Growing Degree Days:Growing Degree Days (GDD) are used to estimate the growth and development of plants and insects during the growing season. The basic concept is that development will only occur if the temperature exceeds some minimum development threshold, or base temperature (TBASE). The base temperatures are determined experimentally and are different for each organism.
To calculate GDDs, you must first find the mean temperature for the day. The mean temperature is found by adding together the high and low temperature for the day and dividing by two. If the mean temperature is at or below TBASE, then the Growing Degree Day value is zero. If the mean temperature is above TBASE, then the Growing Degree Day amount equals the mean temperature minus TBASE. For example, if the mean temperature was 75° F, then the GDD amount equals 10 for a TBASE of 65° F. You can think of Growing Degree Days as similar to Cooling Degree Days, only the base temperature can be something besides 65° F.
In equation form:
TBASE = Growing Degree Day base temperature
TMEAN = mean temperature, (TMAX + TMIN) / 2