After identifying weeds in the lawn, the best strategy for controlling them is to practice good lawn care management. Crabgrass grows in lawns that are mowed too short or watered too frequently. Instead, keep grass mowed to 2 to 3 inches, depending on the variety, and water infrequently but deeply to promote healthy root development.
When identifying weed grasses, consider the form the grass takes. Crabgrass forms an open, matted rosette, while foxtails grow upright. Panicum and Proso millet grow in a tangled mass. Several commonly grown grasses become weeds when they appear in Kentucky bluegrass lawns. Tall fescue and creeping bentgrass grow in clumps in these lawns. Nutsedge has an upright, compact habit, while knotweed is open, with sparsely formed leaves.
Tall fescue is identified in Kentucky bluegrass lawns by its coarse leaves, while creeping bentgrass is finer and lighter. Zoysiagrass appears as thick, straw-colored patches. Quackgrass has thick, coarse leaves, with claw-like edges. The leaves of barnyard grass are smooth and mostly hairless.
The inflorescence, or flowering top, of a grass weed provides a strong identity marker. Crabgrass produces an open, finger-like inflorescence, while foxtail produce a bushy, curved top that resembles a fox's tail. Barnyard grass produces a spiked, bristly inflorescence with stems growing alternately. Wild oat produces short, fringed tops.
Foxtail, wild oats and barnyard grass are most often found in rural landscapes, in fields and along streams. These grasses rarely invade suburban lawns. Crabgrass, tall fescue, creeping bentgrass, knotweed and quackgrass can cause major problems in lawns. Crabgrass and knotweed also invade the edges of driveways and cracks in the sidewalks.
Pre-emergent herbicides are effective at controlling annual grasses such as quackgrass and crabgrass, although they will also prevent seeded grasses from growing. These chemicals are applied in spring before the soil warms to 55 degrees Fahrenheit and grasses germinate.
Perennial grasses, such as tall fescue and creeping bentgrass, are harder to control. Hand-pull small patches of grass or apply a nonselective herbicide, such as glysophate. These products will kill other grasses as well. Re-seed the lawn once the weed grasses have died.