To study the growth of plants, the cycle of insects, and when we can harvest crops, we devised growing degree days.(GDD) There are a few ways to calculate them, but I’ll discuss the one we like. Add the high and low of the day, and divide it by two. For example, if the high of the day is 63 degrees, and the low is 43, you would add them and get 106. Divide that by 2 and you will get 53. GDD are accumulated over 50 degrees Fahrenheit. So we would accumulate 3 GDD for that day. Forsythia starts to bloom at 58 GDD and is in fool bloom at 94 GDD. Eastern tent caterpillars hatch at 92, Eastern pine sawfly at 144. The Bradford pear, which is very common in local landscapes, starts blooming at 142 GDD, which is right about now.
