When to Put Down Pre Emergent [Learn Which Month Is Best]

when to put down pre emergent

Knowing when to put down pre emergent is half the battle when it comes to successfully treating your lawn against weeds and rogue grasses.

Generally speaking, early spring and autumn are the best times to put down pre emergent. That’s because they are both times of the year when the ground is over 50°F, dry, and rainy days occur. Certain pre emergents may be applied during any time of the year with success, targeting specific pest species.

Read on below to learn more about when to put down pre emergent, and why, as well as the answers to several closely-related questions!

How Often Should You Put Down Pre-emergent?

Pre emergents are best applied twice per year, once in the spring, and once in the fall. That said, you may apply some products more than twice (some should be applied up to four times per year).

However, most weeds start and stop growing around spring and fall, which is why these two times of the year are so crucial for putting down pre emergent.

Grow Your Yard Tip
We’ve reviewed the best pre emergent herbicides here, check it out!

Can You Put Down Too Much Pre-Emergent?

If you follow the instructions that come with your pre-emergent product, there is little to fear as far as putting down too much. But, it is possible to go overboard, in which case run-off may occur and the product is wasted.

It is important to note that using too much pre emergent may even be an environmental violation (depending on the product and the state or country you use it in).

Does the Rain Wash Away Pre-Emergent?

Rain doesn’t wash away pre emergent unless it was applied with water. Even in those cases, it must rain for hours before it begins to wash away the pre-emergent.

That is why it is suggested to apply the pre-emergent only when the lawn is completely dry.

We look for a short warm spell in the forecast followed by a day or two of light to moderate rain showers, starting at the end of February (right up until May if need be).

What if You Don’t Soak Pre-Emergent in with Water?

If you fail to water pre emergent well enough, it won’t soak into the soil of your lawn. That means weed and wild grass seeds may still germinate and infest the lawn.

Although it is best to apply pre emergent when your lawn is dry, it should be followed up with a thorough watering so the active ingredients penetrate the top two to three inches of the lawn’s soil (where most of the seeds of pest species lie in wait to germinate)

If you put down pre emergent, and don’t water it well enough for it to soak into the lawn, you’ll soon notice a poor-looking lawn rather than one with fewer weeds.

How Long Before it Rains Does Pre-Emergent Need To Be Put Down?

There is no particular amount of time before it rains that you should put down pre emergent. However, it is recommendable to put it down at least a couple of hours or more before the waterworks start up.

That way the entire lawn is dry during the application process and you are able to thoroughly spread the pre emergent across the lawn.

If the lawn is wet when you put down the pre emergent, the product may stick to the blades of grass and other plants (keeping it from penetrating the seed zone of the soil).

How Much Rainfall is Bad After Putting Down Pre-Emergent?

We like to keep an eye on the weather channel each spring, and again in the fall when we are planning on putting down pre emergent on the lawn.

When we see a day or two, that should accumulate half an inch to an inch of rain, we give operation “apply pre emergent to the lawn” a green light.

If more than an inch and a half of water falls within 24 to 48 hours of treating your lawn, it is too much. Pre emergent will either wash away or become too diluted, at any rate, it won’t be as effective (if at all).

Can You Put Down Pre-Emergent Twice?

Most pre emergent products are designed for at least two applications per year, though some call for a single annual application.

Some pre emergent products, however, are made for lawn applications as often as every 3 months and as little as once every 9 months.

Applying pre emergent products to your lawn more than the label recommends puts your lawn at risk of having its natural growth disturbed.


The best time to put down pre emergent products on your lawn is when the lawn’s soil is somewhere between 50°F and 60°F, and dry. It also helps if there is a bit of rain in the forecast, if not, you need to follow up with a garden hose or sprinkler system.

The bottom line is that the pre emergent must soak deep enough into the lawn to prevent weeds and rogue pest species from emerging and wrecking your lawn, which won’t happen if the ground is too cold, or is already wet when you put it down.

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