One reviewer noted that Preen isn't a crabgrass killer but a preventer ... two early-spring applications when Preen should be most effective. Scotts Turf Builder Halts crabgrass preventer has ...

This herbicide is safe for cool-season turf grasses that have just been seeded and is most effective ... herbicide to treat crabgrass compared to broadleaf weeds, and total control may require ...

That timing is between late March and mid-April. There are several products available on the market for crabgrass control. The most widely used products are Barricade, prodiamine, and Dimension ...