When to spray
Spray mid-morning to mid-afternoon, with a steady 3–15 km/h breeze at boom height, Delta T between 2 and 8, and no inversion. Get those right and the chemical lands where you point it. Here's how to read each one — and why the calm, quiet evening is the most expensive time you can spray.
The label is the law. The bands below follow GRDC and APVMA guidance, but the legal limits — wind, droplet size, buffer zones — are printed on the label of whatever is in your tank. Where they differ, the label wins. CropWindow is a planning aid: confirm conditions at the boom before you open the taps.
The wind band: 3–15 km/h, and never dead calm
You want a steady breeze between about 3 and 15 km/h, measured at boom height — not from a weather station three paddocks away. With a drift-reducing setup (coarser nozzles, higher water rates, lower boom), most labels let you work up to 20 km/h. Above that, fine droplets blow off target and you're paying to spray the neighbour's place.
The end that catches people out is the bottom. Dead calm is not the safe end of the range — it's a warning sign. Below about 3 km/h there's nothing to push fine droplets down onto the target and disperse them, so they hang in the air. If an inversion has set in, that suspended cloud drifts as one concentrated parcel and can land kilometres away, on somebody's vines or somebody's cotton. A light breeze shows you where the spray is going and puts it on the weeds. Still air hides it and saves it up.
Watch gusts too. An average of 12 km/h with gusts to 25 is a hold, not a go.
Time of day: the window has edges
The reliable window runs from mid-morning to mid-afternoon, and each edge has its own reason.
- Morning start — after dew burn-off. Wait for the dew to lift and a working breeze to set in. That's usually when the overnight inversion has broken up and the air is mixing again. Spraying onto a dripping-wet leaf also dilutes and runs your chemical off before it can work.
- Afternoon — watch the Delta T peak. On a hot day, Delta T climbs through early afternoon and can push past 8, where droplets evaporate before they hit the leaf. Ideal is 2–8; 8–10 is workable only with coarse droplets and high water rates; above 10, park it. The full story is in the Delta T guide.
- Evening — the inversion is re-forming. From about an hour before sunset the surface starts cooling and the inversion sets back in. The still, cool evening that feels perfect is the setup for the worst drift there is.
Inversions: why the quiet night bites
A surface temperature inversion is a layer of cool, still air trapped near the ground under warmer air above. It forms on clear, calm nights as the ground radiates its heat away, and it holds from around sunset until an hour or more after sunrise. Spray into it and your fine droplets don't settle — they sit suspended in that cool layer and slide sideways with whatever gentle drainage flow is running, sometimes 10–20 km before they come down.
That's why night spraying is treated as high risk. Research cited by GRDC found spraying at night with the same rig and settings can leave up to five times more chemical suspended in the air than the same job by day. Spraying during a hazardous inversion is an offence under the label conditions on your drum — and the label is the law. Plenty of labels prohibit or warn against night application outright. The calm you're chasing after dark is the hazard itself.
You can't prove an inversion has cleared without measuring temperature at two heights, but the signs are easy to read:
- Smoke or dust that flattens out and drifts sideways instead of rising — the smoke test. Dust hanging behind the ute on a still evening is the classic tell.
- Mist or fog pooling in the low ground.
- Sound and smell carrying further than usual — you can hear the neighbour's pump.
- Heavy dew or frost forming on a clear, calm night.
GRDC's rule of thumb: if today's maximum minus tonight's forecast minimum is 10 °C or more, there's roughly a 90% chance of an inversion at sunrise. See the signs, don't spray.
Rainfast: the dry hours your chemical needs
Rainfast is the dry time a product needs on the leaf before rain can wash it off. Typical windows run from about 1 to 6 hours — premium glyphosate formulations can be rainfast in well under an hour, older and generic ones can want up to 6, and other chemistries differ again. The number that counts is the rainfast statement on your label.
Rain inside the window doesn't half-work — it washes the job off the leaf and you're back to do the whole thing again: chemical, diesel, hours, and a set of weeds that are now bigger and harder to kill. When CropWindow scores a spray day, hours of forecast rain inside your rainfast window read as a hold for exactly that reason.
2,4-D and volatile products: heat is the enemy
2,4-D and the other phenoxy herbicides add a problem the wind band can't fix: volatility. On a hot day the deposit can lift off the leaf as vapour hours after a textbook application and drift onto grapes, cotton or vegetables downwind. That's why APVMA label conditions for 2,4-D are mandatory and specific — apply only in 3–20 km/h wind at the site, never under a hazardous inversion, minimum very coarse droplets, stepping up to extremely or ultra coarse over the warm months (roughly October to mid-April) near sensitive areas.
With volatile products, cooler is safer. Skip the hot afternoons entirely, and read the exact label of the drum in your shed — the wording changes between products and years.
The quick read
Typical starting bands, drawn from GRDC and APVMA guidance. Your label and your judgement come first.
| Factor | Go | Careful | Hold |
|---|---|---|---|
| Wind at boom height | 3–15 km/h steady | 15–20 km/h with drift-reducing setup | Below 3 km/h, above 20, or gusty |
| Delta T | 2–8 | 8–10, coarse droplets only | Below 2 or above 10 |
| Inversion | Broken up — mid-day mixing | Shoulders of the day, wind dropping | Present or forming — smoke hangs flat |
| Time of day | Mid-morning to mid-afternoon | Early morning, late afternoon | Dusk through dawn |
| Rain | Rainfast window clear with room to spare | Rain near the label's rainfast period | Rain due inside the rainfast window |
CropWindow runs this table for you, every hour of the week — wind, Delta T, inversion risk and rainfast checked against your thresholds, with a red, amber or green call and the reasons behind it.
Fair questions
Spray timing, asked and answered
What wind speed is too much for spraying?
The commonly used band is roughly 3 to 15 km/h at boom height, with many operators and labels accepting up to 20 km/h with drift-reducing setups. More wind than that moves fine droplets off target. Just as important: dead calm is not safe — very still air often means a temperature inversion, which can carry drift kilometres.
Can I spray at night?
Often you shouldn't. Clear, still nights are when surface temperature inversions form, and spraying into an inversion is how drift ends up on your neighbour's place. Some labels expressly prohibit night application. If the evening breeze has died to nothing, treat that as a red light.
What does rainfast mean?
Rainfast is the dry time a product needs after application before rain stops mattering — commonly one to six hours depending on the product. Rain inside that window can wash the chemical off before it's absorbed, which wastes the pass and can mean re-spraying.
Know the spray window before you fill the tank
CropWindow checks wind, Delta T, inversion risk and rainfast against your local forecast, hour by hour, and gives you the day with the reasons. Free in early access.
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