Yesterday I used around 3 litres (approximately 1 gallon) of glyphosate (generic Roundup) around my garden. It was in the evening around 6pm.

This morning I awoke 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?


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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    Glyphosate interfers with the synthesis (EPSPS) of aromatic amino acids (phenylalanine, tyrosine, and tryptophan). This synthesis not found in animals. – rockerBOO Mar 13 '15 at 6:02

From pioneer.com:

"Foliar absorption of herbicides occurs in a liquid phase only; once a water droplet has dried on the leaf surface and herbicides have crystallized little to no additional absorption occurs. Therefore, any environmental condition speeding the drying of spray droplets on a leaf surface will reduce absorption. Low humidity and high winds can greatly reduce drying time, thereby allowing little time for absorption to occur. Conversely, high humidity with little wind slows the rate of drying and lengthens absorption time. Rainfall shortly after (< ½ hour) glyphosate application can wash spray droplets from the leaf surface. A foliar application should be "rain fast" once droplets have dried on the leaf surface.

Temperature, soil moisture, and solar radiation that optimize plant growth facilitate absorption and translocation of glyphosate. When photosynthetic rates are high photoassimilate produced in leaf epidermal cells is rapidly loaded into the phloem, other organic molecules like glyphosate are similarly loaded, and both are quickly translocated to sink organs. The rapid removal of glyphosate molecules from epidermal cells maintains a high concentration gradient that increases absorption rate. The time of day glyphosate is applied can also impact its efficacy. Applications made between 9 a.m. and 6 p.m. tend to maximize glyphosate activity. Short-lived temperature spikes (> 90 ºF) can also enhance absorption by reducing cuticle viscosity and allowing easier passage of foliar-applied herbicides."

From Purdue University:

"Glyphosate must penetrate the leaf surface to provide effective weed control. While absorption occurs relatively quickly, rain after an application can wash glyphosate off before it has a chance to enter the leaf. The rain-free period required to prevent reduced activity is in uenced by the susceptibility of the target weed and the glyphosate rate. Small weeds of a sensitive species will require a shorter rain-free period than large or dif cult to control weeds. A 30-minute rain-free period may be adequate under ideal conditions. When spraying larger weeds, however, several hours between application and rain may be required to avoid reduced activity. Differences in rainfastness among glyphosate products are generally small. Adding more surfactant appears to have marginal benefits on the rain-free requirement."

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  • I like how thorough your answer is, but to be a good answer, you'll want to tailor it to this specific question by writing it yourself (and probably toning down the reading level). – Merchako Jun 12 '19 at 23:23
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    Well, let's work on the attention span... – Danijel Jun 13 '19 at 13:18
  • This post explains the mildly-negative view that this community has toward posting long quotes without adding anything meaningful. meta.stackexchange.com/questions/89091/… – Merchako Jun 14 '19 at 7:25
  • I find this information very interesting as it illuminates some of the mechanisms involved in killing plants. The pioneer site has a lot of interesting and in-depth information on herbicides. – Tim Nevins Jul 17 '19 at 22:23

Glyphosate is a systemic herbicide. It really only needs a few hours to get into the plant before a rain. It is not a quick killer in basic formulation, but absent resistance, it kills the whole plant. Farmers started using it in the 70s using contact applicators to brush it onto Johnsongrass. Roundup would take a week to kill it, but it got it all, down to the rhizomes that produce new plants underground.

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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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I am a new spray tech, however I have been told by a 10 year+ tech, 6 hours is needed between rain and application. He is one of the people I pay attention to when he talks. Not just his experience, but his candor.

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    Hi Spray Tech, we are looking for answers with a little more detail, perhaps quoting manufacturer or research, Can you add more? – kevinsky Jan 19 '19 at 13:44

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