Growing degree days are essentially a measure of “heat units” and a way to relate daily air temperatures back to plant growth and expected crop stage. Tracking growing degrees can be a more accurate predictor of crop growth than calendar days. For wheat, GDD are determined by using a base value of 32 degrees, under which no growth is expected to occur. The max and min daily temperatures are averaged, and the base value is subtracted from that result to determine GDD for that day.