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Yesterday I used around 3 litres of glyphosate (generic roundup) around my garden. It was in the evening around 6pm. This morning I woke to the sound of rain.

Will the rain have diluted the glyphosate or will my weeds die?

To make this more generic:

To be 100% effective how long before rain should I poison? 
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2 Answers

up vote 4 down vote accepted

Glyphosate needs to be applied while the plants is actively growing and transpiring moisture, which requires sunlight. This means you need to apply glyphosate in the morning so that it will take effect during that day. Glyphosate it deactivated very easily so applying at night, even if it doesn't rain, is likely to be ineffective.

So, apply in the morning on a day that is expected to be sunny and warm with no rain.

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Wait a second, I just looked up Glyphosate and it says no one knows how it works... Have you tried this yourself? –  Coomie Oct 23 '12 at 5:55
    
That's right, we don't know exactly what it does to kill plants on the cellular level but the instructions based on testing by the manufacturer usually say apply while actively growing. Spring is the optimum time for most weeds. –  kevinsky Oct 23 '12 at 12:32
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@coomie I have tried this myself, in that I used to think Glyphosate didn't work as I was spraying it in the evening after I had got home from work. My Dad set me straight and now I wait for a dry, hot day and spray in the morning. It works much better this way in my experience (which accords with the instructions that I of course hadn't bothered to read!). –  Bogdanovist Oct 23 '12 at 23:05
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Depends on the formulation. I understand from here and here that many products sold as glyphosate based are mostly "inert" ingredients. One of the functions of these is to prevent the active ingredients from washing off by acting as surfactants and stickers described in excessive detail here.

In practice if it had at least a few hours before being washed off you should see some effects. At worst you will have to reapply on the tougher weeds.

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