Use Spectracide Weed Stop For Lawns Plus Crabgrass Killer3, ready-to-use spray, 1 Gallon to kill over 470 types of weeds as listed, including crabgrass. The product kills by contact, producing visible results in 5 hours. This product kills down to the root and, when used as directed, kills listed weeds including crabgrass — not the lawn. This product kills broadleaf and grassy weeds, including dandelion, chickweed and clover as well as crabgrass and yellow nutsedge — see product label for complete list of weed types. Apply when daytime temperatures are between 45°F and 90°F. Do not apply to Bermudagrass when daytime temperatures exceed 90°F. For best results, mow lawn two to three days before treating.