How Long Before Rain Can I Spray?

Most herbicides need between 1 and 6 hours without rain after spraying to soak into the leaf and start working. The exact time depends on the product, so check the table below and the label for the one you're using. If the forecast is tight, give yourself an extra hour or two, or wait for a better day.

• ✓Most herbicides need 1 to 6 hours rain-free after spraying
• ✓Glyphosate labels say 2 hours, but we'd allow 4 where you can
• ✓WHACK-2,4-D 800, Dicamba 500 Select, Decision and GA200 need the longest, 6 hours
• ✓Buckshot is the odd one out: it actually needs rain to work

You want to spray today, but the forecast is looking iffy. How long before rain is it safe to spray? It's one of the most common questions we get, and the honest answer is that it depends on the herbicide. Some need an hour. Some need six. One actually wants the rain.

This guide gives you the rain-free time for each herbicide we sell, what can shorten or stretch it, and how to plan a spray day when the weather won't make up its mind.

What does rainfast mean?

A herbicide is rainfast once enough of it has soaked into the leaf that rain can no longer wash it off. Until then, it's sitting on the leaf surface, and a decent shower can rinse it onto the ground before it does any work.

The time it takes is called the rain-free period. Product labels usually state it as "do not spray if rain is expected within X hours" or "rain within X hours may reduce effectiveness". It's a minimum, not a target. The longer the spray has to soak in and move through the plant, the better the result usually is.

Rain-free times by product

Here's how long each Rainbow & Brown herbicide needs before rain, shortest to longest. These are taken from the current product labels unless marked with an asterisk.

* The label doesn't give a rain-free time for these products. The figure shown is our own guide from years of spraying.

Treat the table as a starting point. Always read the label for the product you're using, because that's the final word.

Why it's tricky in NZ

New Zealand weather changes fast, and a "fine morning" can turn by lunchtime. That makes rain-free times harder to judge here than in places with settled weather.

• Fronts arrive early. A southerly change forecast for evening can be through by mid-afternoon, especially on the east coast.
• Spring is prime spraying time and prime shower time. Weeds are growing hard, which is when sprays work best, but showers roll through most days.
• Summer thunderstorms are hard to pick. A hot, still afternoon can produce a downpour the forecast only hinted at, so the longer-rainfast products are best sprayed in the morning.

What changes the rain-free time

The rain-free time on the label assumes reasonable conditions. A few things can shorten it or stretch it.

• Wetters. An organosilicone penetrant like SuperWetter helps spray spread and soak into the leaf faster. The G-360 and G-540 Glyphosate labels both allow a shorter rain-free time when one is added. Not every label allows extra wetter, so check first.
• The weed. Waxy, hairy or dusty leaves slow absorption. Soft, actively growing leaves take spray in much faster.
• Weather on the day. Warm, humid conditions help uptake. Cold or very dry conditions slow it down, and a weed under stress takes in less.
• How hard it rains. A light drizzle after the rain-free time has passed is rarely a problem. A heavy downpour inside the window is the one that hurts.
• Wet leaves before you start. Dew or leftover rain on the leaf dilutes the spray and can make it run off. Some labels, like Dicamba 500 Select, specifically say not to spray wet weeds.

Planning a spray around rain

The trick is to work back from when the rain is due, not forward from when you'd like to start. Here's the approach we use.

1. Check the rain-free time. Find your product in the table above and on its label. If you're tank-mixing, go with the longest time of the products in the mix.
2. Check the hourly forecast and rain radar. Daily forecasts are too vague for this. MetService's hourly forecast and rain radar give a much better picture of when rain will actually arrive.
3. Add a buffer. Allow an extra hour or two on top of the label time. If the gap isn't there, leave it for another day.
4. Wait for dry leaves. Let the dew burn off or the last shower dry before you start, then spray the most important area first.

Always follow product label and local regulations.

Common mistakes

Most rain-related spray failures come down to a handful of avoidable calls.

Quick checklist

Run through this before you fill the tank.

• I know the rain-free time for the product (or the longest one in my mix)
• I've checked the hourly forecast and rain radar, not just the daily outlook
• There's a gap of the rain-free time plus an hour or two
• The leaves are dry, with no dew or leftover rain
• The weeds are actively growing, not drought or cold stressed
• If I'm adding a wetter, the label allows it

Next step: before your next spray day, look up the rain-free time for the product you're using, and check the label.

FAQs

Further reading:

• Which Glyphosate Do I Use?
• Spray Rate Calculator
• Herbicides Near Waterways