Crabgrass is a type of annual grass that appears in the warm season. It is native to Europe, although it has colonized much of North America as well, to the dismay of many gardeners. The stubborn rooting grass can be difficult to eradicate when it colonizes, and it tends to overwhelm native species with an aggressive seeding pattern and rapid growth habit. As a result, most gardeners try to prevent crabgrass from emerging in their gardens, and take serious steps to eradicate it when it does.
The term can actually refer to several grasses, all of which are in the genus Digitaria. The two most common problem species are Digitaria sanguinalis and Digitaria ischaemum. Both have high growing branching stems, although they can also adapt to live close to the earth in mats. These mats will spread and put out roots within a season. If allowed to flourish, crabgrass tends to choke out the grasses surrounding it, and will form depressions in a lawn that are reseeded with the invader's seeds, allowing the grass to spread even further in the next season, especially if given ample water.
A number of steps can be taken to eliminate crabgrass, although constant vigilance is required. The first is to grow and maintain a healthy lawn that does not allow this grass to take root. Homeowners should plan on seeding, fertilizing, and watering in the early spring, before crabgrass emerges, so that the lawn can get a head start. Seeding should be done densely to create a thick mat of grasses to keep other species from getting a foothold, and water deeply and infrequently. Also, homeowners should keep the turf relatively high when they mow, as a lawn that is around 3 inches (7.6 centimeters) high will shade the crabgrass seeds, preventing them from sprouting.
When individual outbreaks of crabgrass occur, gardeners should make sure to remove each one by hand, pulling up the entire root system. Some gardeners use herbicides to control invading grasses, applying preemergent herbicides before it appears or control herbicides when it does pop up. The use of herbicides should only be undertaken as a last resort, however, as it can have an impact on the health of the rest of the garden, along with pets and children. Application of vinegar may also help, but homeowners should be prepared to kill the entire section of the lawn to which the vinegar is applied, and reseed or sod quickly before crabgrass can root again.