Creating a healthier vineyard begins with cover crops, typically a mix of grasses, mustards, or legumes chosen to suit local growing conditions.
One of the biggest boons a cover crop can deliver to vineyard soils is organic matter, grape growers heard during the B.C. Grape Council’s annual conference last summer in Penticton, British Columbia, Canada.
“Organic matter is a big deal,” said Stan Grant of Progressive Viticulture in Turlock, California. “It buffers the supply of mineral nutrients, because it has the ability to absorb mineral nutrients. This same absorption capacity also imparts a bit of resistance to large changes in soil chemistry—acidity, alkalinity, and to some extent salinity.”
A balanced soil is more hospitable to beneficial microorganisms that enhance soil health and helps prevent damage to vine roots by nematodes and phylloxera.
All organic matter helps boost soil nutrients in the vineyard and the number of soil-dwelling beneficial organisms, Grant said, but at a cost of $50 to $500 an acre, cover crops have the most valuable impact with several benefits beyond organic content.
“With careful management and sufficient time, the benefits of organic matter to vineyard mineral nutrient economy will far exceed the costs,” Grant said. “They are long-term investments which provide many benefits, although they’re not always easy to quantify. And, in my opinion, they are among the most important contributors to sustainable wine grape growing.”
In addition to attracting and conserving mineral nutrients, cover crops draw them closer to the soil surface through their root system. This makes nutrients more accessible to vine roots. Moreover, legumous plants like beans, peas, and clovers can fix atmospheric nitrogen and make these available in the soil when they decompose.
Glenn McGourty of the University of California-Davis and a farm advisor in Ukiah, California, said cover crops can fix all the nitrogen a California vineyard needs—about 30 pounds an acre each year. Since perennial cover crops slough off about half their roots each year, the nutrients return directly into the soil.
“They’re putting a lot of organic matter right where you need it,” McGourty said.
And the amount can be significant, depending on the type of farming system used.
Conventional vineyards might produce up to 1.6 tons an acre a year of organic matter, whereas an organic or biodynamic vineyard will contribute between 0.8 tons and 4.0 tons per acre from a combination of composts and cover crops.
In addition to contributing nutrients, cover crops also prevent erosion, retain soil moisture, increase organic matter and soil fertility, increase diversity of soil organisms, and attract beneficial insects. There may also be an aesthetic value, depending on the seed mix used.
McGourty suggested that perennial cover crops are more suited to the arid conditions in the Okanagan Valley, while their extensive root systems provide effective competition to vines in areas where excessive vigor is an issue.
He was particularly keen on wheatgrasses, however, noting that slender wheatgrass is a good spring crop, while crested wheatgrass is a tough plant that’s typically used on range land. Siberian and streambank wheatgrass (sodar) can stabilize the vineyard floor.
“Wheatgrasses in general would be a group I think would be important for you to trial in this part of the world if you want a perennial cover that doesn’t take a lot of moisture out of the ground and can exist on pretty low rainfall conditions,” he said.
The ability of some cover crops to provide habitat for beneficial insects is also worth noting, McGourty added. Beneficials that may be attracted include pollinators as well as predators of some pests. Plants such as mallow may even help control cutworm, according to research that Dr. Tom Lowery of the Pacific Agri-Food Research Centre in Summerland presented at the Penticton conference.
McGourty kept his focus on pollinators, however, noting that there’s an especial need to give habitat to these insects given regular reports of collapses in honeybee populations. Bumblebees, leafcutter bees, and syrphid flies may all find a home in vineyards with carefully selected cover crops.
“The honeybees are having a tough time,” he said. However, syrphid flies can help control some of the pests and are also good pollinators.