Bermudagrass (Cynodon spp.) and crabgrass (Digitaria spp.) are both fast growers that will bully and overwhelm other plants if left to spread unchecked. In some parts of the U.S., bermudagrass can be made to behave itself to produce a thick, attractive turf if it's maintained correctly. Crabgrass, though never desirable as turf, is not especially difficult to control.
Uses in the Landscape
Bermudagrass is a warm-season grass that's hardy in U.S. Department of Agriculture plant hardiness zones 7 to 10. It's commonly used as a turf grass in the southern United States, and it's also sometimes grown in pastures as forage for livestock.
The two species of crabgrass common in the U.S. (Digitaria sanguinalis and Digitaria ischaemum), however, are annuals with no redeeming landscape or commercial uses. They spread readily and invade turf grass, flower beds, residential gardens and farm fields, crowding out desirable plants.
Appearance and Growth Habit
Bermudagrass is a low-growing plant that spreads via stolons that grow above ground and rhizomes that grow underground; it grows quickly and will fill in bare spots in turf with little encouragement. As a turf grass, it has a medium texture and smooth leaves with fine hairs at the base. During the peak of the growing season its leaves are dark green, but they will turn brown after a frost.
Crabgrass is also low-growing, but it spreads when nodes on its stems take root where they rest on the surface of the soil; it can also spread with the dispersal of its numerous seeds. Mature plants have smooth, dark green leaves up to 5 inches long, and plants tend to grow together to form large, dense clumps.
Aggressiveness and Invasiveness
Both crabgrass and bermudagrass have aggressive growth habits and both can be a problem in the landscape. Crabgrass thrives even in harsh conditions, so it can spread even when other plants struggle. However, its seeds need light to germinate, so it may have trouble getting established in dense turf or densely planted beds.
Because of its spreading habit, bermudagrass can also be invasive, and it will spread from the lawn into beds and paved areas if it's not controlled with edging. When bermudagrass gets into places where it's not wanted, especially in turf that consists of other grasses and in garden beds, it's difficult to eradicate, so it needs to be managed carefully when it's used as a turf grass.
Controlling Crabgrass in Bermudagrass Turf
If you maintain bermudagrass turf properly, the turf grass will be able to effectively compete with crabgrass, and the crabgrass will have a hard time getting a foothold. Mowing the turf to the proper height will prevent crabgrass seeds from germinating; bermudagrass turf hybrids such as "Santa Ana" and "Tifway II" should be maintained at a height of 1/2 to 3/4 inches, and "Tifgreen" should be mowed to between 1/4 and 1/2 inch. Bermudagrass will be most able to fend off crabgrass when it gets at least six hours of sun per day, so it is more vulnerable in shady spots.