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When To Apply Pre-Emergent Herbicide

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Pre-emergents stop weeds from growing before they’re even spotted in your lawn. Keep reading to learn more about the preventative measure, the best time to apply it, and how it can protect your lawn.

Author Image Written by Brenda Woods Updated 04/03/2024

To protect your yard from damaging weeds, you may want to incorporate an effective pre-emergent into your lawn care routine. These herbicides kill weeds before they have a chance to take root.

However, for pre-emergent to be effective, it needs to be applied at the correct time of year. Otherwise, it could inhibit grass growth while being ineffective against nuisance plants. In this article, we’ll detail when to apply pre-emergent herbicide, discuss application techniques, and walk you through some helpful tips for working with these products.

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What Is Pre-Emergent Herbicide?

A pre-emergent herbicide is a synthetic chemical application that prevents weeds from sprouting in your healthy lawn or flower bed. While this treatment doesn’t interfere with the weed’s seed germination, it does stop growth by stunting the formation of new roots.

Types of Pre-Emergent Herbicides

Pre-emergent herbicide typically comes in two different forms: liquid pre-emergent and granule pre-emergent. While each type kills sprouting weeds, they each have unique characteristics and application methods.

Liquid pre-emergents are usually sold in pump-type sprayers or hose connections. One benefit of this typet is its ease of application, as its fast flow rate allows it to evenly coat your lawn. While many lawn care experts prefer liquid pre-emergents, there are a few benefits to a granular treatment.

Granular pre-emergents require little to no mixing compared to their liquid counterpart, which makes preparation a little easier. Additionally, if you already own a lawn spreader for fertilizing, you can use it to apply granular pre-emergent.

How Does Pre-Emergent Herbicide Work?

Pre-emergent herbicides work by creating a makeshift barrier between germinated weeds and your lawn. This protective layer prevents weeds from breaching the topsoil, eliminating them before they can sprout. In addition to preventing existing weeds from growing, pre-emergent also reduces the chances of future problems by reducing the weed’s seed production.

When Should You Apply Pre-Emergent Herbicide?

The right time to apply your pre-emergent herbicide varies depending on the type of weeds in your lawn. For example, some weeds emerge in the fall while others, such as dandelions, grow in the early spring. You should also time pre-emergent application based on the climate where you live.

Spring Application

Spring applications help prevent the growth of spring and summer weeds, such as crabgrass and foxtail. For early spring weeds, wait until soil temperatures have been at least 55°F for at least 48 hours. Usually, this means an air temperature of 65–70°F. Often, this means homeowners in the southern U.S. can apply these herbicides much earlier in the year than northern homeowners. Those in particularly hot climates may need to start applying as early as mid-February.

However, it’s important to hold off on applying until your summer fruits and vegetables have sprouted, as pre-emergent can harm ungerminated plants. Additionally, some types of weeds sprout in the summer, so you may need to apply a different pre-emergent right before the heat sets in.


Fall Application

By the time cooler weather comes around, the protection from your spring pre-emergent has diminished. Fall applications keep new weeds from growing or surviving the winter, which is more important in southern climates where winters are milder. Apply this pre-emergent before soil temperatures reach 55 degrees for five or more days in a row. As with spring, plan around any other treatments you apply so they don’t work at cross-purposes.

What Is the Best Pre-Emergent Herbicide?

The right pre-emergent will depend on your yard and the type of weeds that grow. Here are some common active herbicide ingredients and the weeds that they target:

This chemical is typically used on crabgrass and dandelions. Herbicides that contain prodiamine can also be used to eliminate various broadleaf weeds and invasive grasses in your lawn.
Although this herbicide is usually administered in agricultural settings, some homeowners use it to eliminate yellow foxtail and intrusive vines, such as morning glory.
This is used to stunt the growth of vining plants, broadleaf weeds, and invasive grasses. It can also inhibit turfgrass growth, so only use it on well-established, healthy lawns.

In addition to the type of weeds, it’s also important to consider the size of your lawn and the amount of effort you’re willing to put into treatment. For example, if you have a small area that you need to treat quickly, a granular pre-emergent may be a good option. However, if you’re looking for an even application across a large area and don’t mind a little pre-application mixing, a liquid pre-emergent could be the better choice.

Important Tips for Using Pre-Emergent Herbicide

Before applying pre-emergent weed control, consider the following application tips:
Apply pre-emergent evenly—To completely protect your lawn from weeds, you’ll need to ensure no corner of your lawn is untouched.
Don’t forget water—Herbicide applications are activated by water, which distribute chemicals throughout your lawn’s soil. To save water, you can apply pre-emergent before an expected rainfall. However, if rain isn’t in the forecast, be sure to water your lawn after application.
Limit lawn activity—It’s important to ensure that the soil holding the pre-emergent is undisturbed after application. As such, you shouldn’t aerate after application and you should hold off on mowing until after the pre-emergent has been watered.
Maintain your lawn—Maintaining a healthy lawn can help the turfgrass naturally compete with common weeds. Routine maintenance should include mowing, proper irrigation, and pest control. If you don’t have time to fertilize or apply herbicide, consider hiring a lawn care service, such as TruGreen or Lawnbright.

FAQ About Applying Pre-Emergent Herbicides

Is a pre-emergent an herbicide?

Yes. Herbicides are chemicals that kill weeds and other unwanted plants to protect surrounding vegetation. Unlike post-emergent herbicides, pre-emergents stunt the growth of weeds before they sprout.

What happens if I apply a pre-emergent too late?

If you apply a pre-emergent after the weeds have already germinated, it will be ineffective in preventing weed growth during the spring or summer. Pre-emergent will only work when applied before weeds take root.

Can I apply a pre-emergent in my flower bed?

Yes. Pre-emergents can be applied on lawns and flower beds to prevent weeds from growing and damaging healthy grasses, flowers, and vegetable gardens. Only apply pre-emergents after any flowers or plants have started to grow to prevent the chemicals from killing ungerminated seeds.

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