What are growing degree days?

Growing degree days are a concept of phenology that uses threshold temperatures to predict when plants and pests will reach maturity. It is based on the idea that plants and pests develop only above a certain threshold temperature. By tracking the high and low of each day, we can assign each day a specific number of units, and, knowing the maturity rate of specific plants and pests, translate that into an accurate prediction of when they should mature.  The method is more accurate than simply relying on a calendar.