When to Apply Crabgrass Preventer: A Lawn Care Guide

By: Ken Wilson

An excellent pre-emergent herbicide can effectively manage crabgrass– a tough grassy weed. Most annual weeds, like crabgrass, begin to sprout in the spring. The soil temperature in your lawn starts to warm up around this time, creating an excellent germination condition.

If you're one of the many homeowners wondering when is the best time to apply crabgrass preventer? Ideally, this must be done in the earliest part of spring, when the soil temperature reaches about 55 °F. While not on damp grass, apply the herbicide immediately before it is expected to rain. The pre-emergent should be used before it starts to rain so that the herbicide is washed into the ground.

When to Apply Crabgrass Preventer

As previously mentioned, the best time to apply a crabgrass preventer is when the sun first emerges in the early spring. Crabgrass seeds get activated when the soil temperature is between 55 and 70 degrees Fahrenheit and begin to germinate. (Related: 10 of the Best Crabgrass Killers That Won’t Kill Your Healthy Grass)

To ensure that the herbicide is washed down into the soil to prevent weed germination, use the pre-emergent when it is expected to rain in a few hours. Refer to the list below for a quick guide on the ideal crabgrass preventer application timing:

  • Soil temperature: Apply when the soil is between 55 and 70 °F
  • Before or after rain: Ideally, apply it a few hours before the rain starts
  • Mowing: Apply it after the first two mowings of the season
  • Environmental sign: The best time to apply crabgrass preventer is when forsythia and dogwood begin to bloom

3 Signs the Timing is Right to Put Down Crabgrass Preventer

We talk to homeowners occasionally, and one of the most common questions we get from them is whether the timing of their crabgrass application is appropriate. This is understandable.

Predicting when spring will arrive can be challenging, leaving most people perplexed and unsure whether to lay down their pre-emergent. That said, here are some indicators to determine when to use a crabgrass preventer:

1. The Temperature of the Soil gets above 55 °F

Temperatures begin to climb as soon as winter ends and spring arrives. Crabgrass and other weeds become active and begin to germinate at this time. As such, you might want to watch for 55 °F if you start taking soil temps. When the soil is at least 55°F or above for three straight days, applying a crabgrass preventer at a depth of 1 to 2 inches is advised. Crabgrass seeds begin to sprout when the soil is between 60°F and 70°F.

2. Dogwoods and Forsythia Blooms

Around mid-March until mid-April is the blooming period for forsythia shrubs. At this point, spring officially arrives after a long, chilly winter. If you notice a cascade of forsythia blossoms that are a bright yellow color, consider it the best time to apply a crabgrass preventer herbicide to inhibit the growth of weeds.

Keeping an eye out for when your flowering plants and shrubs bloom may be more straightforward than measuring soil temperatures. Often, flowering trees (like pears) are in full color when soil temperatures begin to rise. Forsythia shrubs are a strong indicator of when to apply crabgrass preventers. Crabgrass germination will occur AFTER the forsythia has finished blooming. Skilled turf managers often use this plant as a signal to start applying a pre-emergent crabgrass preventer.

3. Late February to Early April

The spring calendar is another excellent sign to watch when deciding when to plant crabgrass pre-emergent. Since spring doesn't officially begin on the same day every year, an ideal time to apply a crabgrass pre-emergent herbicide will be determined using a seasonal calendar.

For instance, if spring is predicted to start on March 20, it's a good idea to work on your lawn months prior and apply herbicides to prevent both broadleaf grassy lawn weeds from emerging early.

Types of Crabgrass Preventers

The seeds left over from the previous growing season are prevented from sprouting using pre-emergent herbicides, AKA crabgrass preventers. For your reference, here are the following categories of pre-emergent crabgrass preventers:

  • Synthetic Herbicide ─ Eliminates crabgrass using chemical pesticides.
  • Liquid ─ To effectively cover a large lawn, use a liquid preventer. It works well to eliminate seeds in the sidewalk, driveway, and patio tight cracks. A concentrated liquid solution can need mixing and a sprayer.
  • Granular ─ For tiny lawns, granular formulas perform well when spread out with a spreader and then watered in. Granular versions do, however, take longer to work than liquid ones.
  • Organic Herbicide ─ If you are concerned about using synthetic chemicals, you can eliminate crabgrass using organic means like corn gluten or vinegar. These must be used at precisely the proper time and temperature and operate much more slowly. The grass you're trying to grow may also die if you use vinegar to kill crabgrass.
  • Selective Herbicide ─ This substance usually contains chemical components that only eliminate the weeds you wish to eradicate while leaving other healthy plants and grasses alone. If you just want to kill crabgrass, use treatments that contain chemicals like dithiopyr, benefin, and oxadiazon.
  • Non-selective Herbicide ─ This product typically kills all types of weeds, but it can also affect grasses. Use this product if you want to kill more than crabgrass.

If you’re looking for the strongest weed killers to use, we’ve compiled a list for your convenience.

Applying Pre-emergent Before or After Rain

If you're wondering when to apply a crabgrass preventer (before or after rain), an ideal time is to use it just before it starts to rain. Apply crabgrass preventer to the lawn so the rainwater can wash the herbicide into the soil and come into touch with the weed seeds. Early in the spring, when rain is expected within a few hours, is the ideal time to begin the pre-emergent application.

If, for some reason, it does not rain, water the lawn right away or two to three days after applying the pre-emergent. This ensures that the product has been absorbed into the soil and has prevented the germination of crabgrass.

Important Note: To ensure adequate coverage and crabgrass prevention, you may need to repeat the applications one or two more times– if there is a lot of rain.

How to Apply Crabgrass Preventer

Always read the product's instructions for precise directions when applying crabgrass preventers. Here are some pointers to keep in mind:

  • Before a rainstorm is expected, use granular formulations so that the rain will help integrate the preventer into the soil.
  • Do not apply the crabgrass preventer if heavy rains are predicted. This could only dilute the formula.
  • Using a crabgrass preventer and seeding your lawn simultaneously is not recommended. The herbicide's efficiency may be decreased in doing so.
  • Do not apply the product to grass that's already damp.

Frequently Asked Questions (FAQs)

Can I use pre-emergent to kill existing weeds?

It is common knowledge that most pre-emergent herbicides do not work against crabgrass and existing weeds.

Can you apply pre-emergent on wet grass?

Crabgrass pre-emergent will usually stick on the leaves when used on wet grass. Some products, including Crabgrass Barrier, suggest applying it on damp grass. However, you might have to water your lawn after applying the herbicide. This is because you would want the herbicide deep in the soil to stop the crabgrass from sprouting. Be sure to water your lawn with a sprinkler to rinse the herbicide from the earth if it fails to rain.

Do you water after applying a crabgrass preventer?

Yes, it is an excellent practice to water down herbicide to ensure that it goes into the soil and gets into contact with crabgrass seeds. Whether it is on wet or dry grass blades, always make sure to water your lawn soon after applying solid pre-emergent herbicides.

How long do crabgrass pre-emergents last?

Crabgrass pre-emergents last for 2 to 6 months. They can stop the germination of weeds on your lawn for a long time. However, not all of them work the same, so be sure not to apply a pre-emergent that may hinder your grass seeds from growing.

How often can you apply a crabgrass pre-emergent?

It is highly recommended to apply it twice a year since most weed prevention herbicides can last up to 6 months or half a year. You must allow at least 2 to 6 months between applications, and remember that you should not use it on newly seeded lawns. Other pre-emergents can result in poor or slow germination for some grass seed species.

About the author 

Ken Wilson

Long time career in the home services industry from remodeling to patio construction. Currently residing to in SWFL and active contributor to multiple home & garden publications.