Best Weather to Spray Weed Killer: Temperature, Wind and Rain

Weed killers work best on actively growing weeds, on a calm, mild, dry day, with no rain until the product has had time to soak in. Get the weather wrong and the spray either washes off, drifts onto plants you wanted to keep, or sits on weeds too stressed to take it in.

Temperature

Most weed killers used on lawns and driveways work best when daytime temperatures are roughly between 60 and 85 °F. In that range, weeds are growing and move the product from the leaves down to the roots.

• Too cold (below about 50 °F, or after a frost): weeds grow slowly and take up less product, so results are slow or patchy.
• Too hot (above about 85 to 90 °F): weeds under heat or drought stress close up and absorb less. Some lawn weed killers, especially ester formulations of 2,4-D and similar products, can also turn into vapour in the heat and drift onto nearby plants. Labels often give a maximum temperature; follow it.
• After a dry spell, water the area a day or two before spraying if you can. A weed that is wilted won’t take in much.

Wind and drift

Drift is spray carried away from the target. It is how a garden bed or a neighbour’s tomatoes get damaged.

• A light, steady breeze (many labels say about 3 to 10 mph) is better than none: it carries droplets predictably and away from you.
• Dead-calm mornings and evenings can be worse than a light breeze. Cool air near the ground (a temperature inversion) can hold a fine mist and move it sideways over long distances.
• Above about 10 mph, or in gusts, wait for another day.
• Use a coarse spray setting and keep the nozzle low. Fine mist drifts much further.

Rain and “rainfast” time

The rainfast time is how long the product needs on the leaves before rain won’t wash it off. It varies a lot between products: some ready-to-use sprays say 10 minutes to an hour, others need several hours. Look for it on the label, usually as “rainproof in…” or “rainfast in…”.

• Plan for no rain for at least the rainfast time, plus a margin. Forecast showers often arrive early.
• Don’t water the lawn or run sprinklers during that time either.
• Pre-emergent weed preventers are the exception: many need about half an inch of rain or watering within a few days to move them into the soil. Check the label.

Dew and time of day

Spraying onto heavy dew dilutes the product and lets it run off the leaves. Late morning, after the dew has dried and before the heat of the day, is usually best. Many lawn products also say not to mow for a day or two before and after spraying, so the weeds have enough leaf to take the product in.

A quick checklist

• Temperature within the label range, and not after a frost or during a heatwave
• Wind light and steady, not calm or gusty
• No rain or watering for the rainfast time plus a margin
• Dew dried off; weeds not wilted
• Pets and children kept off the area for as long as the label says

Our weed killer forecast checks temperature, wind and rain hour by hour for your ZIP code, and the lawn fertilizer forecast does the same for feeding. The product label is always the final word, and it is also a legal requirement for pesticides in the US.

A DIY Day is an independent site. Figures above are typical label ranges; always follow the instructions on the product you use. Spotted a mistake? Tell us.