Can You Spray Roundup On Wet Grass | Weed Killers

Weeds are the bane of many a homeowner. Even if your lawn is fairly healthy and low-maintenance, it may be necessary to treat tough weeds once or twice per year. So when you do take the time to spray Roundup on your lawn, you want it to be worth your while.

Unfortunately, there are a few things that can interfere with Roundup and its effectiveness. One of the most common mistakes people make when treating weeds is spraying Roundup weed killer on wet grass. Despite its usual efficacy, a little bit of moisture can deem Roundup almost useless against weeds.

To achieve the desired results every time you spray Roundup, you need to time your applications around things like rain, dew, and irrigation systems. You also need to allow adequate time for the herbicide to dry before the grass gets wet again. I realize that can all sound overwhelming at first, so I’ve broken down the how and why of using Roundup on wet grass below.

Can You Spray Roundup On Wet Grass

For the best results, Roundup should only be sprayed on dry foliage. Applying Roundup to damp grass will dilute the herbicide and make it less effective. There’s also a risk of the herbicide running right off of already wet foliage. Since Roundup needs to sit on weed leaves to take effect, this is obviously undesirable.

However, these guidelines don’t apply to all herbicide formulas. While herbicide sprays are best applied to dry grass, granular herbicides call for the opposite. Granular herbicides must be applied to damp grass so that they adhere to the weed foliage.

Can You Spray Roundup Before It Rains

Ideally, Roundup should only be used on clear, wind-free days. Since Mother Nature is sometimes unpredictable, however, it’s not always possible to schedule our herbicide applications to align with the perfect weather. 

Fortunately, most Roundup products are specifically designed to withstand rain shortly after application. As long as you allow time for the herbicide to dry (necessary drying time varies with each formula), it will maintain its efficacy even after rainfall.

Personally, I still recommend erring on the side of caution. You should only spray Roundup before rain if you’re 100% confident it will have enough time to dry. If the skies are overcast and the forecast shows rain on the horizon, it may be best to delay your herbicide application until another day.

How Long Before Rain Should I Spray Roundup

All Roundup products I’m familiar with require a specific amount of time for the herbicide to dry. You should be able to find this information on the label of your chosen Roundup formula. I always recommend heeding the guidelines printed on any given herbicide packaging over the more generic ones I’m about to mention.

The vast majority of Roundup sprays become rainfast within 30 minutes to 3 hours. Again, the exact amount of time needed depends on the specific formula. The only way to know precisely how long to apply your Roundup produce before rain is to reference the package label.

Using Roundup After It Rains

Roundup is perfectly safe to use after rain as long as you give the area plenty of time to dry. I don’t recommend spraying Roundup immediately after it rains for a couple of reasons:

I briefly mentioned that spraying Roundup on wet grass is likely to dilute the product. Using a diluted herbicide is pretty much pointless when the goal is to eradicate unwanted weeds. It’s more likely than not that you’ll need to re-apply Roundup several days later once the area has dried to get the results you want. So I suggest waiting for the grass to dry in the first place to prevent wasting product and your valuable time.

Another compelling reason to wait until the grass has dried is that weeds love to sprout after rainfall. Traditional Roundup products are only effective on actively growing weeds, so spraying even a day earlier will have no impact on these new weeds. 

By waiting just a couple of days after a significant rainfall to spray herbicide, you can eliminate both old and new weeds with a single application.

Is Roundup Effective If Sprayed On Wet Grass?

Based on what we know about Roundup and how it works to kill weeds, it’s safe to say that using the product on wet grass will at the very least weaken its efficacy. However, whether or not wet grass is likely to make Roundup completely ineffective is up for debate.

At the end of the day, Roundup is a strong chemical and should be used as responsibly as possible. So I recommend only applying Roundup to completely dry grass. This ensures that the herbicide works to its full potential. 

While Roundup may still work on slightly damp grass, there’s a very good chance that you’ll need to apply more of the herbicide overall to get the desired results. That equals more wasted product and also increases the risk of run-off to storm drains and nearby waterways.

It’s important to note that grass can become wet in more ways than one. I know we’ve focused primarily on rain so far but damp grass caused by morning dew or sprinklers poses a similar problem. Take all of these potential sources into account when timing out your next Roundup application.

Verdict: Using Roundup On Wet Grass

I’ve never run into a weed problem that was so urgent it needed to be treated that day. So if you have to choose between spraying Roundup on wet grass or just waiting until another day, I definitely recommend the latter!

Applying Roundup to wet grass is ultimately a waste of time and product. It’s best to wait until the grass is dry — when you know the Roundup will work as intended on tough weeds — than to risk needing to re-treat the area at a later time anyway.

If you’re expecting rain later in the day, there may still be time to spray for weeds. Check the label of your Roundup formula for the suggested waiting period for rain fastness. Unless rain is forecast in the next hour, there’s a good chance you’ll be able to apply Roundup with plenty of time to spare.