When to Apply Pre-emergent Crabgrass Killer

By Mandi Rogier

Crabgrass is a common problem in lawns, as this common grass weed can pop up stubbornly year after year. To prevent crabgrass from growing in your lawn, you can head off its growth cycle before it germinates and attack it with a pre-emergent crabgrass killer. Timing is essential with this type of weed control, as a pre-emergent herbicide that is applied too late in the season will be useless against established weeds.

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Germination Time

Pre-emergent crabgrass killer must be applied before the crabgrass seeds are allowed to germinate. Germination time will vary from one region to another. Crabgrass germinates in the spring. When trees and shrubs in your area begin to bloom, this is a sign that crabgrass will soon be germinating. The exact time to apply pre-emergent weed killers will vary each year. An unseasonably warm winter is a call to apply your pre-emergent killers earlier than usual. Some pre-emergent crabgrass killers can be applied the previous winter, to take the guesswork out of application time.

If you are working with a newly seeded lawn, you must wait until the lawn has been mowed at least three times before applying a pre-emergent crabgrass killer. If this stipulation pushes you into late spring, you may need to consider a post-emergent killer instead. Applying weed killer to a newly seeded lawn too soon can damage the delicate new grass.


Pre-emergent crabgrass killer should be applied evenly over the entire lawn. Even if you have only noticed crabgrass in certain areas of the yard in previous years, take the time to cover the entire lawn. Spots that are skipped can allow stray crabgrass seeds to germinate, grow and spread. Do not aerate your lawn after application and wait three months before reseeding any part of the lawn.


You may be able to prevent the growth of crabgrass in your yard with more natural methods, rather than using a pre-emergent weed killer every year. A dense lawn will naturally prevent the growth of crabgrass. This grass will limit the light and water that is available to low growing weeds. You can also help to prevent weed growth by keeping you lawn at a height of two to three inches. Longer grass provides more shade to the surface of the lawn while crabgrass seeds need light to germinate.