The University of California at Davis describes the potential exposure of grapevines to herbicide carryover drift as "high" in areas where grapes grow year-round. The danger of excessive herbicide exposure is reduced during the grapevine's dormant season, which varies according to local climate conditions. Growers can take steps to reduce herbicide exposure and ensure the health of vines and grapes.
Grapes exposed to unwanted herbicides show a range of symptoms including leaf yellowing, malformation of leaves, reduced cane production, reduced berry size and yield, and vine death. Damage may involve a few plants or the entire vineyard. Damage depends on the type of herbicide spray drift, the health and age of vines, and the intensity of exposure. Spray drift is influenced by droplet size and the environmental conditions during spraying.
Weather condition, application timing and types of equipment should be considered when using herbicides in vineyard. Fresno County farm adviser Steve Vasquez, speaking at a San Joaquin Valley grape symposium, recommends taking into account the stage of growth of grapevines before making a spray application. "Vines that are showing signs of bud swell are extremely sensitive to post-emergent herbicides." He encourages growers to be aware of surroundings and to keep the spray targeted at weeds.
Herbicide droplet size influences potential damage to grapevines. Droplets with a mean diameter of less than 200 microns, or about the diameter of a sewing thread, are the most susceptible to drift, according to Vasquez. He recommends using spray nozzles with an orifice size less than 02 to operate at a pressure of 15 to 40 lbs. per square inch. Types such as Drift Guard and Air Induction can reduce drift by 50 percent to 95 percent.
Low-Risk Weed Control
Many grape growers practice low-risk weed control methods to reduce the need for herbicide spray. The Pest Management Alliance of California vineyard owners and the California Department of Pesticide Regulation has identified several best practices currently used for weed control. They include the French plow, mowing or mulching, heating or flaming, and subsurface drip irrigation. Major wineries such as Mondavi, Kendall-Jackson and Gallo support this group's efforts.
Sustainable Viticulture Methods
A Cornell University report on sustainable viticulture weed management recommends monitoring the vineyard three times during the growing season, mapping weed infestations and planting cover crops between rows. "The best way to prevent new weed problems is to keep good records," according to the university Extension's website. The most critical time for weed control is between the grape's bud break and onset of ripening.