The Morton Arboretum is releasing information monthly on growing degree days. Plants, insects and fungi all develop at various times depending on temperature. Development will speed up or slow down depending on the rise and fall of temperature. Several studies have worked to understand the relationship between heat and development. These studies and information from them help anticipate flowering of trees and shrubs and the emergence of insects based on how many growing days have accumulated.

Growing degree days can be calculated
[(max temp + min temp)/2] – 50 = GDD

If the number resulting from this calculation is above zero then that is the number of degree days for that day. If the result is zero or below, then the number of growing degree days is zero for that day. When 100 GDD have been accumulated, it is expected that certain insects will emerge and plants will begin to flower. Various sources link insect emergence with certain stages in the life of indicator plants, this is possible because plants also respond to heat. These calculations, and the continued release of information from The Morton Arboretum, will provide useful information on how to protect trees and other landscapes from insects and pests.

For more information please contact Ellen Clark, Urban Forestry Communication Specialist, at EllenA.Clark@wisconsin.gov.