It is time to think about preemergent herbicide applications for summer annual weed control in turfgrass environments. Preemergent herbicides offer a great option for select annual grass and broadleaf weed control in warm- and cool-season turf. Preemergent herbicides are commonly used for crabgrass and goosegrass control but also control other grass and broadleaf weeds propagated from seed. As with any herbicide, one must be mindful of the herbicide mode of action. Specifically, with preemergent herbicides, application timing, application coverage, and single versus split applications among other factors are crucial to the results obtained.
Preemergent herbicides are typically applied late winter for control of many summer annual weeds, particularly annual grasses including crabgrass and goosegrass species. Application timing is critical with these products to obtain desired results. Specifically, smooth and large crabgrass germinate when 24 hour mean soil temperatures (four inch depth) reach 55 degrees F whereas goosegrass germinates when 24 hour mean soil temperatures reach 60 degrees F. Since these herbicides control susceptible species as they grow through the herbicide treated zone, the herbicide barrier must be established prior to weed seed germination. In most areas in NC, this occurs in mid- to late-March. If you are not able to track 24 hour mean soil temperatures on-site, you can visit this site and find a site in your geographic region to track 24 hour mean soil temperatures.