Everything You Need To Know About
Pre-Emergent Herbicide

By Beth Krietsch

If you’re looking to upgrade your spring gardening toolkit and haven’t yet tried pre-emergent herbicide, this may be a good place to start. Pre-emergent herbicide is a type of weed killer that works by preventing weed seeds from germinating and is only effective when applied before weeds appear.

How does pre-emergent herbicide work?

Pre-emergent herbicides work by creating a chemical barrier in the top layer of soil that coats seeds and prevents them from growing roots and shoots. Though pre-emergent herbicides work on many types of lawn weeds, they’re most commonly used to reduce the presence of crabgrass.

What to buy

To figure out which type of herbicide to buy, you’ll want to take note of what type of weeds tend to grow in your lawn. The same variety of weeds typically pop up year after year, so taking note of the types of weeds that grow on your lawn can help you figure out which type of pre-emergent herbicide to buy the following season. Your local garden store can help you determine which herbicide will target your weeds.

Keep in mind that no pre-emergent herbicides are able to target all varieties of weeds. Some pre-emergent herbicides kill a broad array of common lawn weeds while others target specific weeds like crabgrass or chickweed.

To more accurately target the spectrum of weeds present on your lawn, Oregon State University recommends using two types of herbicide, such as one that controls grass weeds and another that controls broadleaf weeds.

You’ll also need to decide whether you want to use a liquid or granular herbicide. Liquid herbicides should be mixed with water (if sold in a concentrate form) and applied with a sprayer, which you can either buy or rent. With granular varieties, you’ll apply by hand or with a spreader or shaker bag.

Water your lawn within a few days of application for the herbicide to be effective, as the water allows the herbicide to spread deeper into the soil. Most pre-emergent herbicides are chemical based, but a few natural and organic varieties do exist. Before applying, know that diluting the herbicide more than the recommended amount can greatly reduce its efficacy. For this reason, it’s important to apply the herbicide according to its directions.

When to apply pre-emergent herbicide

Apply pre-emergent herbicide in the spring, a few weeks before weeds typically appear.

If you’re new to the region or simply unsure when lawn weeds first pop up, a good rule of thumb is to apply your herbicide after the weather falls between 65 and 70ºF for at least four days in a row. You can also seek advice from a local gardening center or university cooperative extension. Many people apply pre-emergent herbicide for a second time about six to eight weeks after the first application. Read the label to find out more about when to best apply.

Things to keep in mind

  • Don’t wait too long—This variety of herbicide will not be effective if weeds emerge before application. It also won’t be effective if applied too early, as rain can cause the herbicide to be washed away or travel too deep into the soil. But if you’re really unsure of when to apply your herbicide, it’s better to apply too early than too late.
  • Maintain undisturbed soil—Aerating your lawn or cultivating it in any way can disrupt the top layer of soil where the herbicide has created a barrier, in the process disrupting the barrier. After applying pre-emergent herbicide, avoid digging or doing anything else to the top layer of soil.

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