While many lawn owners worry about crabgrass once it appears, the best time to take care of the problem is before it starts. Preventing crabgrass is easier in cooler climates but there are steps that can be taken no matter where you live to prevent its growth. However, once it gets started for the year, getting rid of it can become a labor-intensive project.
Crabgrass is an unsightly grass that is often the bane of those who try to keep their lawns in good condition. It grows in the summer in cooler climates, and can grow nearly year round in the southern portions of the country, especially in Florida. It may have purplish stems and tends to form mats in the lawn (see Resources). If you have a lawn with a uniform grass species, generally the crab grass will not be too difficult to spot.
The main effect of crabgrass is that it prevents other grasses from growing in that area, eventually spreading and taking over other areas. Fortunately, this grass cannot deal with cooler temperatures. The first frost should take care of most of it. A freeze will certainly kill it. Crabgrass that is in a freezing area will not come back the next year. That particular grass is completely dead.
Unfortunately, crabgrass is very prolific at reproducing and will seed several times through the growing season. Those seeds will remain in place over the winter and germinate in the summer, or when soil temperatures reach 60 degrees or more. Therefore, applying a pre-emergence herbicide in the spring or before soil temperatures reach this point will go a long way toward taking care of the crabgrass problem.
Also, in order for crabgrass to grow, it must have full light. The best way to prevent crabgrass by natural means is keeping the length of your desired grass between 2.5 and 3 inches. This will discourage the growth of any crabgrass seed that survive the herbicide. Also, if crabgrass is a particular problem in a certain area, planting a shade tree therewith a shade-tolerant grass may be a possibility.
If living in a warmer climate and crabgrass has already taken root in the yard, it may never completely die without direct intervention. Grass killers can be used in the area, but it may just as effective to pull it up live. It will eventually have to be pulled no matter what. Once pulled, put down a pre-emergent herbicide, wait at least month or two, then replant with the desired grass species. If you find the crabgrass reappears before you can replant, try covering the area with a blanket or some other cover to prevent sunlight from reaching it until you are ready to replant.