Growing Degree Days (GDD) replaced an older “days of maturity” rating. While there are many factors that affect plant growth, GDD is a helpful tool.
GDD are determined by comparing the average temperature of a day (Maximum Temperature Vs Minimum Temperature) and comparing it to a base temperature. Unlike Heating Degree Days and Cooling Degree Days where the base temperature is derived by determining the ideal temperature inside a building, the base for GDD is determined by your specific crop to grow.
For corn there is a Modified Growing Degree Day with a maximum limit of 86° and a minimum limit of 50°. The base temp is 50° (As corn grows little as the temperatures approach 50°).
Modified GDD = (Maximum Temperature + Minimum Temperature ÷ 2) – 50
GDD is then calculated for each day in a growing season. The summation determines the amount of GDD available during the growing season measured.
Growing Degree Days summations can be plotted on a map for easy use. From a map you can determine your regions GDD and pick a plant which reaches physiological maturity by the expected minimum amount of GDD. With charts you can also determine the last date you can safely plant and still expect to have enough GDD remaining before a killing frost.