Crabgrass is a common annual grassy weed that is very invasive. It is able to survive in a variety of harsh conditions such as dry or poorly draining soils, making it difficult to control. Crabgrass is often found in lawns that are not properly maintained and are patchy or dying.
There are two main varieties of crabgrass now found in American lawns, says the University of New Hampshire Cooperative Extension: small crabgrass (or Digitaria ischaemum) and large crabgrass (Digitaria sanguinalis). Small crabgrass has a smooth stem and grows prostrate to the ground. Small crabgrass grows between 1 and 6 inches, making control with mowing difficult. Large crabgrass has a hairy stem and grows upright, averaging between 12 and 48 inches when not mowed.
Crabgrass seeds germinate between mid-spring and late summer. According to Cornell University, 50 percent of crabgrass seed will not germinate until the next year. To control crabgrass properly, grass seed production must be curbed.
Cultural practices are the best way to reduce crabgrass growth. Mowing turf grass at a proper height of 2.5 to 3.0 inches reduces crabgrass production by growing a denser turf that blocks out sunlight, reducing crabgrass seed germination. Mow only one-third of the grass blade at a time to reduce disease and patchy spots. Proper fertilization of 2 to 4 pounds of nitrogen per 1,000 square feet of lawn makes the lawn dense. Two applications of fertilizer on the lawn, one in September and one in November after the final mowing, will show the best results.
Pre-emergent herbicides are applied before the crabgrass seed germinates. An application as early as March 1 will often control crabgrass throughout the entire season. Application early in the season prevents any seeds from germinating. Pre-emergent herbicides are best avoided when seeding a new lawn. An application of water after spreading the herbicide activates the product. Instructions on the label of the product are to be followed at all times.
Post-emergent herbicides are applied after the crabgrass has emerged. Small spots of crabgrass are best pulled or hoed by hand, but large areas may require chemical control. Post-emergents are sprayed directly onto the foliage of the crabgrass, so application is best on days where wind is at a minimum to prevent the herbicide from drifting onto desirable grass.