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What Kills Crabgrass and Not Grass?

By Kimberly Richardson
A patch of crabgrass produces 150,000 seeds, all waiting for spring weather.
Ryan McVay/Photodisc/Getty Images

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.

New Lawns

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.


About the Author


Kimberly Richardson has been writing since 1995. She has written successful grants for local schools as well as articles for various websites, specializing in garden-related topics. Richardson holds a Bachelor of Arts in English and is enrolled in her local Master Gardener program.