Finding the right cover crop
Growing cover crops in an arid climate is difficult without a second irrigation system.
Cereal rye mixed with resident vegetation is used as a cover crop in this eastern Washington vineyard.
Cover crops are grown for different purposes and there are different management styles, said Dr. Robert Stevens, Washington State University Extension soil specialist. "It will all be different for each of you."
Cover crops reduce weed pressure, help prevent soil erosion, use excess soil moisture, improve biodiversity, and in the long term, can increase organic matter.
"But for the most part, we live in a desert. If you want to raise your soil organic matter by two percent, move to Iowa," Stevens said. "You can increase organic matter with cover crops over the long term. But buy compost if you want it done quickly."
Growers are essentially growing two crops in a field when growing cover crops, Stevens said during a recent wine grape convention. "That can be very difficult because there are different growth cycles, different management and moisture needs for the two crops."
Growers with legume cover crops are actually growing three crops, including the rhizobia living on the roots that convert nitrogen into forms the plant can use, he said. "You need to think about what you do and what crop you are doing it for."
A grower's irrigation system will determine what cover crop species is grown, stand density, and survival of the crop. Growers in eastern Washington will need to provide moisture to get the stand established, he noted.
In field trials evaluating hairy vetch as a cover crop, fall-planted hairy vetch that didn't receive irrigation was half the size of hairy vetch that was irrigated. "We've got to figure out a way to water the center of the rows," Stevens said. "If you're really serious about having a cover crop that does all you want, you may need a second irrigation system that can deliver small amounts in a timely manner."
In addition to water management, cover crops may require nutrient management to help establish the stand. Seedbed preparation is another area needing attention.
He noted that smaller seeds require more intensive and more uniform seedbed preparation. Poor soil moisture increases the need for good seedbed preparation because seeds need good contact with soil moisture for germination.
Another management issue includes herbicide availability to eliminate unwanted weed species and maintain a cover-free area in the row.
"For the organic grower, cover crops can be challenging," Stevens said. Organic growers must manage frequent mowing of the cover crop and increased cultivation to manage weeds.
Grasses, wheat, and rye have worked well in Concord vineyards as cover crops, according to Stevens. Grass mixtures of several species have also worked well because they are tough enough to handle tractor and harvester traffic and reseed themselves. When legumes are used as cover crops, they can add nitrogen back to the soil, but they can get too vigorous.
Kyle Bair, WSU research associate, has been studying the release of nitrogen in Concord vineyards. Legume cover crops grown as green manure and cultivated into the soil can release nitrogen for use by the crop.
"You want the release to be at the time that the plant needs it," Bair said. The vine needs most of its nitrogen from bloom to veraison, which is also when most plant uptake occurs.
Factors that impact nitrogen release include the type of cover crop, soil moisture, soil temperature, and soil microbes, he added.
Bair compared hairy vetch and yellow sweet clover in a research and commercial vineyard plot to better understand nitrogen release. Both cover crop species were planted in the fall. Both plots received irrigation through overhead sprinklers. He tracked soluble nitrogen levels in the soil after incorporating the cover crops each year in early June and noted that both treatments provided nitrogen when the plant needed it. In some of the plots, the cover crop was reseeded in the spring after it was incorporated in the soil.
He found no effect on yield in the short term (two years) in the Concord trial from the cover crops and saw no difference in Brix between the treatments. When comparing hairy vetch and yellow sweet clover, he found that the vetch was more forgiving as it was easier to establish and hardier than the clover.
Stevens encouraged growers to look at the cover crops grown in their area to see what is successful and to see what their neighbors have done wrong.
Detailed information about growing cover crops can be found in WSU's new extension bulletin Cover Crops as a Floor Management Strategy for Pacific Northwest Vineyards, EB 2010. To order or download the bulletin free, visit: http://cru84.cahe.wsu.edu/cgi-bin/pubs/EB2010.html? id=22otPxSx.