Dear Jeff, 

Great question! As you may know, crabgrass is an annual grass that grows each spring from crabgrass seed that was sown in the soil last fall from prior crabgrass plants. Crabgrass preventer prevents the seed from germinating, and thus must be applied before the crabgrass seed germinates and emerges from the ground. A University of Minnesota article from 2021 recommended applying this herbicide at the end of March: https://extension.umn.edu/yard-and-garden-news/controlling-crabgrass-your-lawn

Given the mild winter and warm temperatures, it would best to test the soil temperature (not the air temperature) to determine when to apply crabgrass preventer. Crabgrass seed will begin to germinate once soil temperatures are above 50°F . Therefore, we would suggest that you measure the soil temperature in the top 2-3 inches of soil in the area where you are trying to prevent the crabgrass. Once the soil temperature reads 50°F or above, it would be recommended to apply the crabgrass preventer.

Good luck!