Grass weeds in lawns are classified as either annual or perennial. Management options differ between the two classes, with annuals generally easier to control.
Tall fescue can be a signficant weed problem in Kentucky bluegrass lawns.
Crabgrass and other annual grass weeds are common problems in home lawns that can be treated through both chemical and nonchemical methods. Proper lawn care practices to encourage a dense stand of vigorous grass is the best way to prevent weeds from invading. For example, mowing height can have a big impact; lawns mowed higher (over two inches) tend to have less problems with annual grasses such as crabgrass. Close-mowed lawns tend to open up, allowing weeds like crabgrass to invade. Light, frequent watering also favors crabgrass. Crabgrass often invades areas seeded in late spring because of bare soil, frequent watering, and the onset of hot weather, which is ideal for its growth.
Herbicides (weed killers) are also available to manage annual weeds. Preemergence herbicides prevent annual grass weeds such as crabgrass from emerging. Timing of application is important, as the weed killer should be applied to soil before the crabgrass emerges from the soil. Crabgrass will germinate when soil temperatures are greater than 55 to 60 degrees F. for 7-10 consecutive days, and continues until soils reach 95 degrees F. Other annual grasses germinate as the soils get warmer than 60 degrees.