Growing degree days for PGR applications

Growing degree days (GDD) have become widely used for timing plant growth regulator (PGR) applications - for suppression of poa and bentgrass growth and poa seedhead production and more recently for suppression of ultradwarf hybrid bermuda. Unfortunately, there are a couple of questions that need to be answered before you can take full advantage of GDD models.

When should I start counting GDDs? January 1st is the easiest point to start counting GDD unless there are some specific instructions that…