Growing Degree Days

Use growing degree days instead of a calendar to determine the best time to scout for and treat for pests. Last April it was cold and snowed. Three years ago March was warm and that carried right into April. Trying to plan what to look for based on a calendar date can be frustrating when dealing with the changes of a New England spring.
Growing degree days in Massachusetts start accumulating on March 1st. After March 1st a simple calculation each day will let you know how many days accumulated.

Calculating GDD is simple math. Add the daily high temperature to the daily low temperature and divide by 2 to get the average daily temperature. If that number is greater than 50 subtract the number by the baseline temperature of 50 degrees. The result equals the number of days accumulated. For example, if today was 65 with a low of 40 the average daily temperature would be 52.5 an accumulation of 2.5 growing degree days.

Gypsy moth eggs hatch after accumulating between 90 and 100 growing degree days. Crabgrass germinates after around 200 growing degree days. Knowing the number of growing degree days necessary for a pest to appear and the number accumulated is a great tool for knowing when to apply preventative materials and when to scout for activity.

Cornell has resources to locate GDD accumulation for different parts of the northeast. The University of New Hampshire has excellent resources about GDD including by pest type, for example GDD’s of landscape pests.