Crabgrass is a common invader of lawns across North America. Crabgrass is a composite of several species, all of which are opportunistic, colonizing open spots in the lawn where the soil is bare, compacted, or otherwise disturbed. The coarse texture and prostrate habit of the plants make this grass especially unappealing to those seeking perfect turf.
BIOLOGY: Crabgrass is a sun-loving summer annual. Seed germination begins in spring, but it may continue throughout the growing season. The spreading stems bear coarse, often hairy leaves and can root at their nodes. Rowers and seed are borne in multiple spikes atop wiry stems. The plants are killed by the first frost, leaving great quantities of seed for the next year. Crabgrass is tolerant of dry, poor soil and occurs in nearly every soil type and crop or landscape.
SYMPTOMS: The coarse texture of crabgrass makes it easy to spot. Its presence is particularly conspicuous in autumn, when frost-killed plants make brown patches in an otherwise green lawn.
CONTROL: Even repeated close mowing will not stop crabgrass from reseeding. Preemergent herbicides, however, will prevent the seed from germinating. These typically include pendimethalin, bensulide, benefin, and trifluralin, among others. Corn gluten is an organic herbicide alternative that has shown some ability to suppress crabgrass germination.
It is important to apply preemergent herbicides at the right time. The exact date will vary by region and even from year to year, but it is typically when the forsythia in your area is in bloom. Depending on your location, a second application may be necessary as the first loses its potency. Reapplication times will vary according to your soil types and rainfall. Check with your local experts for specifics.
Finally, because crabgrass is intolerant of shade, maintaining a vigorous stand of the desired turf grass through proper mowing, fertilization, and soil care is the best defense against this opportunistic invader.