Timing crabgrass preemergence applications in spring

When is the correct time to apply crabgrass preemergence herbicides in the spring?

Forsythia bush blooming in the spring.
Forsythia bush blooming on MSU’s campus on March 15, 2024. Blooming forsythia bushes are a good phenological indicator for preemergence herbicide timing. Photo by Kevin Frank, MSU.

Spring hasn’t even officially arrived and it is already off to roaring fast start. The number one question I’m receiving right now is whether it’s time to apply 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, soil temperature at a 2-inch depth exceeded 50 F for the first time this year on March 13, 2024. This corresponded almost perfectly with the GDDTracker, which indicated East Lansing, Michigan, was in the optimum application window for preemgence herbicide applications on March 12. 

GDD Tracker preemergence model for March 2024.
GDDTracker preemergence model for East Lansing, Michigan, through March 14, 2024. Green shading on the map indicates optimum application timing.

GDDTracker has about half of the Lower Peninsula (from Saginaw across to Mt. Pleasant and areas south) in the optimum application window as of March 14. Compared to 2023, GDD accumulation is significantly ahead of last year. On March 14, 2023, East Lansing had accumulated 78 GDD base 32; this year we’re at 297 GDD. The current forecast indicates we are headed for some much cooler temperatures, which will slow down GDD accumulation and ultimately result in the optimum application window being extended.

Remember, the GDD models are meant to serve as guides, not absolutes. 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. We will stay in the optimum application window for at least two to three weeks, if not longer, depending on whether we have cool or warm temperatures going forward.

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.

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

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