Spring is off to a relatively quick start this year and the questions are rolling in on 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 many professional turfgrass managers 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 12, 2020, was 44 F, well below the range of 50-55 F we’re targeting. In comparison, last year the soil temperature on April 2 was 44 F.

GDDTracker has the entire state of Michigan still progressing towards the optimum application window. 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, it looks like southern areas of the state may enter the optimum application window within a couple weeks, but remember in most years we stay in the optimum application window for at least a couple weeks, if not longer.

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. Compared to 2019, GDD accumulation is significantly ahead this year. On March 12, 2019, East Lansing, Michigan, had accumulated 13 GDD base 32; this year we’re at 107 GDD. My best guess is that we’re still a couple weeks away from most of southern and mid-Michigan accumulating enough GDDs to enter the optimum preemergence application timing, but that will still most likely be early compared to a typical spring. 

A common strategy for dealing with an early spring is to follow-up with a “booster shot” second preemergence application at half the label rate in approximately 30-45 days after the first application.

Remember, the practices that encourage a healthy, dense turf stand, such as mowing high, returning clippings and adequate fertilization, are all part of an effective crabgrass prevention strategy.