Don’t get the drift – spray drift that is

Spray drift is a serious concern for agricultural property owners, their neighbors and crop protectant spray operators. When crop protectants drift off-target, they can cause health issues and property damage, and can harm the environment.

Spray drift occurs when chemical droplets, particles or vapors move through air, during or after application, to a site other than where they are intended. The direct impact is obvious: farmers do not want to contaminate one field while trying to protect another. The effect could inadvertently damage or destroy the crop. Drift also may contaminate a home garden or another farmer’s crops.

Knowledgeable spray operators understand what can cause drift and then they work diligently to mitigate those factors.

Droplet diameter
Crop protectants can be applied using ground or aerial spray equipment. It is the spray nozzles that produce droplets. Spray droplets are measured in microns, which is equivalent to 1/25,000 of an inch.

The miniscule droplets can stay suspended in the air until the wind moves them along. The smaller the droplet size the greater the risk of spray drift. Drift is least likely when the droplets are 200 microns or more in diameter. (For comparison, a human hair is about 100 microns in diameter.)

An operator typically uses a range of droplet sizes. The most effective size is determined by the shape, type and size of the crop that is to be treated. The product label will include droplet size requirements, so follow label recommendations.

Small to medium-size droplets provide good coverage of insecticides and fungicides. Medium-size droplets, when applied with some wind is blowing, effectively treat grasses, stems and narrow leaves. Systemic herbicides often call for large droplets, because they fall most efficiently on broadleaf plants and other large, flat surfaces.

Meteorological conditions
Wind speed and direction are primary causes of spray drift. Generally, wind speeds between two miles per hour (mph) and 10 mph are acceptable for spraying. A reliable wind speed meter is helpful in determining wind speed at the time of application.

Wind can inadvertently carry droplets over sensitive plants, a neighboring field or wildlife habitat. To reduce this damage, leave a buffer zone at the downwind edge of the spray area. It can be sprayed later when the wind dies or changes direction.

Clearly it is best not to spray in windy conditions, but the wind does not have to kick up for drift to occur. On calm days, the risk is inversion. During an inversion, the coolest temperature is at ground level, warms with increasing elevation then becomes cool again. Spray droplets stay suspended in the calm air, only to slowly drift downwind.

Spray drift also can occur when the chemicals evaporate over an unintended site. Evaporation is impacted by temperature and humidity. While droplets evaporate in all weather conditions, cool and damp conditions reduce the drift it causes.

Other factors that can cause spray drift are the area’s topography, as well as whether the chemicals are applied with ground or aerial spray equipment. The chemicals’ characteristics, such as its viscosity and volatility, also play a role.