Is it too windy to spray weeds?
Short answer: spray when the wind is a steady 3 to 10 mph. Above 10 mph, don't. Below 3 mph, be careful too, because dead-calm air can carry spray farther than a breeze.
The wind band: 3 to 10 mph
“Drift potential is actually lowest at wind speeds of 3 to 10 mph (gentle but steady breeze)”— UNL CropWatch “wind speeds of 3 to 7 mph are preferable. Never spray when wind speeds are more than 10 mph.”— UNL CropWatchFairsky turns that into its spray verdict: GO between 3 and 10 mph sustained, and NO when the wind goes past 10 mph. Gusts matter too: Fairsky also says no when gusts top 15 mph (that gust number is a Fairsky default; no source gave one).
Why dead calm isn't safe either
“Calm or low wind conditions may indicate the presence of a temperature inversion.”— UNL CropWatch “Typically, temperature inversions start at dusk and break up with the sunrise because of vertical air mixing.”— UNL CropWatchIn an inversion, cool air is trapped under warmer air and fine droplets can hang and drift sideways for a long way. That's why Fairsky marks hours under 3 mph as borderline (WAIT) and only gives spray verdicts in daylight.
Heat makes some weed killers drift as vapor
“Volatility of chemicals increases with temperatures above 85 degrees Fahrenheit and with decreased humidity and dry conditions.”— SDSU ExtensionFor agricultural dicamba, labels go further: “DO NOT apply if temperatures are forecasted to be 95 °F or above either on the day of treatment OR the day after”— Rutgers NJAES Fairsky's spray rule calls 86–90°F borderline and anything hotter a no. On the cool side, a common homeowner product says to apply “when weeds are young and actively growing and daytime temperatures are between 45°F and 90°F”— Ortho Weed B-gon so Fairsky's minimum is 45°F.
How long it has to stay dry
Rainfast times vary by product. One Ortho Weed B-gon product says it's “rainproof in 1 hour”; a Roundup concentrate listing says “In as little as 30 minutes, it is rainproof.” Ortho's how-to is more cautious and suggests spraying “in dry conditions when rain isn't in the forecast for at least 24 hours.”— Ortho how-to Fairsky's GO needs 4 rain-free hours after you finish. Your product's label wins, and the app lets you enter its numbers.
A quick pre-spray check
- Wind steady between 3 and 10 mph, gusts under about 15.
- Daylight, not the calm hours around dusk and dawn.
- Between 45°F and 85°F (cooler mornings beat hot afternoons).
- No rain for at least the label's rainfast time; Fairsky wants 4 hours.
Sources
- UNL CropWatch: managing pesticide applications to avoid drift
- UNL CropWatch: 5 things to know to avoid herbicide drift
- SDSU Extension: turf herbicide applications in extreme heat
- Rutgers NJAES: herbicide drift and temperature
- Ortho Weed B-gon product page (Scotts)
- Ortho Weed B-gon how-to
- Roundup Concentrate Plus (DoMyOwn listing)
Quotes were checked on the linked pages in October 2026. Thresholds marked as Fairsky defaults are our own conservative choices, not citations. Product labels differ; your label wins.
Questions
What wind speed is too windy to spray weed killer?
University of Nebraska CropWatch says drift potential is lowest at 3 to 10 mph and to never spray when wind speeds are more than 10 mph; 3 to 7 mph is preferable.
Is no wind the best time to spray?
No. UNL CropWatch notes that calm or low wind can indicate a temperature inversion, which typically starts at dusk and breaks up after sunrise. Fine droplets can hang in the air and move off target.
How long does weed killer need before rain?
It depends on the product: one Ortho Weed B-gon product is rainproof in 1 hour, and one Roundup concentrate in as little as 30 minutes. Ortho's how-to advises spraying when rain isn't forecast for at least 24 hours. Fairsky wants 4 rain-free hours; your label wins.
Fairsky is not a substitute for official warnings. For warnings, go to weather.gov.
Live forecasts: National Weather Service / NOAA (public domain), fetched by your browser. Climate averages: NOAA NCEI U.S. Climate Normals, 1991–2020 (public domain). Project thresholds: product data sheets, ACI and university extension guidance, linked on each page; your product’s label wins.