According to Wikipedia, Growing Degree Days (GDD) are a measure of heat accumulation used by horticulturists and gardeners to predict the date that a flower will bloom or a crop reach maturity. So you may be saying “Who Cares Joe”? Well plants care…..plants like Crabgrass, Forsythia, Hydrangea and many other plants that begin to bloom when there has been enough heat accumulation to allow such growth. That’s why we can never guarantee that Crabgrass will start to germinate in the 3rd week of April. This year (due to the early warm temperatures), it was ealier…. and in years past, it has been later. We can never guarantee that grass seed will germinate in x number of days…. It may be longer, it may be shorter and it’s all based on Growing Degree Days.