Spray Drift

Incorrect spray application can lead to contamination and pollution of adjacent land, water, wildlife, properties and people. Understanding the causes of spray drift will help to minimise the risk and ideally eliminate it from occurring.

A range of factors can influence the potential for spray drift, including:
• Prevailing weather conditions, with wind speed being a major factor to consider. “The safest conditions in which to spray are when it is cool and humid with a steady wind of 2 to 4 miles an hour or 3.2 to 6.5 kilometres an hour (light breeze) blowing away from any sensitive areas or neighbours' land.” If wind speed is less or greater than this then the risk of spray drift increases so spraying should not take place. Use a handheld anemometer to and confirm wind speed, as well as noting visible signs of wind movement.
• High temperatures, particularly when greater than 30°C, can create rising air currents which limit the quantity of pesticide from reaching the target, with more of the pesticide being redistributed within an air current away from the target. In addition, hot conditions will reduce the size of spray droplets due to evaporation, so where these conditions prevail do not spray.
• Ground cover. If the land is mostly exposed soil, then the air movement will be greater at nozzle height than if there is good vegetative ground cover.
• The type of nozzle and the fineness of nozzle spray, creating the spray quality: medium to coarse spray droplets reduce the risk of spray drift, especially for use with herbicides, although fine spray droplets will typically be required when using fungicides. In practice, the nozzle producing the coarsest spray quality should be used.
• The speed of application should not create a swirling effect or reduce droplets from adequately contacting the target. Spray at an appropriate speed.
• Equipment pressure: the higher the pressure the greater the risk of spray drift.
• The height of the nozzle/s from the ground.
• Ground conditions to ensure an even and smooth application can take place. Uneven and rutted ground will result in varying application height, with unevenness of application and increased chance of spray drift.
• Equipment condition. This must be checked as part of a pre-calibration process before use and damage or fault must be corrected before use.
• The use of a sprayer shield or shroud can reduce spray drift ensure more of the pesticide is applied to the target.
• The use of air assisted spray booms will force the droplets downwards onto the target, reducing the time spent in the air thereby reducing the risk of spray drift.
• Rotary atomisers at lower speed will typically produce larger diameter spray droplets thereby reducing spray drift potential, although droplet size and speed will vary according to target organism.

(Main source: Defra, 2006, ‘Pesticides: Code of practice for using plant protection products', pp88-92 http://www.hse.gov.uk/pesticides/resources/C/Code_of_Practice_for_using_Plant_Protection_Products_-_Complete20Code.pdf )