Crabgrass is classified as a dicot species, making the name "crabgrass" a misnomer as it is not a true grass. All grasses such as are used on lawns are monocots; that is, they have a single embryonic leaf when first sprouting from seed. This means that crabgrass has more in common with shrubs and flowering bushes than actual grass. This distinction is important in that it makes crabgrass difficult to discourage from growing while presenting herbicidal possibilities that do not harm true lawn grasses. However, to be truly rid of crabgrass is a yearlong process.
Spray the affected areas of your lawn with a post-emergent dicot herbicide such as Acclaim Extra. This herbicide works by attacking the roots of crabgrass and so must be allowed to soak directly into the soil around the weed. This herbicide is not 100 percent effective against well-established crabgrass, but it will help inhibit the growth cycle and kill off young plants. Uproot particularly stubborn weeds by hand with a set of work gloves to remove the stragglers.
Apply fertilizer, either by hand or with a fertilizer spreader, in late fall, when the first frost has begun. The density or amount of fertilizer to be spread is dependent on the brand and clearly marked on the packaging. Crabgrass does not sprout until the ground temperature has stabilized near 55 degrees F, almost 10 degrees warmer than most cold-weather grasses. For this reason, fertilizing the lawn in winter rather than spring will give the desirable grass a head start for the following season.
Use the fertilizer spreader to disseminate corn gluten across the lawn in early spring, before the first blooms have come up. Roughly 1 lb. should be used over every 20 square feet. Wet it thoroughly afterward and reapply weekly until the lawn is ready for its first mowing of the new year. Corn gluten contains an enzyme that, once introduced to the soil, acts to inhibit the germination cycle of crabgrass and other dicots. This will kill the majority of crabgrass seeds before they can sprout. Chemical pre-emergent herbicides will work just as well but tend to cost more.
Dig up parts of the lawn in which grass is thin to a depth of 4 inches. Lay down a thick layer of grass seeds so thick the soil cannot be seen beneath, recover them, lightly tap down the soil and wet it daily for a week. Crabgrass is an opportunistic weed. With no bare areas to take advantage of, flare-ups are curtailed.
Water the grass only when the ground has become completely dry. Crabgrass requires more water to thrive than do most lawn grasses. Cutting off its water supply will keep it from germinating and growing throughout the year.
Raise the blades of your lawnmower to ensure the lawn is never cut to a height of less than 3 inches. Cutting the lawn any shorter than this can weaken lawn grasses and provides crabgrass with the opportunity to grow up in the lawn's place.