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The most common ways that herbicides injury off-target plants are spray drift and herbicide volatility. Spray drift is the movement of airborne spray particles during the application that never settle on the target vegetation, while herbicide volatility is the tendency of a chemical to vaporize into the atmosphere from a treated surface where it moves away from the treated area.
Spray drift is affected by a number of factors; nozzle, application rate, wind speed and adjuvants, to name a few. To minimize herbicide drift, it’s recommended that droplets be larger than 400 microns, as smaller droplets can be easily carried off-site. With wind in particular, when wind speed is less than 2 miles per hour, wind direction is highly variable and can change direction suddenly. And remember, off-target damage potential increases when wind blows toward susceptible crops. Some states have state regulations for proper wind speeds during applications.
Herbicide volatility varies by product and can occur with applications on any surface, but increases on impervious surfaces like rocks and pavement. The main climactic situation where volatility occurs is during periods of high temperatures and low humidity, and calm conditions can contribute to volatility.