Cover Story
POSTEMERGENCE CONTROL OF CRABGRASS and Other Summer Annual Grasses in Turf By Pete Landschoot, Ph.D., Professor of Turfgrass Science, Penn State
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uring late spring and summer, summer annual grasses can be an unsightly weed problem in many Pennsylvania lawns, sports fields, and golf course fairways and roughs. Reasons for severe infestations include high seed populations in soil, thin turf canopies allowing space for weeds to grow, mistimed preemergence herbicide applications, and premature breakdown of preemergence herbicides. Once summer annual grasses begin to emerge, they generally grow faster than cool season turfgrasses and will gradually take over large sections of turf. If not controlled before producing seed, another large weed crop will emerge the following year. The most common summer annual grasses likely to infest turf in Pennsylvania include crabgrass, goosegrass, and foxtail species. Other summer annuals that may be problematic in certain areas are barnyardgrass and Japanese stiltgrass. Seeds of summer annual grasses don’t all germinate at the same time; instead, germination and seedling emergence is staggered over several weeks or months. This timing mechanism ensures that a late frost or some other weatherrelated event won’t kill off the entire population. Depending on where you live in Pennsylvania, you may see crabgrass seedlings emerging from mid-April to mid-July. Also, seed of various species of summer annual grasses germinate at 10 Pennsylvania Turfgrass • Summer 2020
different times of the growing season. For example, goosegrass seed usually germinates and emerges 3 to 4 weeks later than crabgrass. The long germination/emergence period makes control of summer annual grasses challenging. The most effective means of controlling summer annual grasses is through cultural measures that create a dense turf and applications of preemergence herbicides. When these measures fail, postemergence herbicides can be used. Postemergence control of crabgrass and other summer annual grass weeds involves the use of products that kill growing plants after they have appeared in the turf. Postemergence herbicides can be used to treat only those areas where summer annual grass weeds have emerged. Preemergence herbicides, on the other hand, are usually applied over the entire turf area since the applicator does not know where seeds are located, or if they are present.
Postemergence Herbicides for Control of Summer Annual Grasses Dithiopyr is mainly used as a preemergence herbicide, but products containing this active ingredient (Dimension 2EW, Dimension Ultra 40WP, Dithiopyr 40WSB, and others)
