Cumulative daily growing degree days (GDD) for 2005 (open circle) and 2006 (closed circles) in Rock Springs, PA, for the 2005 and 2006 cropping seasons. Growing degree days were calculated by the following equation: GDD = [(T
max + T
min)/2] – T
base, where T
max is the maximum daily temperature, T
min the minimum daily temperature, and T
base is the base temperature (4.4°C) (Nuttonson, 1958).