How long before rain can I spray?

It depends on the product. Check the label for the rainfast period or rain-free interval. As a general planning rule, avoid spraying when rain is forecast within the next few hours, and use the label's figure when it is longer.

What does rainfast mean?

A product is rainfast once rain no longer significantly reduces its effectiveness. The time needed varies by formulation, adjuvant, plant surface and weather, and is given on many labels.

Does a 30 % chance of rain mean I can spray?

It means rain is possible at your location during that period. Consider the expected amount and the product's rainfast period, check the forecast again before starting, and follow the label.

Rain and spraying: rainfast periods and forecasts

How rain affects spraying: rainfast periods on the label, reading precipitation probability, heavy dew, and the runoff risk to streams, ponds and water bodies.

Rain shortly after an application can wash product off the leaf before it has been absorbed or has dried, reducing its effect and sending it into the soil and nearby water. Rain during application also makes coverage uneven and can make machinery and drone operations unsafe. Planning around the forecast is part of every application.

Rainfast periods

A product is rainfast once rain no longer significantly reduces its effectiveness. The time needed varies a great deal with the formulation, the active ingredient, the adjuvants, the crop or weed surface and the weather after application. Some products become rainfast quickly once dry; others need several hours or more.

The label is the authority. Labels commonly state a rainfast period, or a minimum rain-free interval after application, and sometimes recommend adjuvants that shorten it. There is no universal figure that applies to all products.

As a general planning rule, PulveCast's own help text suggests avoiding application when rain is forecast within the next 2 to 4 hours — but where the label gives a longer rain-free period, use the label.

How much rain is a problem

There is no single threshold. A brief light shower after a product has dried may make little difference; heavy, intense rain soon after application can remove a large share of it. Intensity and timing matter more than the daily total, and the answer differs by product. If significant rain falls within the label's rainfast period, the label or the manufacturer's technical advice may say whether a repeat application is allowed.

Reading precipitation probability

The probability of precipitation is the chance that measurable rain falls at a given point during the forecast period — not the share of the area or the time it will rain. A 30 % chance means rain is possible, not that it will be light. In showery, convective weather the forecast can change quickly, so check it again before starting and watch the sky during the job.

Look at both the probability and the expected amount. A high probability of a trace of rain is a different risk from a moderate probability of a heavy shower.

Heavy dew

Dew can behave like light rain on the leaf: it dilutes the spray and can cause it to run off. It also shows the air is near saturation, often with a thermal inversion. See humidity and dew point.

Runoff and water bodies

Rain after spraying does not only reduce efficacy. Product washed off leaves and soil can reach ditches, streams, ponds and wells, where some pesticides are toxic to aquatic life. To reduce the risk:

Do not spray just before heavy rain, especially on saturated soil or slopes.
Respect buffer zones next to water that the label or local regulations require.
Take extra care on sloping fields, bare soil and fields with drains leading to water.

In PulveCast

PulveCast checks the hourly probability of precipitation against your limit (50 % by default, adjustable) and shows the forecast rain amount, turning an hour to caution for a trace of rain and to do-not-spray when more than 0.5 mm is expected.