URBANA, Ill. — Crabgrass is often thought of as a spring-emerging weed, but with enough moisture, crabgrass can germinate any time in the growing season, challenging homeowners.
“Crabgrass is an abundant seed producer, as evidenced by the large seedling populations easily found now,” said Michelle Wiesbrook, University of Illinois Extension specialist. “By now, many spring-applied crabgrass preventers have degraded and are likely no longer doing their intended job.”
As a weed species, crab grass is opportunistic. Both large crabgrass, Digitaria sanguinalis, and smooth crabgrass, D. ischaemum, are commonly found in the Midwest.
Its stems typically grow prostrate, forming a mat. Crabgrass can be found in most warm, moist, fertile lawns in sunny areas where turf is thin or mowed too short.
“Crabgrass likes hot spots next to the driveway or sidewalk, since it tolerates hot, dry, compacted soils,” Wiesbrook said. “From there, the weed may spread aggressively to crowd out desirable grasses.”
Preventing crabgrass is easier than treating it when its grown, Wiesbrook said. A dense, healthy turf is more resistant to crabgrass, so mow grass tall and water deeply. In landscape beds, mulch and other weed barriers can be used to prevent weed germination.
Remove existing plants by hand using proper tools to ensure complete removal of the roots.
Preventative herbicide may be applied in spring before germination. Follow label directions for application.
Crabgrass is best prevented in early spring by using a pre-emergent herbicide prior to seed germination once soil temperatures stabilize at 55 degrees. Irrigation or rainfall is then needed for activation.
“Look for herbicides with these active ingredients for cool season turfgrass: benefin + trifuralin, dithiopyr, oxadiazon, pendimethalin, prodiamine, quinclorac, siduron, and sulfentrazone,” Wiesbrook said.
For crabgrass that has emerged, make two applications of graminicide two weeks apart, Wiesbrook said. Options include fenoxaprop, fluazifop, sethoxydim, and clethodim. Cool-season turfgrass options include bispyribac-sodium, fenoxaprop, mesotrione, quinclorac, and topramezone.
Smaller plants are more easily controlled. Injury can occur to turf if applications are made when temperatures are greater than 85 degrees, so be willing to accept some injury or wait for cooler days to apply.
Misidentification can lead to improper control approaches that waste time and money, Wiesbrook cautioned.
For help identifying grasses, check out the pocket-sized guide, “Identifying Turf and Weedy Grasses of the Northern United States,” available at pubsplus.illinois.edu.