Crabgrass can slowly take over a lawn and cause the grass to die out from lack of nutrients. This weed is green, but it grows in clumps and is unsightly on a lawn. To have a healthy yard, you must first remove the weeds and then reseed to grow back what has died out.
Spray the lawn with a herbicide specifically designed to kill crabgrass. Use a selective herbicide that will not kill your grass. If you have just a few patches of crabgrass, cut the weeds off at the roots with a knife and pull them out. If you sprayed, wait a few days to see if it all has died. If not, respray.
Mow the lawn so that seed can get down to the dirt. Tall grass will trap seeds and keep them from reaching the growing area and cast shadows that blocks the sunlight.
Add a starter fertilizer that will stimulate seed growth.
Aerate the yard to loosen the soil and provide places for the seed.
Spread seed of the same kind already growing in your yard. The package will tell you how much to use for overseeding a lawn that already exists. There is no need to waste the product by spreading too much. Spread in one direction on the lawn and then at a 90-degree angle to the first direction.
Cover with a thin layer of straw to keep away birds.
Water the lawn daily for the first month.