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Weed of the Month: Crabgrass

It’s late summer and crabgrass weeds have invaded some lawns. Unfortunately, crabgrass is very difficult to control with most herbicides once it develops a few tillers or branches. There are several species of crabgrass (Digitaria spp.): tropical crabgrass, smooth crabgrass, India crabgrass, large crabgrass, Southern crabgrass, and blanket crabgrass. All of them are summer annuals that germinate primarily in spring, grow during the summer, set seeds, and then die with the first frost.

Closeup of crabgrass (Digitaria spp.) going to seed in a thin lawn.

Closeup of crabgrass (Digitaria spp.) going to seed in a thin lawn.
Jackie Jordan, ©2019, Horticulture Extension Agent, Clemson Extension.

Crabgrass is native to Europe and Eurasia and is a problem worldwide. The plant was originally introduced into the United States as a potential forage crop in 1849. Crabgrass is tolerant of heat and drought and is quick growing. A single plant can produce 150,000 seeds. The prolific nature of crabgrass makes control efforts a yearly task. Controlling crabgrass in a home lawn requires sound cultural practices and using pre-emergent herbicides at the correct time.

Crabgrass seeds require lots of light to germinate. A thick stand of turf is the best defense for limiting crabgrass weeds. Make sure that the lawn is being cut at the correct height for your turfgrass species. Scalping the lawn will expose the soil and reduces the vigor of the grass. Keep your lawn mower blades sharp, which ensures the turfgrass recovers quickly from mowing. To find information on the correct mowing height for your lawn, please see HGIC 1205, Mowing Lawns.

Proper irrigation is also important for limiting weed infestations. During heat or drought conditions, lawns should receive deep but infrequent irrigations. Watering everyday or every other day provides plenty of soil surface moisture for weed seed germination, including for crabgrass. Find more information on properly watering lawns, please see HGIC 1207, Watering Lawns.

Preemergent herbicides are best applied in early/mid-February and repeated in 8 to 10 weeks for adequate control. Crabgrass seeds start germinating once there are five consecutive days of 70 °F during early spring, but additional seed may germinate as late as June. All granular pre-emergent herbicides must be watered in with ½ inch of irrigation to make sure the product dissolves and the herbicide reaches the correct depth to provide effective control. Adequately irrigate the lawn within seven days of application or else the product is degraded by sunlight. For a list of products to control crabgrass and other grassy weeds, please see HGIC 2300, Grassy Weeds.

If this document didn’t answer your questions, please contact HGIC at hgic@clemson.edu or 1-888-656-9988.

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