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When to Apply Pre-emergent


Timing of this lawn treatment is essential to keep weeds under control all year long.

May 13, 2024

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Timing is everything in so many instances, including your pre-emergent application. Getting to weeds before they have time to break through the soil, so you don’t have to fight with them once they’ve rooted, is a huge time saver. But, if you put down pre-emergent too early, or too late, it can’t do its job. Proper timing maximizes its effects.

Although you can’t guarantee a lawn will be totally weed-free for anyone, laying down pre-emergent at the right time makes it possible to reduce the potential of invasive weeds taking root.


What is pre-emergent herbicide?

Available in both liquid and granular forms, pre-emergent herbicide targets weeds that aren’t visible above ground yet, but have begun to sprout. While pre-emergent won’t damage seeds, it prevents germinated weed seedlings from establishing themselves and growing.

Pre-emergent weed killer targets a variety of weeds, especially ones that tend to dominate such as crabgrass in the spring and annual bluegrass in the fall.

Because pre-emergent herbicides leave weed seeds alone, and those seeds have the potential to lay dormant, you need to apply pre-emergent every year.


When to apply a pre-emergent in spring

No matter what season you’re applying pre-emergent herbicide in, soil temperature is your best indicator of when it’s time to schedule an application. In the spring, it’s best to apply pre-emergent before the soil before hits 55 degrees or higher for five days in a row.

It’s at this temperature that crabgrass begins to germinate.

A good rule of thumb is to apply pre-emergent when soil temperature begin to consistently reach 50 degrees in your area. Consider doing a second application 4-6 weeks later as well to catch any late-germinating crabgrass and other weeds.


When to apply a pre-emergent in fall

Soil temperature also plays a role in establishing when to apply pre-emergent weed killer in the fall. For this season, you’re looking at temperatures as they drop rather than as they rise. 

To target weeds like annual bluegrass and chickweed, the best application window is when soil temperatures consistently begin to drop below 70 degrees. This usually occurs in late summer or early fall. You should typically be done treating lawns in the fall with pre-emergent by around the middle of September.


Types of pre-emergent herbicide 

Whether you select pre-emergent in liquid or granular form, make sure you follow manufacturer directions to prepare the treatment properly. You’ll also need an applicator and can work with either a broadcast spreader or drop spreader.

If you’re planning to use a granular pre-emergent, consider trying:

Select the formula based on what level of active ingredient your job requires as well as the ideal ratio of nitrogen, phosphorus and potassium necessary to maintain the health of the plants and grass you’re fertilizing.

For sprayable pre-emergent fertilizers, you also have a variety of options, including:


How to apply pre-emergent for lawns

Getting started applying pre-emergent herbicide is simple. Mix the product correctly, read through all directions carefully and calibrate your sprayer or spreader. Especially take note of how much pre-emergent to use per gallon of water to get the mixture right. Not all application rates are the same, which is why referring to the product’s label is so important.

Thorough coverage is the key to success when applying pre-emergent, so avoid spraying it only in distinct spots. You want a full layer of pre-emergent to stretch across the entire lawn.

Once applied, you’ll need to water the pre-emergent in to activate it. Either time the application to right before rain is expected or apply about half an inch of water within 7 days of applying. 


Finding the right resources for lawn care

Applying pre-emergent is just one step to maintaining a healthy lawn, no matter the size, shape or type of grass. To get all the tools and support you need, local SiteOne branch associates are available to help answer questions. You can also access a wealth of resources related to agronomics online. When it’s time to buy pre-emergent, shop SiteOne for what you need 24/7 at SiteOne.com or through our SiteOne mobile app. 
 

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herbicide drop spreader liquid pre-emergent broadcast spreader granular pre-emergent