There’s much more to effective weed control than selecting the right herbicide, and factors such as equipment, weather conditions, and application timing need to be considered to keep the sprayed material on target.

That was part of the message to growers and PCAs from Steve Vasquez, Fresno County farm advisor, at a San Joaquin Valley grape symposium at Easton.

Vasquez made the comments in a report prepared by his fellow farm advisor, Kurt Hembree, on trials the pair worked on. “Grapevines are very sensitive to a number of herbicides routinely used in vineyards and/or surrounding crop and non-crop areas,” Vasquez said.

What’s more, he added, since the vines are growing for most of the year, “potential exposure to off-target herbicide movement, or drift, is high. Thus, it’s important that herbicide treatments be made on time and on target.” Otherwise, when something is amiss with an application, symptoms on vines can range from chlorosis of the foliage to complete loss of the vine. Along the way, damage can also be done to production the following year, either in a few vines or across the entire vineyard.

Although vines are less susceptible to herbicide damage during their dormant period, that window of opportunity for weed control is small, said Vasquez.

“It’s important to take advantage of preemergent and postemergent herbicides during this period to help reduce the weed population and spray trips needed in-season when vineyards are more susceptible to damage. Regardless of when spraying occurs, be aware of your surroundings and keep the spray mix targeted at the weeds.”

Drift occurs when spray droplets are carried from the application site downwind and deposited on sensitive plant surfaces such as buds, leaves, or shoots. Label instructions should be followed for the optimum performance of the herbicide, along with prevention of drift and crop injury.

Drift is influenced mainly by droplet size and environmental conditions during applications. High wind, low humidity, and high temperature favor drift potential.

Vasquez noted that droplets with a mean diameter of less than 200 microns, or about the diameter of sewing thread, are the most susceptible to drift, since they are lighter in weight and tend to remain airborne longer.

On the subject of nozzle selection and spray pressure, two things that govern the fineness of droplets and the potential for drift, Vasquez recommended using spray nozzles with an orifice size of less than #02 to operate at a pressure of 15 to 40 psi.

“Use a minimum of 10 gallons per acre, unless otherwise specified on the label. TeeJet brand nozzles are the most widely used and readily available. For vineyards, standard flat fan nozzles work fine for preemergent application, but are not well suited for postemergent sprays.

“Instead, consider using an ‘extended range’ type nozzle, which can produce different droplet sizes under different operating pressures.”

Rather than cranking up the spray pressure to achieve large volumes of water, and more drift hazard, select larger nozzle sizes if more volume is desired, he added. Types such as Drift Guard and Air Induction can reduce drift by 50 percent to 95 percent.

Another way to reduce drift by as much as 95 percent is to use drift retardants, taking care to follow label instructions.

Vasquez recommended 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 postemergent herbicides.”

Height of the sprayer boom and nozzles also influences the distance the droplets may travel from the application site.

A way to keep the boom and nozzles closer to the ground and creating less drift hazard is to treat weeds when they are small, or less than 4 inches tall. When booms are raised or nozzles are angled more upright, the risk of drift is greater.

Another factor in a spray program is the volatility of the herbicide applied, the more volatile compounds being more likely to drift.

Karen Francone, Fresno County deputy agricultural commissioner, was on hand to discuss regulations and best application practices intended to help avoid drift and potential liabilities for growers and applicators.

Offering some pertinent definitions, she pointed out that pesticide drift concerns the entire agricultural industry.

The California Department of Pesticide Regulation and the Federal EPA define drift as “physical movement of a pesticide through the air at the time of application or soon thereafter, to any other site other than that intended for application.”

Substantial drift occurs when the amount of pesticide outside the area treated is greater than that which would have resulted had the applicator used due care. Due care is when the application is performed with as much competence and diligence as anyone could use to prevent drift.

When an applicator does not use due care, he is considered to be negligent. Due care considerations include wind velocity and direction in relation to hazards such as other crops, homes and other inhabited buildings, and busy thoroughfares. Consideration should also be given to routes of school buses to avoid exposure of children.

Due care can include: use of drift control agents, anemometers on-site to determine wind velocity (required on some labeling), size of nozzles, topography, time of day, equipment and methods, chemical properties of the pesticide, and windbreaks.

Pesticide manufacturers, Francone said, are increasingly specifying details such as minimum — as well as maximum — wind velocity on labels. Applicators should follow both required and suggested practices on product labels as proof of their due care.

“In some areas of California, dead calm conditions are often associated with an inversion situation and a minimum wind velocity of 2 miles per hour will help insure an inversion situation does not exist.”

Francone underscored the importance of following the Best Application Practices Manual put together several years ago by the ag chemicals industry as a stewardship program to curb sulfur drift problems. The practices, she said, were instrumental in keeping sulfur off the restricted materials list.

She said her office can assist growers with “courtesy audits” of their pesticide application programs, including recordkeeping.

Francone related a “success story” in the close cooperation between county and state regulators, growers, PCAs, and applicators in dealing with drift issues on the west side of Fresno County.

Four to five years ago about 40 cases of drift occurred in the district, prompting the parties to join in developing answers on how to mitigate drift.

“Over the past three years,” she said, “we’ve had no incidences of off-site movement of materials during the period of Jan. 1 through April 15, when special conditions are in place for herbicide use on the west side. We were able to find a way for industry and regulators to work together to come up with solutions.”

The effort also involved University of California specialists, who learned that many of the drift complaints were actually disease-related problems on crops.