Growing degree days (GDDs) are used to estimate the growth and development of plants and insects during the growing season. Insect and plant development are very dependent on temperature and the daily accumulation of heat. The amount of heat required to move a plant or pest to the next development stage remains constant from year to year. However, the actual amount of time (days) can vary considerably from year to year because of weather conditions. Growing Degree Day (GDD) values are computed by subtracting a base value temperature from the mean daily temperature and are assigned a value of zero if negative. Base temperatures are a point below which development does not occur for the organism in question. Growing Degree Day products are created for base 0, 5, 10 and 15 degrees Celsius.
Base 0 values are commonly used for cereals, base 5 for alfalfa, canola and general plant growth, base 10 for grasshoppers and beans and base 15 values are commonly used for general insect development.
GDD values are only accumulated during the Growing Season which spans from April 1 through October 31.