Can You Seed After You Put Down Crabgrass Killer?

Preemergent Herbicides

• The Colorado State University Extension recommends waiting two to four months after applying most preemergent herbicides before seeding new turf. If you use a crabgrass preventer in spring, wait until cooler fall temperatures before planting seed. An exception is the preemergent herbicide siduron, which controls weed seeds but leaves rye, fescue and bluegrass seed unharmed.

Postemergent

• Newly planted lawns are sensitive to herbicides, and treating crabgrass with a widespread postemergent herbicide can kill your new shoots. Wait until the new lawn has grown enough to warrant three mowings before applying herbicides. If possible, spot-treat crabgrass with a wide-spectrum herbicide like Roundup instead of treating the entire lawn.

Control

• One crabgrass plant, left to grow throughout the summer, may produce roughly 150,000 seeds. Not all of these seeds will sprout at the same time, and many will germinate in following seasons. A healthy, thick lawn chokes out this warm-season annual, despite remaining seeds. Mow high, avoid fertilizing in summer when crabgrass is most aggressive, and water deeply to encourage a healthy root system.