There are many different weeds that may be categorized as crabgrass. It is important to try to identify the weed before trying to treat it. Having a lush healthy lawn will also prevent many problems. 1- Aerate your lawn - Aeration will open the soil to air, water & nutrients to allow your natural grass to grow much better. 2- Fertilize in the fall and spring to promote a thicker denser lawn. 3- Depending on your climate, water generally watering a lawn deeper and less frequently will prevent germination of crab grass seeds. 4- Mow to leave grass a bit longer. Most crabgrass species grow best in lawns that are cut short. A longer natural grass shades the seeds of crabgrass and helps to prevent their growth. 5- Use a crabgrass treatment product. This should be done in the early spring to prevent the initial emergence of the crabgrass. Often waiting to long to apply these products will make them ineffective. These products can often be purchased online at a discounted price. Label instructions on these products will give recommendations for application.