Pennsylvania Turfgrass – Pete Landschoot, Ph.D. Professor of Turfgrass Science, Penn State
During 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 takeover 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 weather-related 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 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.
 
Stage of Growth and Postemergence Herbicide Applications
Knowing the growth stage of the target species and the corresponding herbicide rate are important for successful postemergence control of summer annual grasses. Stage of growth can be determined by examining individual plants and observing the number of leaves on the main shoot, or how many secondary stems, called tillers, have emerged from the crown. Generally, summer annual grasses with 1 or 2 tillers are easier to control with postemergence herbicides than plants with 3 or more tillers. As the number of tillers increases, the product rate will likely increase, or a different herbicide may be needed.
Because summer annual grass seedlings emerge in turf stands at different times during spring and summer, their growth stages will vary. Be sure to examine several plants in different locations of the turf stand to get an adequate assessment of plant maturity. If early in the growing season, the stand may be composed mostly of non-tillered or 1 or 2-tiller plants. In this case, choosing a herbicide product and rate that provides effective control and is safe on turf may be easier. Adding a preemergence herbicide with a postemergence product early in the growing season will prevent late-germinating summer annuals from infesting the stand. Later in the summer, annual grasses may have 4 or more tillers along with some newly emerged plants. In this case, you may need to apply a herbicide at rates that will control weed plants with the higher tiller numbers. Postemergence applications at this stage of growth may require more than one application for comple...