When to apply pre-emergent

Spring applications target summer annual weeds like crabgrass and goosegrass. Fall applications target winter annual weeds like annual bluegrass, chickweed, henbit, and other cool-season germinators. A single spring application will not prevent every weed all year.

Purdue Extension describes crabgrass as a summer annual that germinates around 60 degrees Fahrenheit. Apply spring pre-emergent before the germination window, with enough time for rainfall or irrigation to move the product into the upper soil layer.

The fall application runs on the same idea in reverse. Where spring waits for soil to warm up to the crabgrass threshold, fall works against soil cooling down to the annual bluegrass one. University of Georgia Extension puts it plainly: annual bluegrass seed germinates in late summer or early fall once soil temperatures fall below 70 degrees Fahrenheit, and the herbicide has to be down before that happens. University of Tennessee Extension adds that once soil passes below 70 degrees, germination continues for the next two to three months, so a late application still blocks the seed that has not sprouted yet. Texas A&M AgriLife Extension recommends beginning the fall regimen when soil temperatures reach roughly 70 degrees, which generally lands in September or October in Texas.

Because that window is the one most people miss, buy the product while soil is still in the 70s rather than waiting for the number to cross. Clemson notes that most labels call for a second application 8 to 10 weeks after the first for continued control through the winter.

Check your current soil temperature before applying.

Most pre-emergent herbicides can also prevent desirable grass seed from establishing. If you recently seeded or plan to seed, read the label carefully and choose a product that fits that situation. Purdue notes siduron is an exception for some cool-season seedings, but it is not a season-long solution.

Common active ingredients include prodiamine, dithiopyr, pendimethalin, and dimension-style products. Use the cheapest pre-emergent ranking to compare options by cost per 1,000 sq ft after you confirm grass safety, target weeds, reseeding interval, and label rate.