When to Apply Pre-Emergent Weed Killer?

By Home & Garden Last updated Wednesday, June/12/2024

Sand Burrs

If you've ever tried to pick burrs off your socks or out of your skin, you know these weeds are persistent and stubborn. Southern sandspur or grassbur weeds germinate at about the same time as crabgrass and can be treated with pre-emergent herbicides at the same time, too. Wait until the soil temperature is at least 52 degrees Fahrenheit for at least five days in a row before applying pre-emergent herbicides. In the southernmost areas of the United States, this will be around the first of March. If you live farther north, the soil temperature may not reach 52 degrees until April or, in the northernmost states, May.

Crabgrass seed usually germinates when the soil temperature reaches about 55 degrees Fahrenheit. Crabgrass normally germinates at or close to the time that sandspurs germinates. You don't have to take the soil's temperature to know if crabgrass seeds are ready to sprout, however. If you see forsythia getting ready to bloom in your area, it is time to declare war on crabgrass. Apply herbicide just prior to or during the bloom time of forsythia. The specific date varies not only from region to region but also within a region due to the vagaries of weather.

In addition to crabgrass, grassy weeds like foxtail, fall pancium, barnyardgrass and goosegrass fall prey to pre-emergent herbicides in early spring. Apply the herbicide at least one week before the weeds' anticipated germination date. To prevent annual bluegrass, apply the pre-emergent herbicide in late summer or early fall. To control annual bluegrass, apply pre-emergent in late summer or early fall.

Height matters. Mow your grass at the maximum recommended height. Mow hybrid Bermuda grass and fine-bladed zoysia to 1 inch tall. Keep buffalo grass, common Bermuda grass and coarse bladed zoysia at 1.5 inches tall. Centipede, Kentucky bluegrass and Texas bluegrass can be mowed to a height of 2 inches. Keep St. Augustine, ryegrass and tall fescue mowed to 2 1/2 inches. If you do not know what kinds of weeds are lurking in your lawn, ask you local cooperative extension service for help. Check herbicide labels to be sure the product will work on the species of weeds in your lawn, and follow label directions. Do not apply pre-emergent herbicides to newly sown or sodded grass until after the third mowing. If you use granular herbicide, water it into the soil.