Crabgrass is a persistant weed that, if left alone, will quickly take over your entire lawn. It is difficult to kill Crabgrass, but it is possible to control it if you know how it grows and which herbicides to use.
Pre-Emergent Crabgrass Control
Crabgrass is an annual weed, meaning it grows from seed each spring and dies once cold weather comes. The best way to control crabgrass is to prevent it from growing in the first place. Crabgrass produces seeds during the summer and fall. These seeds stay dormant until the ground warms up in the spring and they sprout. You need to apply the pre-emergent herbicide in early spring to prevent these seeds from sprouting. Once the crabgrass seeds start to grow, the pre-emergent herbicide won't stop them. If you use the herbicide too early, however, it will lose effectiveness. Apply the pre-emergent herbicide to your lawn to prevent crabgrass right after the forsythia bushes stop blooming for the best protection. Also keep in mind that the pre-emergent herbicide creates a barrier over your yard, so aerating or digging will puncture this barrier and allow the crabgrass to grow.
Killing Crabgrass After it Grows
Use a post-emergent herbicide specifically designed for crabgrass to kill crabgrass that is already growing. Make sure the herbicide is formulated for your grass type so that you don't kill your lawn as well. Post-emergent herbicide works best on crabgrass when it is young. If your crabgrass is well established, you may have to treat it several times before you can kill it. You can also simply wait until it dies in the fall and treat your yard with pre-emergent herbicide in the spring to prevent crabgrass from coming back.
You can use a general herbicide to kill crabgrass, if the problem is not widespread. Spray the general herbicide carefully because it will kill any plant it comes in contact with, including your grass.
Organic Crabgrass Control
It is hard to control crabgrass with organic methods, but you can make your yard inhospitable to crabgrass to prevent it from taking over. Fertilize your yard in late fall so the nutrients go toward your lawn and not the crabgrass. Water your lawn deeply, but less often, to encourage your grass to develop deep roots. Crabgrass has shallow roots, so you can give your lawn the water it needs while denying water to the crabgrass. Reseed bare spots in your lawn so that crabgrass and other weeds don't take root. If you used pre-emergent herbicide in the spring, you will need to wait until fall to reseed. Mow your lawn to height of 2-3 inches to crowd out weed seedlings. Pull out any crabgrass that begins to grow by hand. Make sure you remove all of the roots, or the crabgrass will return.