Crabgrass is a light green weedy grass which usually appears in summer. Crabgrass likes hot, sunny, dry conditions and this is why it is typically found in areas where the lawn is thin and sparse. The weed shoots up seed heads similar to bahia grass as small y-shaped seed pods. The seeds can lay dormant for up to seven years in the soil. It is also drought tolerant.
A proper maintenance routine based on your species of grass is the best way to prevent weeds. Mowing your grass at the proper height taking off no more than 1/3 of the blade at a time. Watering in fewer longer intervals for established lawns rather than many shorter intervals. Fertilizing in the proper amounts of nitrogen based on your species of grass at the right time of year. Prevention using proper maintenance is better than trying to get rid of crabgrass once it is established.
While prevention is the best method of controlling crabgrass you can kill the weed once it establishes itself. A pre-emergent herbicide applied to your lawn kills the seedling after germinating. The seeds do live in the soil for up to 7 years. Being consistent with applying a pre-emergent herbicide will effectively lessen the amount of new weeds emerging over time. Pre-emergents are generally applied twice a year but its best to follow the manufactures instructions. Because crabgrass is drought tolerant a regular watering schedule will minimize crabgrass and discourage growth.