A spray window is a stretch of hours when the weather lets a crop protection product reach its target and stay there. Most application problems on Canadian farms come down to the weather on the day: strong wind that carries droplets off target, a calm evening inversion that lets fine spray travel sideways, heat that evaporates droplets before they land, or rain that washes product off leaves before it is absorbed. This guide explains how to read each of those factors when you plan a spray day.
Wind: the Variable That Matters Most
Wind is what moves spray off target. Too much wind pushes droplets onto neighbouring crops, shelterbelts, yards and water. Too little wind can be just as risky, because dead-calm conditions often mean a temperature inversion (more on that below). As a planning range, many Canadian extension guides treat steady winds of roughly 3 to 15 km/h as workable, with drift risk rising quickly in stronger or gusty wind. Some product labels set lower maximums, so the label always decides.
Practical wind checks:
- Measure wind speed at boom height in the field, not from the nearest town forecast
- Watch direction as well as speed, and know what sits downwind: sensitive crops, homes and water
- Gusty wind is harder to manage than steady wind at the same average speed
- Coarser droplets and a lower boom reduce drift, within what the label and nozzle chart allow
Temperature Inversions: Why Calm Isn't Safe
On clear evenings with little wind, the ground loses heat quickly and chills the air right above it, while warmer air sits on top like a lid. Fine droplets can hang in that cool layer and drift sideways for long distances before settling, sometimes well past the field edge. Inversions commonly form around sunset and can last until the sun warms the ground the next morning, which is why an evening that feels perfectly still can cause more off-target damage than a light daytime breeze.
Signs an inversion may be present:
- Wind below about 3 km/h, especially at dusk, overnight or early morning
- Clear skies with temperatures falling quickly in the evening
- Dust or smoke that hangs in a layer or creeps slowly along the ground
- Dew or ground fog forming in low spots
Temperature, Heat and Humidity
Products work best inside the temperature range on their label. In cool weather, weeds grow slowly and many systemic herbicides move through the plant less effectively. In hot, dry weather, droplets evaporate faster and shrink into finer, more drift-prone particles, and heat-stressed weeds can take up less product. Hot afternoons also tend to be the windiest part of the day, so many operators plan around mild, moderately humid periods once any overnight inversion has broken.
Rain and the Rain-Free Period
Rain soon after application can wash product off leaves before it is absorbed. Each label states a rain-free (rainfast) period, and it varies a lot between products. Before you start, check the hourly forecast for the whole job plus the full rain-free period after the last acre, not just the hour you start. Heavy dew can matter for some products too, so read the label directions on leaf wetness.
Reading a Spray-Window Forecast
A forecast cannot replace an in-field reading, but it does help you plan the day, line up operators and water, and avoid wasted trips. A useful spray-window forecast checks every hour against wind, temperature and rain together, instead of showing a single daily summary, and it should be anchored to your farm rather than a regional station.
A Pre-Spray Weather Checklist
Before and during every job:
- Read the label: wind limits, temperature range, rain-free period and buffer zones
- Check the hourly forecast for the whole job plus the rain-free period
- Measure wind speed and direction at boom height before you start, and again during the job
- Look for inversion signs at dawn and dusk
- Note sensitive crops, homes and water downwind
- Record time, wind, temperature and product with each application; many provinces require records for commercial applicators, and they are useful for everyone
Frequently Asked Questions
What wind speed is too high to spray?
The product label sets the legal limit. As general planning guidance, many Canadian extension sources treat roughly 15 km/h as the point where drift risk climbs sharply and advise against spraying in stronger or gusty wind. Always follow the label maximum and your own in-field reading.
Why shouldn't I spray when there is no wind?
Very calm conditions, especially at dusk, overnight and early morning, often mean a temperature inversion. Fine droplets can stay suspended in the cool air near the ground and travel a long way before settling, so a dead-calm evening can cause more off-target damage than a light breeze.
How long does spray need before it rains?
That is the rain-free or rainfast period on the product label, and it differs widely between products. Check the label, then make sure the hourly forecast stays dry for that whole period after you finish, not just when you start.
Can a weather app tell me it is safe to spray?
No forecast can guarantee safe conditions. A farm-level forecast helps you plan the day, but you still need to follow the label and check wind speed, direction and inversion signs in the field before and during application.
See your farm's own 24-hour spray-window forecast, scored hour by hour on wind, temperature and rain. Try AgriIntel free for 14 days.
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