Crabgrass at a distance might look like grass, but when you get up close, you will see that it grows in groupings and fans out, causing your yard to look messy and unhealthy. If you want to improve your lawn by overseeding, you will first want to remove the infestation of crabgrass. Once the crabgrass is gone, you can concentrate on creating a thick, lush yard by growing new seeds.
Cut roots of crabgrass clumps with a knife and pull them out. Do this if you don't have many patches to deal with or have the time to spend on this method.
Spray a selective herbicide that states that it is lethal to crabgrass. Selective herbicides are created to kill weeds and not grass, so it is okay if it makes contact with the lawn. This is best if your yard is overrun with the weed.
Wait a few days to insure that all crabgrass was killed. If not, spray again and wait another few days.
Place black plastic over large areas of crabgrass where there is no grass growing. Put heavy objects on the corners to secure it. The plastic will keep light off of the weeds. Without light, the weeds will die. This method is effective, but can take a week or more with stubborn plants.