Timing crabgrass preemergence applications in spring
When is the correct time to apply crabgrass preemergence herbicides in the spring?
Spring is off to a slow start this year but there are always questions regarding the optimum timing for applying crabgrass preemergence herbicides. There are several different methods used for determining the optimum application timing from tracking growing degree days (GDD) to looking for forsythia bush flower bloom. One system used by both professional turfgrass managers and do-it-yourselfers to determine application timing is to track GDDs using the website GDDTracker.
GDDTracker starts counting GDDs on Feb. 15. The crabgrass preemergence model uses GDD (base 32 degrees Fahrenheit) to indirectly measure soil temperatures in a turf situation (enter your zip code under the map and then click on “Crabgrass PRE”). The GDD model attempts to predict the optimum application timing for when the 0-2 inch depth soil temperatures consistently reach 50-55 F. Applications made at this time provide adequate time for the preemergence herbicide to be applied and watered in before crabgrass germination occurs.
Summer annual grasses such as crabgrass require proper soil temperature and moisture to germinate and establish. Eighty percent of germination will occur when soil temperatures at the 0-2 inch depth are consistently between 60 and 70 F. For preemergence herbicides to be effective, they need to be applied before the soils reach this optimum temperature range. At the Hancock Turfgrass Research Center on Michigan State University’s campus, we have yet to record a soil temperature of 50 F at a 2-inch depth. The most recent soil temperature on March 31, 2022, was 40 F, well below the range of 50-55 F we’re targeting. In comparison, last year the soil temperature on March 31 was 49 F.
GDDTracker has almost the entire state of Michigan still progressing towards the optimum application window with only the southern most areas near the Indiana border currently in the optimum application window as of March 31. Compared to 2021, GDD accumulation is behind this year. On March 30, 2021, East Lansing had accumulated 351 GDD base 32; this year we’re at 232 GDD. My best guess is that we’re still about a week away from most of southern and mid-Michigan accumulating enough GDDs to enter the optimum preemergence application timing, which is probably about typical for this part of Michigan.
Remember, these models are meant to serve as guides, not absolutes, and keep in mind the range of the optimum application window extends from 250-500 GDD. You don’t have to apply the first day the model indicates optimum. Based on the weather forecast, I'd expect most of mid-Michigan south to the Indiana border to be in the optimum application window within a week. In most years we stay in the optimum application window for at least a couple weeks, if not longer, so you'll have some time to make applications.
In addition to using soil temperatures and GDDTracker, a good environmental indicator for preemergence timing is when forsythia bushes are blooming with their bright yellow flowers. Every year I watch a forsythia bush near my office that happens to be located on the south side of a building, so it’s a warm site and usually blooms a little early. It's still early, so there is not even a hint of yellow flowers.
Although this year is definitely not an early spring, a strategy that some professional turf managers use is to make a second preemergence application at half the label rate approximately 30-45 days after the first application.