Crabgrass is an annual grass that grows out from a central root system into a wide, gangly, spreading shape. In some very warm climates, it can even survive over the winter. Crabgrass is invasive, sprouting in yards and on golf courses, and continuing to spread whenever and wherever the turf has suffered from drought or disease. To make matters worse, different types of crabgrass prefer different conditions, so combating it successfully depends on determining which crabgrass is growing in your lawn.
Darcy E. Partridge-Telenko, Bryan Unruh and Barry J. Brecke of the University of Florida IFAS Extension write that this type of crabgrass (Digitaria serotina) has short, hairy leaves. It spreads by sending out creeping stolons, which are branches that spring from the base of the plant. Wherever the stolons touch soil, they root and produce more plants. Blanket crabgrass can be either an annual or a perennial, depending on the climate in which it is growing. It prefers to grow in places where the soil is damp to out-and-out wet.
India crabgrass (Digitaria longiflora) has smooth leaf blades that rarely exceed one inch in length. As it grows, it forms mats of grass. Indian crabgrass prefers dry soil.
This species (Digitaria ischaemum) has large, smooth leaves. The stems have nodules along their length that will take root when they touch adjoining soil.
Southern Crabgrass and Large Crabgrass
The horticulturalists write that southern crabgrass (Digitaria ciliaris) and large crabgrass (Digitaria Sanguinalis) look very much alike. Like smooth crabgrass, both species spread by sending out stems with nodes that take root when they touch the soil. The leaves of both varieties are more than two inches long and hairy.
This species (Digitaria bicornis) has long hairy leaves that resemble both southern and large crabgrass, but the branches that produce the seeds all emerge from the same point on the stems.
Partridge-Telenko, Unruh and Brecke contend that the best way to control crabgrass is to keep your lawn healthy enough to resist its invasive nature. You should mow your lawn to the recommended height for the variety of grass you've planted, be especially vigilant about discarding lawn clippings when crabgrass is producing seeds, follow recommendations about fertilizing your lawn, and have your soil tested periodically to make sure its chemical makeup favors the grass you're growing on it.
If crabgrass has gotten such a foothold in your lawn that your only option is to apply a herbicide, do so before the crabgrass starts germinating in the spring. Apply the herbicide exactly according to the instructions on the packaging.