Growing Degree Days

The life cycles and growth rates of many biological organisms are determined by temperature. As temperatures increase, activity increases. One of the ways we measure the biological activity of plants and insects is Growing Degree Days or GDD. Knowing the GDD allows us to monitor a specific number and apply plant protectants (like insecticides) at the appropriate time in an organisms life cycle to maximize control. It works for plants too, like crabgrass.

What is GDD and how is it calculated? We assume that an organisms growth rate increases as the temperature rises above a predetermined base temperature. Each organism may be given a specific base temperature.

There are several methods to calculate GDD, but the most common is to subtract 50 degrees from the mean daily temperature. So, GDD = (Thigh + Tlow)/2 - 50.  For a high of 80F and a low of 60F the GDD for that day would be: (80+60)/2 - 50 = 20GDD. Once the total GDD exceeds the organisms lifecycle threshold, treatments can be applied. Knowing these activity thresholds is important and we monitor them depending on our target pest and optimal treatments.