Japanese stiltgrass (Microstegium vimineum) is a C4 summer annual grassy weed commonly invasive to forests. It can be problematic in lawns. Often it is problematic in shaded areas adjacent to an infested forest. But we have observed infestations in full sun turfgrass areas where you would normally expect crabgrass to be the predominant summer annual. It seems this weed is becoming increasingly problematic in lawns. But why I am I writing about a summer weed in February?
Japanese stiltgrass is the first summer annual grassy weed to emerge. Preemergence herbicides commonly used to control the summer annual crabgrass (dithiopyr, prodiamine, and pendimethalin) were effective for Japanese stiltgrass control in our research. However, stiltgrass emerges sooner than crabgrass and these preemergence herbicides are only effective when applied before stiltgrass emergence. In our 2022 research we applied these preemergence herbicides on March 11, 2022 in Pittstown, NJ. Stiltgrass typically emerges in March, often about a month before crabgrass. Germination of the seed and subsequent emergence is probably highly variable from year to year and depends on soil surface temperatures. Do not expect seedlings to be killed by frost. While Forsythia full bloom can be used as an indicator to apply crabgrass preemergence herbicides, there is no information available regarding soil temperatures or phenological indicators for stiltgrass emergence.
Our research found that while preemergence herbicides applied before stiltgrass emergence (in early March) were effective, those same treatments applied around crabgrass emergence in early May were not effective. A mixture of the postemergence herbicide Acclaim Extra (9 fl oz/A) and a preemergence herbicide was effective for stiltgrass control when applied in early May at crabgrass emergence. For stiltgrass control in June, July, and August, higher rates of Acclaim herbicide will be necessary for control. Acclaim Extra is the only product registered for postemergence stiltgrass control in turfgrass. Field research found it to be the most effective control option. See this RCE factsheet for more information about Japanese stiltgrass identification and other control options in non-turf areas.
In summary, to control Japanese stiltgrass, preemergence herbicides need to be applied a month earlier than is typical. If this is not feasible, tank-mixing preemergence herbicide with low rates of Acclaim Extra is an effective option.