Spray drift: causes and how to reduce it

Spray drift is the movement of a pesticide through the air away from the target area, during or shortly after application. It wastes product, leaves the target under-dosed and can damage neighbouring crops, gardens, water and wildlife. In many countries the operator is legally responsible for off-target damage.

Particle drift and vapour drift

• Particle (droplet) drift is spray droplets carried off target by the wind at the time of application. It depends mostly on droplet size, wind and release height.
• Vapour drift happens when a product evaporates from the spray or from treated surfaces and the vapour moves off site, sometimes hours after application. It depends on the product's volatility and on temperature. Some herbicide formulations are known for it, and their labels often carry specific temperature and timing restrictions.

Good weather choices reduce both, but vapour drift is mainly controlled by product choice and label conditions.

Main causes of particle drift

1. Small droplets. Fine droplets are light, fall slowly and are easily carried by the wind. They also evaporate faster and get smaller as they travel. Droplet size is the factor most under the operator's control — see droplet size and nozzles.
2. Wind speed. The further and faster the air moves, the further droplets travel before landing. See wind speed for spraying.
3. Release height. A boom set higher than needed, or a drone flying high above the canopy, gives droplets more time in the air.
4. Thermal inversion. Under an inversion fine droplets stay suspended in a layer near the ground and can move a long way.
5. High Delta T. Hot, dry air makes droplets shrink quickly, turning medium droplets into fine ones mid-flight. See Delta T.
6. Travel speed and pressure. High ground speed creates turbulence behind the sprayer, and higher pressure on many nozzles produces a finer spray.

Practical ways to reduce drift

• Use a coarser droplet class that still gives adequate coverage for the product. Air-induction and other drift-reduction nozzles produce fewer fine droplets.
• Lower the boom to the lowest height that still gives an even pattern for your nozzle spacing and angle. For drones, keep to the manufacturer's recommended height above the canopy.
• Slow down. Lower ground or flight speed reduces turbulence and helps droplets deposit.
• Keep pressure within the nozzle's range for the droplet class you want.
• Leave buffer zones next to sensitive areas and water, as the label or local rules require.
• Consider drift-reducing adjuvants where the label allows them; they can reduce the proportion of fine droplets.
• Watch wind direction. Spray when the wind is blowing away from sensitive areas — homes, susceptible crops, apiaries, waterways — and treat the downwind edge last or under better conditions.
• Avoid inversions and very high Delta T.

No single measure removes drift. The combination of the right weather window, the right droplet size and a sensible release height is what keeps product on target.

Further reading

• Introduction to Pesticide Drift — US EPA
• New Nozzles for Spray Drift Reduction — Ohio State University Extension

In PulveCast

PulveCast rates drift risk each hour as optimal, caution or avoid by crossing the forecast wind with your nozzle's droplet class, and combining it with Delta T and humidity. In automatic mode it starts from your crop preset's ideal droplet class and suggests a coarser one when the wind would put drift risk in the red.

More guides

• Delta T for spraying: what it is and why 2 to 8 °C
• Delta T chart: how to read it
• Thermal inversion and spraying
• Wind speed for spraying
• The best time of day to spray
• Humidity and dew point for spraying
• Rain and spraying: rainfast periods and forecasts
• Temperature and spraying
• Droplet size and nozzle selection
• Weather for drone spraying
• Kp index, RTK and GNSS for spray drone operators
• Pre-spray checklist