As vineyard acreage in western Washington increases, more farmers are seeking strategies for controlling weeds, especially in new vineyards. Heightened interest in organic wine grape growing on the west side has vintners looking for certified organic weed control methods.

Researchers Carol Miles and Tim Miller and graduate student Callie Bolton from Washington State University's Northwestern Washington Research and Extension Center in Mount Vernon recently completed a two-year trial of sustainable weed control management on WSU's first certified organic vineyard.

The 2.5-acre vineyard was planted in 2009 in order to conduct research in organic wine grape production and disease management for local growers considering an organic operation. But weed control eclipsed other concerns, according to WSU Extension vegetable specialist Miles.

"Weed management in new wine grape vineyards was identified by local growers and winemakers as the primary constraint to establishing organic production in the region," she said.

Several treatments studied

Two early ripening wine grape varieties, pinot noir precoce and madeleine angevine, grow at the Mount Vernon vineyard. Miles, Miller and Bolton studied several weed-control treatments recommended by local growers and winemakers: rototilling and mowing; in-row cultivating with a Wonder Weeder, a new vineyard cultivator produced in Burbank, Wash.; and planting in-row and between-row cover crops of winter wheat and winter peas and a combination of the two.

Preliminary results showed that the cover crops were perhaps too effective because they not only competed against the weeds, but also with the young grape plants.

"All cover crop treatments tended to reduce shoot growth (new vine length), vine pruning weights and vine diameter of both grape cultivars," Miles said.

Timing important for cover crops

When a vineyard is being established in the first three years, this competition is undesirable. Later, however, it's important for maintaining a healthy balance between fruit load and vine growth, said Miller, a WSU Extension weed scientist.

Grape vines tend to grow too thickly — at the expense of the grape clusters. "De-vigoring," or slowing down excessive vine growth, helps keep that balance.

"In western Washington vineyards, cover cropping might be an excellent tool to use for weed management once vines start to produce fruit," Miller said, "but probably not early in the life of a vineyard, when one wants fast establishment and lots of vine growth."

Miles added that while it is likely best not to have in-row cover crops for a newly established vineyard, a comparison with cover crops planted only between rows was not included in their study: "So we cannot say that cover crops overall should be avoided during the establishment years."