Crabgrass is identified by its star-shaped growing pattern, its fibrous root system and its tendency to sprout into a dense mat that covers any other plants around it. Crabgrass identification requires looking at factors such as growing conditions and environment, natural variations in crabgrass plants, and the current growth stage of the crabgrass.
As a seedling, crabgrass resembles a corn plant. The leaves are about one-fourth of an inch wide and sprout at an angle from the stem. Soon after, side shoots develop that grow outwards from the stem, fall to the ground and grow in a star-shaped pattern. This is useful in identifying crabgrass from normal grass because ordinary grass grows up from the stem, not out to the side.
Growing conditions change how crabgrass appears. Given ideal conditions of plenty of sun and water, crabgrass grows up and out quickly and is very thick. If the crabgrass does not get much water, it appears
thin but still sends out side shoots. In conditions of less sunlight, the crabgrass appears scraggly and weak and does not send out side shoots.
Crabgrass growing in a lawn is identified by the thick clumps and matted areas that differ from the lawn grass. After a lawn is mowed, crabgrass grows back before the lawn grass does. So areas that stand out as taller are potentially identified as crabgrass.