Good wine is grown with all types of farming systems, but the grape must have the right amount of sugar and ripen in a balanced way to create high-quality wine. To do so, grape vines must be comfortable and healthy.
“The more stresses on the vines, the more unbalanced grapes you’re likely to get,” explains Glenn McGourty, University of California Cooperative Extension wine growing and plant science adviser in Ukiah, Calif. “I've seen really good wine made from grapes grown in a vineyard without a single blade of grass, and I've also seen people who cover crop year-round and never till. It’s up to the grower to figure out the needs of his site for what he produces.”
Cover crops reduce stress
Healthy soils are the first step to healthy vines. In California, soil is fairly active in winter, when the ground is moist, but nearly dormant in the summer. McGourty recommends growers use winter cover crops that stop growing when it gets hot, but leave plant residue behind to protect moisture in the soil.
“Also, if tillage can be avoided, that's a really good thing,” McGourty explains. “Because when soil is tilled, it breaks up aggregation and exposes the soil to the sun. It's just like your skin; you want to keep it protected whenever you can.”
In addition, cover crops improve water infiltration. When raindrops land directly on bare soil, it breaks down aggregates and seals the soil. This causes water to run sideways and erodes soil. Cover crops protect soil from raindrop impact, allowing the soil to remain porous and absorb more water.
The roots of cover crops secrete nutrients and emit starches, sugars, glues and waxes that feed microbes. Microbes cycle nutrients into the plant root zone and decompose organic matter into humus, which coats soil particles with stable organic compounds.
Species for vine happiness
Planting a mix of cover crop species creates biodiversity. Many growers favor cover crop mixes with oats, mustard and winter peas. Legume cover crops add nitrogen to the soil providing up to 30 pounds of nitrogen per acre, an amount consumed annually by a vineyard. McGourty also endorses short-season grasses, rose clover and subterranean clover.
COVER CROP ACTION: On the left is an image of Dundale pea, which was tested as a cover crop in the SARE study. Another, on the right, was Trios 888 triticale. The aim was to determine soil moisture impact, organic matter value and other properties of cover crops in vineyards.
Vineyard Team of Atascadero, Calif., completed a two-year Sustainable Agriculture Research Education (SARE) study to test how much water different plant species use when those species function as cover crops. The study plots had individual plantings of Dundale pea, Paraggio medic, UC 937 barley, Blando brome and Trios 888 triticale.
“Declines in the Paso Robles groundwater basin and several years of very little rain,” explains Craig Macmillan, Vineyard Team technical program manager, “increased awareness of water conservation in vineyards. Growers came to us with the question, ‘Do cover crops take away our limited water from vines?’ We found there was no difference in soil moisture level from bare soil to cover crop, and that all five cover crop species were equal in their consumption of water with clean cultivation. We were surprised to learn this.”
Disk, mow or graze?
Vineyard Team also researched how winter cover crop termination methods affect soil moisture. A control of clean cultivation was measured against five other methods: no till and mow after seed set, mow at bud break, mow and disk at bud break, mow 30 days after bud break, and chemical mow at bud break.
“We did not find significant differences,” Macmillan reports, “including against the clean cultivating control. And that was counterintuitive. If one patch has little or no vegetation, and another patch has quite a bit of vegetation, I expected the vegetated place to have less soil moisture.”
Statistical significance in soil moisture depletion was found at only one site between clean cultivation and late-mow treatments in the first year of the study, and between early-mow plus disk and clean cultivation in the second year. “We don’t know how to interpret this,” Macmillan explains, “because clean cultivation had the most soil moisture in year 1 and the least moisture in year 2.”
The study did find that the fallow/no-till treatment produced significantly more plant biomass than other treatments. “Basically, we found there’s no reason to let soil moisture depletion be a guiding factor in cover cropping practices,” Macmillan concludes.
One cover crop removal method not included in the study is livestock grazing. Many growers in Mendocino County hire sheepherders to intensively graze their flocks on the vineyard cover crops.
“They’re woolly lawn mowers,” McGourty says of sheep, “and this year it really paid off. It was a wet spring, and growers weren’t able to get tractors into the fields to mow or till until really late.
“It is a limited time period available to graze vineyards, as sheep will damage vines if they have leaves. But some growers design their vineyards with the canopy trained higher to allow sheep to graze anytime.”
Cover crops for carbon
Cover crops sequester up to 3 to 4 tons of carbon per acre. Soil microorganisms digest and return half of it to the atmosphere, and the other half remains in the soil. Most farmed soils in California hold 1% to 1.5% of carbon, and cover crops build this to 3% to 4% over a five-year period.
“Getting carbon back into the soil goes beyond healthy vines,” McGourty says. “It benefits the broader environment. There may be a time — and I think it's coming upon us — where we can be paid by carbon offset fees to farm carbon in our vineyards.”
Cover crops may seem just a farming practice to reduce soil erosion, add organic matter for soil moisture and add nitrogen, but it grows up the chain to consumers’ pockets. Many wineries reduce the carbon footprint of their operations, or go carbon-neutral, for their business’s conservation effort. It is also a market-savvy move to attract the green dollars of consumers seeking wine that matches their own conservation ethics.
Hemken writes from Lander, Wyo.