Hello Mary,

You are correct.  The pre-emergent herbicides need to be applied when soil is 50-55 F in order for the herbicide to be activated before the crabgrass seeds begin to germinate.  Naturally, this is about the same soil temperature when the forsythia starts to bloom.  The more accurate way to determine your soil temperature and timing for pre-emergent herbicides is the MSU Growing Degree Days tracker:  GDD Tracker 4.0 - Turf Pest Management Tracking and Alerts with Growing Degree Day Models for Michigan, Illinois, Indiana and Ohio - Project initiated by Michigan State University

Enter your zip code and select Crabgrass PRE in the right panel.  This will show whether you are in the optimal zone for pre-emergent herbicides.  Unfortunately, this year the forsythias seem to be a little later compared to the soil temperature.  Right now, most of Wayne County seems to be past optimal application time for pre-emergents.

Does that mean you can't apply?  Not necessarily.  You just need to do it very soon and the effectiveness may not be as good as if you had applied earlier.

Hope this helps.