Temperature and spraying

Air temperature affects an application in three ways: how fast the droplets evaporate, how some products behave once they are on the plant, and how well the plant takes the product up. Hot and cold weather create different problems, and neither is solved simply by looking at the thermometer.

When it is too hot

• Evaporation. Warm air, especially warm dry air, evaporates water from the droplets quickly. Droplets shrink, drift more easily and may not reach the target. This is what Delta T measures, and why Delta T usually peaks in the afternoon.
• Volatility. Some herbicides, including certain growth-regulator formulations, can turn into vapour from the spray or from treated surfaces, and the vapour can move off site hours later. Higher temperatures increase this risk. Labels for such products often set temperature limits or timing restrictions. See spray drift.
• Crop and weed stress. Plants under heat or drought stress often close their stomata and thicken their leaf cuticle, which can reduce uptake of systemic products. Some products also carry a higher risk of crop injury in high temperatures.
• Operator and equipment. Heat affects operator safety in protective equipment, and drone batteries have operating temperature limits.

Many labels state a maximum temperature, or advise against application in hot conditions. Where they do, the label is the limit.

When it is too cold

• Slow uptake and activity. Plants grow slowly in cool weather, and many systemic herbicides move more slowly through them, so control can be slower or weaker. Some labels advise waiting for warmer weather or for plants to resume active growth.
• Frost. Plants damaged by frost, or that have just come through a frost, may not respond well. Common guidance is to wait until plants have recovered and are growing again; the label may give specific advice.
• Low Delta T and inversions. Cold mornings are often humid and calm, with a low Delta T and a thermal inversion. The risk is fine droplets hanging in the air, not evaporation.
• Freezing. Near or below 0 °C (32 °F), water can freeze in lines and nozzles, and ice on leaves prevents good deposition.

Temperature and Delta T together

Temperature alone does not tell you whether evaporation is a problem. At 30 °C (86 °F), Delta T may be about 6 with 60 % humidity or about 9.6 with 40 %. At 20 °C (68 °F) and 80 % humidity it is about 2.5. So a hot but humid day can be sprayable, and a mild but very dry day may not be. Use temperature to judge plant activity and volatility, and Delta T to judge evaporation.

Practical points

• On hot days, spray in the cooler parts of the day, with coarser droplets where the label allows, and avoid volatile formulations when temperatures are high.
• On cold days, wait for weeds to be actively growing and for the air to warm and mix.
• Measure temperature in the shade at spray height, not in the cab or next to machinery.

In PulveCast

PulveCast checks each hour's temperature against a minimum and maximum you can set (0 °C and 40 °C by default, or the limits loaded for a supported drone model) and computes Delta T from the same temperature and humidity.

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
• Spray drift: causes and how to reduce it
• Wind speed for spraying
• The best time of day to spray
• Humidity and dew point for spraying
• Rain and spraying: rainfast periods and forecasts
• Droplet size and nozzle selection
• Weather for drone spraying
• Kp index, RTK and GNSS for spray drone operators
• Pre-spray checklist