Get 'em before they sprout!
The best weapon you have against this annual weed is crabgrass preemergence herbicide (also called crabgrass preventer). You apply this product in the spring before the crabgrass seed sprouts. The most cost-effective way to apply a preemergent herbicide is to use a fertilizer with crabgrass preventer added to it. These combination products are readily available in the spring and cost about $20 for a 5,000-sq.-ft. bag at garden centers. Apply it when you would normally apply your first application of fertilizer, and do it just before it rains to work both the fertilizer and the herbicide into the soil. The fertilizer will help thicken the turf. Thicker turf helps to squeeze out crabgrass plants missed by the herbicide. Common brands include Ferti- Lome's Weed-Out, Sta-Green's Crab-Ex Plus and Scotts Turf Builder.
So how do you select the exact instant for application? Pay attention to your grass. Fill up the spreader with preemergence granules between the second and third mowings of the year. The window is short, only about a week and a half, when the soil hits the ideal temperature, about 52 degrees F. You can also buy an inexpensive soil thermometer (sold at garden centers) to monitor soil temperature. If you're going to err on timing your crabgrass preemergence application, err on the early side. While you'll lose some effectiveness, you'll still probably kill lots of crabgrass. If you go late, you're likely to miss early sprouts.