Growing degree days (GDD) are a measure of heat accumulation used by horticulturists, gardeners, and farmers to predict plant and animal development rates such as the date that a flower will bloom, an insect will emerge from dormancy, or a crop will reach maturity.
In the absence of extreme conditions such as unseasonal drought or disease, plants grow in a cumulative stepwise manner which is strongly influenced by the ambient temperature. In other words, GDD values provide a best case outlook as to plants' pace to maturity.
As we enter the Grand Solar Minimum, temperatures are decreasing. The data should demonstrate this. I have created this tool to analyze the drop off in GDD between 2018 and 2019 seasons across the US.