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Optimal Spraying In Diverse Weather Conditions

Weather conditions can have a big influence on the effectiveness of spray application. Rain, wind, temperature and humidity all need to be taken into consideration before any spraying is done.

Ineffective spray application is not just about wasting money. It also the concerns for the environmental effects of spray drift and run-off.

By monitoring the weather and changing your spray application methods to best suit the conditions, you can achieve more accuracy and less wastage

Wind speed and direction should be monitored at all times (particularly when air blast spraying) and spraying should only be conducted when the wind direction is consistent and the speed is no less than 2 kilometres an hour or no more than 15 kilometres an hour (as a general rule).

You should not spray when there is no wind, as fine droplets will stay suspended in the air and drift at a later time.

Ways to overcome wind factors include spraying with drift-mitigating nozzles producing larger droplets, using wind deflectors, driving at a slower speed and spraying closer to the target.

A simple way to determine wind speed and direction is with the smoke from a fire or a smoke plume generating device. Temperature, relative humidity and wind speed can also be measured using a readily available handheld weather station.

  • Temperature can also have a big effect of spray application.
  • As a general rule, try to spray when the temperature is not too hot or too cold.
  • Avoid spraying in temperatures above 28 degrees Celcius, as some herbicides tend to vaporise at high temperatures, releasing harmful vapours.
  • From a temperature perspective, Ideal spraying conditions are early mornings after overcast nights.

You should avoid spraying when temperature inversion conditions exist (cooler air is trapped beneath warmer air) as droplets can be trapped at a higher level and carried out of the target area.

Humidity is another variable in achieving effective spray application. If humidity is too high, it suppresses droplet evaporation and leads to extended life. If it is too low, it causes rapid evaporation of smaller droplets, leading to increased spray drift or the possibility of unacceptably high residue levels on plants.

Rain can have good or bad effects on spraying, depending on the product being sprayed. Some products work best when rain water carries them into the soil, but as a general rule, rain should be avoided, as leaves that are still wet from rain may not be able to absorb the required amount of product.

Pesticides should also not be applied if rain is likely to wash them away from the site of application. For this reason, rain-fast periods should always be checked on product labels.

Some general weather-related guidelines for spray application are:

  • Only spray in steady wind between 3 and 15 kph
  • Avoid spraying in temperatures above 28 degrees Celsius
  • Do not spray in inversion conditions
  • Record onsite weather conditions at the time of spraying
  • Always read the product label for ideal application conditions

There are many factors apart from weather that can contribute to environmental contamination. Nozzle wear is common and your spray equipment should be checked for wear and serviced regularly, if a nozzle flow is greater than 10% above theĀ  ISO specification for the particular nozzle, the nozzle needs to be replaced.

Spray applicators have a responsibility to minimise run-off and spray drift and, as far as weather is concerned, watching local forecasts, consulting product labels and monitoring conditions in the field are the best ways to achieve that goal.

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