Crabgrass is a headache-inducing annual weed. By the time it becomes noticeable it has grown too large for most granular herbicides to handle. Use the right control at the right time to eliminate this pest.
Pre-emergent herbicides kill crabgrass before its roots dig into the soil. Pre-emergent herbicides interfere with the seed's germination, but are useless after crabgrass germinates and do not affect growing grass. Treat your lawn before soil temperatures reach 55 to 60 degrees Fahrenheit, and treat again six to eight weeks later for control through the summer.
Post-emergent herbicides act best against very young crabgrass sprouts. Once the sprouts have 2 to 5 tillers, the crabgrass is strong enough to shake off most herbicides. Use a product with methyl arsenate (MSMA) up to three times during the season, spacing each treatment 7 to 10 days apart. Large-scale treatments are hard on your lawn; however, spray hotspots with a broad-spectrum herbicide instead.
Young or sprouting lawns are susceptible to herbicide damage. If you choose to use a pre-emergent with bluegrass or fescue seed, use a product containing siduron. Siduron targets crabgrass without harming the germinating lawn seed. Wait until you have mowed your growing lawn twice before using post-emergent herbicides.