage to emerging shoots and will be terminated by cultivation between budbreak and bloom. This trial will be conducted over the winters of Years 1, 2, and 3.

Data collection. Temperature and humidity data will be collected at 15-minute intervals at the experimental site throughout the entire project using an ExactLog logger (iButtonLink Technology, Whitewater, WI). Every two weeks during the winter, measurements of stand density in each of the 5 cover crop plots will be made by randomly placing a 0.25 m² quadrat and counting the number of plants. At the same time, density and species of native/weed vegetation will be recorded. Vine phenology, especially the date of budbreak and bloom, will be closely monitored during the growing season to determine the effect of winter cover on these critical dates. Soil samples from the top 0.4 m of soil will be taken from each plot before the trial and just prior to cover crop termination in the spring of each project year. These samples will be sent to the OSU Soil Health Lab for a complete soil health assessment which includes analyses of mineral nutrient content, physical qualities, chemical properties, and biological activity.

During the growing season, periodic physiological measurements will be made including stem water potential and stomatal conductance to determine the impact of a perennial cover on vine physiology and water stress. Stem water potential will be determined using a Scholander-style pressure chamber (Model 615, PMS Instruments, Albany, OR) and stomatal conductance will be determined using a porometer (LI-600, LI-COR Biosciences, Lincoln, NE). At veraison, when berries start turning red, leaves will be sampled from all plots and sent to a local commercial lab for nutrient testing to determine the effect of a perennial cover on nutrient status. Yield and fruit composition will be determined at commercial harvest as in the winter trial. Pruning mass of vines will be determined following each winter of the project.

Winemaking. In all three years of the study, the winemaking collaborator will make nonreplicated wines for the control and cover crop treatments. At commercial harvest, grapes from all blocks will be pooled by treatment and vinified in a commercial winery using the same winemaking process for both wines. The wines will be used as an outreach tool, including sensory workshops at the local grape symposium to demonstrate the effect (or lack of effect) of a perennial cover on the organoleptic qualities of the wine. This is a critical outreach tool for the perennial cover crop trial because growers and winemakers are unlikely to adopt a perennial cover crop if the unique qualities of wines produced in the Rocks District are unfavorably altered. If fruit composition is significantly altered in Year 1, replicated wines may be made in Years 2 and 3. The sensory analyses of these wines lie beyond the time range of this project.

Data analysis. Data from all three years of the trial will be analyzed using a two-way analysis of variance for a randomized complete block design, with treatment and year as the two main factors and block as a random factor. Analyses for vine phenology, soil composition, nutrient status vine yield, fruit composition, and pruning mass will be conducted using R software for statistical computing (https://www.r-project.org/). Data may be presented at outreach events prior to the completion of the trial, but only the properly analyzed data from all three years will be included in academic and Extension publications.

Objective 3: Trial a perennial cover crop in a very rocky soil to determine changes in vegetative growth, vine water relations, nutrient status, fruit composition, wine quality, and soil quality.

Experimental site and design. The perennial cover crop trail will be hosted in a commercial vineyard block of Vitis vinifera L. cv. Tempranillo located in Milton-Freewater (45° 56' 46" N, 118° 24' 38" W) with Freewater very cobbly loam soil. The experiment will be arranged in the same randomized completed block design as in the winter cover crop trial, with two treatments, five blocks and 10 total plots. The cover crop treatment will be seeded initially with perennial ryegrass (Lolium perenne) in in early spring of Year 1 using a 421-S Multi-Purpose Chest-Mounted Spreader (Solo Incorporated, Newport News, VA) and lightly incorporated. The control treatment will be clean cultivated. The cover crop treatment will be mowed each year prior to budbreak to prevent frost damage to emerging shoots. This trial will be conducted over Years 1, 2, and 3. The cover crop treatment will be reseeded in the fall of Years 1 and 2 if there is insufficient cover after the initial seeding.

Data collection. Temperature and humidity data will be collected as in the winter trial. After the initial seeding in Year 1, biweekly measurements of stand density will be conducted in the cover crop plots as in the winter trial. Vine phenology will be monitored in all plots and years. Soil samples from the top 0.4 m of soil will be taken from each plot before the trial and in the spring of each project year. These samples will be sent to the OSU Soil Health Lab for a complete soil health assessment.

At the time of commercial harvest in each year, a subset of 10 vines from each plot will be harvested and weighed to determine vine yield and cluster mass. A 10-cluster subset from the combined vine yields will be taken for analysis of berry mass and fruit composition. Fruit samples will be submitted to the ETS testing lab in Walla Walla, WA to determine total soluble solids, pH, acid composition, nitrogen compounds, potassium, and concentrations of key phenolic compounds. The data on fruit composition will be critical for demonstrating how cover crops impact the quality characteristics of grapes and wine, which is ostensibly more important than yield. Pruning mass of vines will be determined following each winter of the project.

Data analysis. Data from all three years of the trial will be analyzed using a two-way analysis of variance for a randomized complete block design, with treatment and year as the two main factors and block as a random factor. Analyses for vine phenology, vine physiology soil composition, vine yield, fruit composition, and pruning mass will be conducted using R software for statistical computing (https://www.r-project.org/). Data may be presented at outreach events prior to the completion of the trial, but only the properly analyzed data from all three years will be included in academic and Extension publications.