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What Kills Crabgrass and Not Grass?

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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.

Plant Grass Seed After Crabgrass Killer?

Post-emergent herbicides containing glyphosate are commonly used to kill crabgrass that is already growing. New grass seed can be planted one week after killing the crabgrass with glyphosate. Always wear protective clothing when using herbicides, and avoid the treated area for the length of time recommended by the manufacturer. It is applied at a rate of 7.5 ounces to 3.3 pounds per 1,000 square feet, depending on the product formulation. Cool-season grass species should be seeded in early fall, although they can also be seeded in early spring. Remove any sticks, rocks or other debris. Water the soil until it is wet and leave it for one week to settle prior planting the new grass.

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