When to Put Crabgrass Preventer Down

Warm spring weather encourages brightly-colored tulips, brilliant forsythia and, unfortunately, crabgrass (Digitaria spp.). Once the broad, rough clumps of crabgrass appear, it's often too late for granular herbicides to make much impact against this weed and other grassy annual pests. Instead of fighting crabgrass throughout the summer, apply a crabgrass preventer at the right time and literally nip the problem in the bud.

Crabgrass germinates when soil temperatures -- not air temperatures -- rise to roughly 60 degrees Fahrenheit and remain at that level for 3 to 7 days. Apply crabgrass preventer one week or more before the ground reaches this critical temperature. If you fail to hit this window, the crabgrass sprouts and the preventer, or pre-emergent herbicide, has little to no effect on the rapidly growing weed. This window varies from region to region, but usually occurs between early March and mid-May.

If your lawn has essentially lost the war against crabgrass, consider applying crabgrass preventer a second time in late May or early June. Crabgrass flourishes in hot summer weather when your cool-season fescue or Kentucky bluegrass is wilting in the heat, and a second application prevents further weed seed germination when your lawn is vulnerable. The exact timing depends on the weather and the active ingredient in your crabgrass preventer -- read the label carefully for guidelines.

Concrete, stone and other solid landscape structures absorb heat from the late-winter sun and warm adjacent lawn areas early. Watch carefully for these warming areas, especially adjacent to sidewalks and driveways, and southern exposures, and apply crabgrass preventer to these areas one to two weeks before your general lawn application. Be aware, however, that some preventers may damage your lawn if you accidentally over-apply the herbicide. Apply according to the label, and avoid re-applying to previously treated areas.

Most granular crabgrass preventers require water to become active. Always water in crabgrass preventers, or pre-emergents, immediately after application. The water spreads the herbicide into a barrier within the soil's surface, and this barrier prevents root growth of the germinating seeds. Too much water, however, breaks down this barrier. Avoid applying during rainy weather. Also, aeration and dethatching break the herbicide barrier and allow seed germination, so schedule your lawn chores accordingly.