Things that need to be sprayed: Weeds. Things that don't: organic fields, a neighbor's garden, etc. What can you do to make sure your herbicide isn't drifting from the target? Extension professionals have several tips.
The percentage of drift-retardant use has gone up significantly in recent years, says Bob Hartzler, Iowa State University Extension weed scientist. “Due to liability reasons, a lot of applicators routinely will add a retardant,” he says. However, when used with newer drift-reduction nozzles, it's often difficult to see a benefit, says Hartzler.
Droplet size greatly affects herbicide drift. “Small droplets take more time to fall to the ground and can thus drift farther,” says Hartzler. “It's desirable to use a nozzle that produces large, uniform droplets. Switching from standard flat-fan nozzles to turbulence-chamber or venturi nozzles increases droplet size and can greatly reduce the amount of drift. However, some herbicides require smaller droplets for consistent results. The label usually recommends an appropriate nozzle type.”
“Keeping sprayer pressure as low as possible but within nozzle specification produces larger droplets from the same nozzle operated at a higher pressure,” says Hartzler. “Using high carrier volume also allows the use of larger nozzles, further reducing drift.”
Keep in mind, though, that a pressure reduction will require recalibration and adjustments in sprayer speed and carrier volume, he says.
The shorter the distance spray has to travel to reach its target, the less chance there is of drift. “Boom height depends on nozzle angle and spacing,” Hartzler says. “110° nozzles at 20-in. spacing should be 15-18 in. above the target to produce a uniform application rate across the boom.”
“Drift symptoms from some herbicides are more visible than others,” says Chris Boerboom, University of Wisconsin Extension weed scientist. “Some of the contact herbicides might still cause some symptoms, but are not as visible and don't affect growth for as long. However, switching herbicides might be an option in only a limited number of cases.” He also suggests that a pre-emergence herbicide could be laid down as a buffer around some highly sensitive sites.
“If you have a highly sensitive site, consider delaying the application until the wind direction shifts away or delay the application until early morning or late evening when wind speeds may be slower,” says Boerboom. “An application with a consistent 3-5 mph wind blowing away from a highly sensitive site is ideal. Herbicides never drift upwind; studies have proven this.
“These steps are drift-reducing technologies, not prevention strategies,” he says. “The best combination of technologies will not prevent drift in a high wind.”
Hartzler suggests using a wind meter. “A wind meter is a small but important investment in drift management. Keep records of environmental conditions at the time of application, these can protect you when questions regarding off-target movement occur.”
According to Boerboom, “The risk of drift is certainly real with numerous residential properties and vegetable, organic and other sensitive field crops,” he says. “Be careful and be smart out there; use drift-reducing technology such as drift-reduction agents, spray nozzles that increase droplet size, higher spray volumes and lower boom heights.”
Hartzler reminds that any spray drift is illegal. “Chemical trespass issues are receiving increased attention,” they say. “Managing spray drift is the responsibility of the applicator. However, minimizing spray drift is in everyone's best interest.”