Can You Apply Pre-Emergent In The Rain?

Pre-emergent is a great tool when it comes to maintaining a beautiful lawn. When used correctly, pre-emergent works to kill weeds before they begin to sprout through the soil. Several factors play into how effective pre-emergent can be, including temperature, moisture, and the type of pre-emergent used.

You don’t want to spend hours pulling weeds, and we don’t want you to (sounds like unnecessary back pain). Though it might sound like it now, it isn’t impossible to have a gorgeous lawn that isn’t infested. Property of your dreams, sprouting right up.

How Does Pre-Emergent Work?

To understand what affects pre-emergent, let’s take a look at how it works first. Pre-emergent is designed to stop the weed before it breaks through (hence the name). The herbicide creates a chemical barrier that coats the weed seeds, preventing them from germinating and growing deep-set roots. 

That being said, there are a couple of things to keep in mind. If you just planted grass, do not apply pre-emergent as well. The herbicide works by preventing seeds from growing roots. In short, you will kill everything, grass included. A general rule to remember is to wait at least four months after planting grass to apply pre-emergent. 


Notice the “pre” in pre-emergent. It is designed to stop the germination of seeds, but it is ineffective when they have already taken root. In other words, the weeds you see on your lawn are blissfully unaware of any pre-emergent applied after germination. 

Does Rain Affect Pre-Emergent?

Rain does affect pre-emergent, though not necessarily negatively. In the right conditions, rain aids the absorption of pre-emergent into the soil. Let’s break it down.


Timing is everything. If applied too late, pre-emergent is ineffective. The best time to apply pre-emergent is when the soil temperatures hit 50 to 55 degrees in the first few days. This is when the seeds begin to germinate, and pre-emergent will be the most effective. 

We know what you’re thinking; how do you measure soil temperatures? Though you may look a bit crazy (we promise we’re not laughing), any household thermometer will do. Insert the thermometer 4 inches into the ground and wait 5 minutes before reading. If this doesn’t sound particularly joyful, that’s okay. When air temperatures are around 60 degrees, the ground is 50 to 55. 


Prickly Weed

For the pre-emergent to sink into the ground, about 0.5 to 1 inch of moisture is required. So, some rainfall is beneficial, as it increases the effectiveness of the herbicide. Too much rain, however, has the opposite effect. This causes the herbicide to become excessively diluted and, in some cases, wash away completely. 

On the topic of water, pre-emergent should only be applied when the ground is damp or dry. If the ground is wet when pre-emergent is applied, it will not soak in, rendering it ineffective. How you water can make or break the effectiveness of the pre-emergent. Here are some tips to increase the herbicide (and your) success rate.

  • Begin with dry or slightly damp ground.
  • Give the pre-emergent 24-48 hours before watering.
  • Do not overwater; no more than 1 inch of moisture on the pre-emergent in the few days that follow. 

Can I Apply Tenacity Before It Rains?

Syngenta 46256 Tenacity 8oz Herbicide, Clear

Yes, Tenacity can be applied before it rains. Tenacity typically is rainfast within one hour after application. Most pre-emergents, such as Tenacity, can be watered in; however, as mentioned above, it is recommended that you wait 1-2 days after application. If rainfall occurs before or just after application, the pre-emergent may run off the grass, not sinking into the ground.

What type of Pre-Emergent Should I Use?

Now that you’re well educated on pre-emergent and when to apply it, what type should you use? This depends on the weed you are looking to get rid of. Different ingredients target certain weeds. Not sure what pre-emergent to use? Find the weed you’re looking to remove below;

  • CrabgrassCrabgrass sprouts up in many patches and can be very difficult to get rid of. It’s making you crabby, we know. There are several different types of pre-emergent that work wonders when it comes to getting rid of it.

The chemical Prodiamine prevents cell division, not allowing the seeds to sprout up. In other words, this chemical works to prevent crabgrass right from the beginning.

  • Thistle- Thistle is a pain, quite literally. They have an incredibly intricate root system, which is why they tend to be so invasive. Oxadiazon inhibits the developmental function of thistle, hindering the ability to grow such complex root systems.
  • Chickweed & Bluegrass- For chickweed, bluegrass, and most other unwanted plants, Dithiopyr & Trifluralin will do the trick. These particular chemicals also prevent cell division, inhibiting any form of mitosis.
Scotts WeedEx Prevent with Halts, Crabgrass Preventer, Pre-Emergent Grassy Weed Killer, 5,000 sq. ft., 10.06 lbs.
  • Scotts WeedEx Prevent with Halts prevents crabgrass all season long
  • This pre-emergent grassy weed killer stops crabgrass before it starts
  • Apply in early spring for season-long prevention of crabgrass, or apply in fall to prevent winter...
  • Do not apply this lawn care product to Dichondra and Bentgrass

The chemicals listed above can be found in the ingredients of certain pre-emergent herbicides and are not the name of the herbicide itself. So, if you’re not sure what kind of herbicide to get for your lawn, check its contents. Look for the ingredient lists that include the specific kind of chemical necessary to target the weed plaguing your lawn. 

Last update on 2024-06-16 / Affiliate links / Somes Images and Data from Amazon Product Advertising API