Crabgrass is a notorious weed across the United States, especially in lawns. It is an annual grassy weed. It has pale, bluish-green blades that can reach 2-5 inches high and are sometimes slightly hairy. Plants grow in flat, broad clumps that crowd out preferred plants, especially turf grasses in summer. The prostrate stems radiate out like the legs of a crab (hence the name) and root as they go.
Crabgrass sprouts from seeds in early spring and summer from seeds deposited potentially over many years. One crabgrass plant can produce thousands of seeds that germinate over many years in your lawn. Crabgrass seeds can also find their way into sidewalk and driveway cracks, along with flower beds and vegetable gardens. Germination begins when soil temperatures reach 55-60ºF and stay there for 7 days. Germination continues until temperatures reach about 95ºF. Crabgrass likes to germinate in bare areas of the lawn. Flower heads consist of slender, arching, finger-like spikes originating from the near the top of the stems. Crabgrass is very fast growing in warm weather. The foliage usually stands out as lighter green than most turf grasses. Plants die in fall.
The first step in controlling crabgrass is to maintain a healthy, vigorous lawn with proper fertilizing, watering and mowing. Lawns that are mowed too low are particularly susceptible to invasion by crabgrass. Check with your local extension office, but the heights for most fescue and bluegrass lawns should be 3 inches.
Existing crabgrass plants can be controlled with appropriately-labeled post-emergent herbicides. Be sure to check the label as some post-emergent herbicides cannot be used on all turf types. They are most effective when plants are young. The combination of properly-timed pre-emergent herbicides and post-emergent herbicides are key to prevention. Your local cooperative extension can provide information on proper timing of pre-emergent applications in your area. For more on herbicides (link to weed and feed).