Can You Spray Weed Killer on Wet Grass – What You NEED to Know


Sometimes, nature can get in the way of our home and garden maintenance schedule.

Particularly when it rains. Trying to do anything when it’s wet outside can put a real dampener on things.

Like killing weeds.

But it’s important that we work with the weather and conditions in order to achieve horticultural success.

And one of the questions that professional gardeners get asked a lot is – can you spray weed killer on wet grass?

We’ve got the answer coming right up.

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Wet Grass and Weed Killer? Too Long, Didn’t Read

We promised you the answer, and so for everyone in too much of a hurry to read the full article – here it is:

Yes, you can put weed killer on wet grass.

But it’s not the best time to do so, and problems can occur.

man spraying weed killer on grass

Aside from this, you can see significantly different results depending on multiple factors – including the type of herbicide, the weeds you’re trying to eradicate, and how wet the conditions actually are.

As there are a few caveats, we would highly recommend you keep reading for more information.

How Rain Effects Herbicides

Depending on the type of herbicide you’re using, damp or wet conditions are going to have some kind of effect on its ability to do its job.

Water can dilute liquid herbicides, for example, which will lower its potency, and you’ll likely notice a drop in its performance as a result.

Furthermore, a common problem (with herbicide use in general) is that rain can wash away liquid weed killers, and harmful chemicals can travel to unwanted areas, such as desirable plant beds, and waterways.

As such, applying liquids to leaves and grass shortly before a downfall will almost certainly cause run-off, and the product is washed away to be deposited in less effective – and potentially damaging locations.

But can you spray weed killer after it rains?

Applied immediately after rainfall, herbicides might not travel as far as they would when it’s actually raining, but the residual water still on the leaves and grass can hamper their effectiveness.

This is why it’s important to spray such chemicals only in optimum conditions – when it’s dry, for example.

It’s also important to refrain from cutting the grass after it’s rained, but in particularly wet locations, this might be something of a challenge.

If you absolutely have to get the mower out, check out this article on how to cut wet grass.

But not all garden maintenance is hampered by the rain.

Most granular herbicides, for example, need water to activate, and manufacturers recommend you thoroughly water the area where the product has been applied.

lawn moss and weeds

As such, if you know it’s going to rain shortly after application, you can save yourself time and money by getting the hose out, and letting nature do the hard work for you.

Rainfastness

When it comes to applying most herbicides, you need to be aware of your particular product’s “rainfastness.”

This is the length of time you need a rain-free period before the herbicide is “dry,” and from this moment it will not be affected by subsequent rainfall in any way.

Not all herbicides are the same, and the time it takes them to dry before a rain shower can vary.

Let’s take Roundup, for example – one of the most popular and recognizable herbicides. Their website claims that all of their products are dry within 30 minutes to three hours.

Obviously, this window varies significantly, and you need to be aware of the rainfastness of the herbicide you’re actually using.

This herbicide rainfast times PDF might help as a general guide, and although it is specific for Washington State, it should give you a ballpark figure for general herbicide rainfast times.

Having said that, I would encourage you to err on the side of caution and check the instructions on the bottle in your hand – with specific regard to the location you’re in.

A Word of Caution

Herbicides can be potent and harmful chemicals, and they need to be applied strictly according to their directions.

Failing to do so can result in damaged crops, dead desirable vegetation, and irate neighbors – at the very least.

In a worst-case scenario, you could end up with seriously ill pets and children, as well as poisoning watercourses – which will almost certainly result in legal action.

This guide to the best pet-safe weed killers might help if you’re at all concerned about using harsh chemicals. Alternatively, go here to find out how to remove weeds on your lawn without herbicides.

While you can spray herbicides on wet grass, there is still a risk that the chemicals will end up somewhere you don’t want them to.

And if you want to avoid this, plus any possible repercussions from irresponsible herbicide use, it’s best to wait until you have optimum conditions.

And the same can be said if you’re using the best weed killers for driveways, too.

In fact, it’s probably even more important to wait for the right conditions when using chemicals on concrete.

So, let’s find out what they are.

Optimum Conditions for Spraying Weed Killer

When it comes to spraying weed killer, it’s critical that you adhere to the manufacturer’s distribution instructions.

But for the most part, the optimum conditions for application for most herbicides are generally the same or very similar.

clover lawn being watered

Wait for completely dry, warm days. For post-emergent herbicides, the mercury should be somewhere between 65 °F and 85 °F.

For the best pre-emergent herbicides, you’re typically looking at soil temperatures over 55 °F.

Ideally, there shouldn’t be a breath of wind, as even a brief breeze can carry a sprayed chemical into an undesirable location.

And always use the recommended application method for the size of the area you wish to cover and that particular chemical.

Last, but by no means least, it’s obviously very important you’re using the right product for the job you need it to do.

Come rain or shine, if you’re not using the correct weed killer, you’re not going to see productive results.

This article on the most common lawn weeds and how to get rid of them should point you in the right direction.

The Best Time of Day to Spray Weed Killer

You will likely be spraying systemic herbicides during the growing season in order to combat unruly and unwanted vegetation from taking over.

A whole article could be written on the best time of day to spray weed killer, but as a general rule of thumb, it’s in the early morning and late afternoon in warm climates – such as through the summer.

And try late morning, midday, and early afternoon in cooler conditions, in early spring and late fall.

Winter herbicide application isn’t recommended, as the products can freeze and become ineffective. But if you are applying during this time, do so at midday, when the temperatures are the highest they will be.

Using Surfactant

Some liquid herbicides – like many from this selection of the best commercial weed killers on the market – may also require a surfactant to increase their effectiveness.

This is an additional chemical that is diluted into the weed killer in order to break the surface tension of the liquid, which makes it more effective when soaking into leaves, grass, and other plant life.

Recently, I’ve been having a running battle with Creeping Charlie on my lawn, and I’ve been using Tenacity selective herbicide with a surfactant to improve results.

Of course, I’ve been making sure that I’m only applying this when the grass and ground is already dry – so if the clouds are forming, I need to have finished the application 30 minutes to an hour before the heavens open.

And as this is quite a large job that takes a lot of time, I invariably find myself putting it off for a few days, so I can have guaranteed clear skies and dry conditions.

Speaking of large jobs, this article might help you choose the best weed killers for large areas.

Just remember – if you have a lot of land to cover, you’re going to need even more time if you’re trying to finish before a downpour.

However, with some herbicides, rain can act as a natural surfactant/wetting agent, which helps the chemical enter through the leaves and kill the weed down to the root.

How Soon After Rain Can You Put Weed Killer on Wet Grass?

Again, it is possible for you to apply a herbicide on wet grass, but its effectiveness will likely be diminished if it’s too wet.

If you really don’t have time to spray in the window before rainfall, I would recommend waiting as long as you can afterward, so you can at least give the grass a chance to dry out.

Remember – it doesn’t always have to be rain that causes your lawn to become wet, as a particularly dewy morning can also leave blades very damp.

raindrops on clover lawn close up

Either way, if you’re spraying soaking leaves and grass, you might as well be pouring the herbicide down the drain – and a stack of money with it.

Can You Spray Weed Killer on Wet Flower Beds?

You can, but it’s going to operate similarly to when it’s sprayed on your lawn.

However, some of the best weed killers for flower beds work best when the soil is damp – particularly the granular variety.

Post-emergent granular weed killers – for example – work by forming a barrier in the soil, which needs to be activated by water.

Alternatively/additionally, you might want to try these awesome weed barrier groundsheets, which can be highly effective when used with pre-emergent granules.

But as with all herbicide applications, follow the instructions to the letter, and you won’t go far wrong.

FAQs

Can you spray glyphosate after rain?

Glyphosate, the potent ingredient commonly associated with Roundup products, is a harsh chemical that can do serious damage if it’s sprayed anywhere it isn’t wanted.

As such, I would highly recommend you refrain from applying it after rainfall, even if you think it’s safe to do so.

As most Roundup products are rainfast between 30 minutes to three hours (check the bottle), you should always try to spray the chemical well before rain is forecast.

Do I need to use weed killer on grass?

No, you don’t need to use herbicides at all, if you don’t want to.

There are alternative methods for using harsh chemicals on grass – for example – and you can follow that link for some top tips and advice.

Generally speaking, I think the use of herbicides should be as a last resort, if the area you need to cover is just too big, and/or you genuinely know that nothing else is going to be effective.

Is it better to spray weed killer before or after rain?

Good question, and although we’ve covered it – it’s worth repeating, as it can still be a little confusing.

If the chemical you’re using is going to be rainfast (dry, and unaffected by water) in plenty of time before rain is forecast, then it’s better to spray the weed killer beforehand.

However, if you don’t have enough time, don’t risk it, and wait as long as possible afterward, to allow the leaves, grass, and/or soil to dry.

Can you spray insecticides on wet grass?

For pest control products, I would offer the same response as I would with herbicides. In fact, when it comes to using any such chemicals, they generally don’t play well with rain.

Always check the bottle and stick to the instructions.

Summary

Can you spray weed killer on wet grass? Yes, you can.

But you might see a mixed bag of results depending on the herbicides used, the weed you’re targeting, and how damp the conditions are.

I highly recommend waiting until it’s dry before spraying any chemicals in the garden, but let me know your thoughts on the matter in the comments.

I wish you the very best of luck with your weed-killing endeavors.

Andy Gibson

My name's Gibson. Andy Gibson. I like to think of myself as the Bond of the backyard, that is if yard work ever became sexy. I write about everything about indoor and outdoor gardening and the dread-it-but-still-need-to-do-it chores around the yard, like cleaning out the gutter guards.

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