Stopping weeds before they gain a “roothold” in your lawn is one of the smartest cultural practices you can use. Applying a preemergence herbicide does just that — it doesn’t stop the weed seeds from actually germinating, but it kills the new seedlings before they “emerge” by providing a chemical barrier at the soil surface. Preemergence products are often sold in spring as crabgrass preventers, since they control summer annual grassy weeds such as crabgrass and goosegrass.
The time to apply spring preemergence herbicides is about when dogwoods begin to bloom (early March in southeastern North Carolina). Unless you have a fescue or bluegrass lawn, avoid “weed and feed” products, since applying fertilizer too early to a warm-season lawn can increase the risk of cold damage from late freezes.
Before you apply a preemergence herbicide, though, do assess your lawn for winter damage. If you suspect substantial damage and plan to reseed your cool-season lawn with fescue or bluegrass, do not apply a preemergence herbicide (which could prevent the new grass seed from emerging). If you decide to re-sod those areas (particularly with a warm-season lawn), you might as well wait until you’re preparing the soil surface later this spring and then spray the weeds with a postemergence herbicide to remove plant tissue and improve the sod-to-soil interface.
Another factor to consider is the amount of rainfall your lawn receives this spring. If your locale experiences a lot of rain soon after you’ve applied your preemergent, you may experience a herbicide “break” (or failure). If you spot weeds sprouting, consider applying a second preemergence herbicide (to stop additional germination) as well as spot-spraying a postemergence herbicide to the young weeds.