Can You Spray Weed Killer On Wet Grass | Does It Work?

Come spring or fall, there’s much to do to keep our beautiful lawns and garden borders looking great and in healthy condition. Twice each year, you can always catch a gardener or landscaper whistling along through this green space or that with either a bag of seed or an herbicide sprayer in hand. Ironically though, reseeding and de-weeding should be done around the same time that dark clouds, full of precipitation, begin to loom above. But as the old adage goes, you can’t control the weather. 

So, how do we accomplish what we need to? When each morning, we’re greeted with either a heavy layer of dew (or frost) on our lawns or saturated soil from the previous night’s downpour. We know we can throw seeds down, regardless. But, can you spray weed killer on wet grass? And if so, how effective is it? 

Can You Spray Weed Killer On Wet Grass 

Yes, you can. However, results will greatly depend on which weed killer or preventer you choose. Is it a pre or post-emergent?  An organic option or synthetic formula? Some are granular while others are water-soluble. Some target specific weed types while others are broad-ranging.

Some are even herbicide and fertilizer combinations and these work quite differently than herbicides, alone. 

Each type will come with its own recommendations to achieve optimum results. Including whether it can be used on wet grass or before/after it rains. 

Does Rain Stop Weed Killer From Working 

Let’s look at this a little closer. Grass can become wet for a variety of reasons, right? So, let’s start with rain. Applying granular weed killers around the time it rains is actually recommended, as this facilitates absorption. A decent rainstorm would save you time and money.

The only time rain would stop granular options from working as expected is if they’re applied on a slope and heavy rain washes them away. (This scenario could also see those granules contaminating nearby water sources.)

Alternatively, if the granules end up sitting on top of a dense lawn and not coming in contact with the soil. If this happens you will need to work the granules into thick grass not only so the herbicide gets through the grass, but also to prevent blade burn and therefore, achieve a better result.

Water-soluble options are already diluted for easy application. If applied just after a rainfall, further dilution may render the active ingredients ineffective. Most water-soluble options recommend applying before a rain with a minimum of 30 minutes drying time for it to remain “rain-proof”.

How Soon After Rain Should I Spray Weed Killer 

Every herbicide that is not formulated to work with rainfall should include what’s known as a “rainfast” period on its label. This represents how much time should pass between application and rainfall. Following this guideline will ensure the best possible product performance. 

Keep in mind that these directions are typically based on lawns growing in healthy soil and environmental conditions. Weeds growing in poor soil may require a longer period of time between application and the onset of rain. This is due to these weeds taking longer to absorb the herbicide.  

Herbicide Application With Dew On The Ground 

Similar to after a rain, morning dew can provide just the right environment for certain kinds of weed killers to perform at their best. Granular options can start working immediately upon coming in contact with morning dew.

It’s also commonly understood that weeds appear happiest after receiving a bit of that morning moisture and are ripe to receive more. But, in this case, it may be in the form of a liquid herbicide that’s formulated to work in wet conditions. 

Ones that are not considered effective for use in wet conditions may get diluted by morning dew beyond the point of effectiveness. So, be sure to read the manufacturer’s rainfast recommendations. 

Can You Use Weed Killer Before Rain Is Due 

Some weed killers can be applied prior to a downpour and some can not, depending on a few different factors. If choosing a product with a rainfast aspect to it, be aware that rainfast recommendations can vary between products.

It’s best to monitor your local weather forecast and apply this kind of herbicide within the suggested timeframe. Confirming the active ingredient in your herbicide choice will also help to determine the best time. Keep reading for more information on each application type. 

Herbicide Spray 

A liquid herbicide is considered ‘rainfast’ when it has fully dried on the surface of weeds. This applies to both pre-emergent and post-emergent products. For post-emergents, ingredients like glyphosate, acifluorfen, and dicamba have a faster rainfast listing. Roughly between 30 minutes and 4 hours.

In contrast, there are various 2-4-D amine products that can take up to 8 hours to become rainfast. 

Most pre-emergent products actually perform better when applied just before a rain. Adequate rainfall assists this type with absorption to reach full effectiveness. Overly heavy storms, however, could result in excessive herbicide run-off with little to no result. 

Granules 

When eliminating weeds, I personally prefer granules for their ease of use. No need to keep the weather app perpetually open on my phone. Because, as mentioned, many are formulated to work more efficiently when applied prior to the rain. 

The only thing to keep in mind is that pre-emergent granules could be washed away in a strong storm and end up in a place you actually wanted to plant seed later on. Pre-emergents will keep those seeds from germinating too.

Post-emergent granules shouldn’t cause this issue as they’re typically formulated to focus on the biology of mature weeds and not your lawn. 

Is It Better To Spray Weeds Before Or After Rain 

Water-soluble herbicides work best when applied in dry weather and are allowed to completely dry on the target area. This allows time for weeds to absorb the herbicide and do their job. If it is not given time to dry, its effectiveness washes away with the rain. So, spraying weeds while it’s raining or as storm clouds are gathering above, is not advised.  

That said if you find yourself pressed for time and it comes down to spraying before or after, my recommendation is that before is always best. This way you’re allowing for at least some product absorption. Spraying after will only further dilute the herbicide and reduce effectiveness. 

How Long After Spraying Weeds Can I Water My Lawn 

Granular weed killers should be applied when the grass is damp and left alone to work for a minimum of 48 hours before you water your lawn. Existing moisture will help the herbicide stick to the weeds. Watering too soon will only serve to wash it away. 

Most liquid applications don’t need to be watered in to activate. You can simply check the rainfast designation on the label to determine when to water. Or, a solid rule of thumb is to wait a full 24 hours before watering. 

Best Time Of Day To Spray Weeds 

Interestingly enough, the time of day you spray can either make or break the effectiveness of your chosen herbicide. In colder climates, spraying around midday after temperatures have warmed up a little has shown to support optimum results. In warmer climates, early morning and late afternoon applications have proven the most successful. 

Herbicides may work differently from region to region, due to climate and soil differences. Finding one that works with the rainfall frequency and dew point in your area will certainly start you on the road to success.