Controlling Troublesome Crabgrass in Turf

August 11, 2010 at 8:15 am

The follwing is a repost from the 8/5/10 OSU Extension BYGL Newsletter

Crabgrass is flourishing in some lawns this year, and in some cases these are lawns that have been ‘crabgrass free’ for several years. What’s happening? According to Dave Gardner (Turfgrass Science, OSU Department of Horticulture and Crop Science) the answer can be summed up in three words: hot, wet, and humid. The 2010 growing season has been unusually hot and humid with many areas experiencing successive days above 90F. And until recently, much of Ohio has had consistent rainfall events keeping soils wet.

It is hard to mistake crabgrass. Crabgrass has spreading stems and wide, flat leaf blades that lie on the ground. The spike-like seed heads are arranged like fingers which is described by the name of the genus, Digitaria. Crabgrass is an annual plant, which means that once seed is produced, the plants die. This life cycle presents a serious problem for lawns that are dominated by crabgrass; the lawns become dominated by dead crabgrass plants in the fall. Of course, this also presents an opportunity since the best time to seed turfgrass is in late summer to early fall. The new turfgrass plants will rise just as the crabgrass declines.

Crabgrass is a “warm-season” plant meaning that conditions have been ideal for their growth and development. Most grasses used in Ohio lawns are cool-season plants, meaning that high temperatures stunt their growth; consequently, they are at a disadvantage when competing head-to-head with warm-season plants. This is particularly a problem in lawns with thinning stands of turfgrass. Short-term control of nutsedge and crabgrass usually focuses on herbicide applications. Long-term control focuses on growing dense stands, utilizing high populations of turfgrass to maximize competition with these weeds.

A herbicide application targeting crabgrass may be combined with a turfgrass “rejuvenation” program, as long as care is taken to observe the waiting period between the time of the application and the time of seeding. For example, quinclorac (e.g. Drive) may be used effectively as a postemergent selective herbicide for late-season crabgrass control. The targeted crabgrass must have 6 or more tillers; it will not control crabgrass that has just started to tiller. Seeding of Kentucky bluegrass may be done 28 days after a quinclorac application, and seeding of turf-type tall fescue or perennial ryegrass may be done 14 days after an application.

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