In the past ten years, our working knowledge of cover crops and their culture has grown tremendously. In the beginning, the emphasis was on growing annnual cover crop species used mostly in field crop rotations to enhance fertility, break disease cycles, and grow copious biomass. Many were impressive, but didn't really fit the architecture of most vineyard sites. Additionally, many were producing large amounts of nitrogen, in quantities far greater than optimum rates recommended for vineyards.
Through trial and error, we have learned which species are best for which situations. A recently published book, "Cover Cropping in Vineyards" ( University of California ANR Publication # 3338) is a summary of much of this experience and research.
The focus of my research has been finding plant materials that are adapted to specific sites, rather than a "one size fits all" approach. My initial work was done on fertile soils along the Russian River, where perennial grasses and legumes were used to reduce tillage and improve soil physical conditions, as well as provide beneficial insect habitat and firm footing in wet weather. Often, these choices of covers reduced vine vigor, which was a plus when vines were overly vigorous. We also found that large annual legumes produced excessive nitrogen when farmed every row, but were useful when farmed alternately every other row with perennial sod forming species.
Presently, I am conducting research in Lake County, where no-till farming is the preferred approach, especially on hillsides. Lake County is a high elevation (over 1300 feet) high rainfall area, and soil erosion is a great concern, as all creeks lead to Clear Lake, famous for its summer algal blooms. An added challenge is cold fall and winter temperatures, making the germination window somewhat narrow. Self seeding annuals such as blando brome, subterranean clover, rose clover and burr clover are favored, along with triticale for a pioneer cover crop in newly planted vineyards. We are still conducting research in this area to find the best adapted species.
Regarding nutrition of grapevines, much work still needs to be done. Evidently, some legumes are quite effective at cycling potassium in the root zone, and probably have a beneficial effect to the grape vines as cover crop roots decompose, leaving potassium complexed with organic matter in forms that are more exchangeable than on clay micelles. Research needs to be conducted on this to see if it is so. Mustards are reputed to be calcium accumulators in the soil, and grow well in mixes and rotations of small grains and legumes.
Lastly, the physical effects on soil structure are often overlooked by plant nutritionists. Oxygen is a key element that is rarely mentioned, but absolutely essential for vine growth and wine quality. By reducing tillage, and moving into a sod culture, numerous benefits await the vineyardist (assuming that they have adequate water for their vines!)