It is easiest to control crabgrass before it germinates using preemergence. May 1 is typically when crabgrass is expected to germinate throughout much of Kansas, but this does vary depending on location and weather patterns. Many homeowners use signals from nature to determine preemergence timing.
A common guideline is to apply crabgrass preemergent when redbuds are in full bloom. While this is an indicator, it’s not going to provide 100% accuracy since microclimates exist within each landscape. In warmer areas of the landscape, crabgrass is likely to germinate earlier.
Soil temperature is a useful indicator to predict crabgrass germination. Using the K-State Mesonet you can access the average soil temperature at a two-inch depth over the past seven days. When the daily average is 55 degrees F for about five days, research suggests this is the time to apply crabgrass preemergence.
https://hnr.k-state.edu/extension/horticulture-resource-center/common-pest-problems/documents/Crabgrass.pdf