Crabgrass requires resignation on the part of the homeowner to the fact that winds will blow and birds will fly, making weed control an ongoing process. Most lawns, no matter how well kept, eventually face the threat of invasion by this sprawling annual weed grass. Eliminating crabgrass requires management strategies, not only for effectively killing a grass living among other grasses, but also for making the environment inhospitable for future outbreaks. The best defense is a good offense--a thick, healthy lawn.
Plant a lawn grass that is adapted to your area and fertilize during its period of most active growth. Cool season grasses need nitrogen during early spring and late fall; fertilize warm season grasses from late spring through early fall. Fertilize only at the rate dictated by package directions when it will do the most good for lawn grass.
Apply pre-emergent herbicides containing pendimethalin, bensulide, benefin or trifluralin, which are formulated for use on crabgrass, according to package directions. Pre-emergents keep seeds from germinating, so they'll keep other grass seed from starting, too. Apply pre-emergent herbicides in very early spring when crabgrass germinates and before lawn grasses begin their growth.
Dig the first crabgrass you spot with a long tool called a dandelion digger. Crabgrass roots are shallow, with thicker roots called rhizomes that spread from the crowns of parent plants to sprout offspring. Eliminate all the offspring, too.
Mow the grass frequently to keep crabgrass from setting seed on five-inch flower stalks. Keep the grass at the optimum height for the variety: 1.5 inches for St. Augustine grass to 2.5 inches for Kentucky bluegrass, two common turf grasses. Avoid cutting grass too short, which will weaken its crown.
Water lawns infrequently but deeply. Daily "sprinkling" encourages shallow roots, which favors the spread of crabgrass's shallow rhizomes. Give turf grasses an inch of water a week in a few drenches to help develop deep roots. A strong root system encourages strong crowns that can choke out crabgrass and other weeds.