What growing degree days are
Plants and insects develop according to the heat they accumulate rather than the number of calendar days. Growing degree days (GDD), also called heat units, add up the warmth above a base temperature below which growth stops. Farmers use them to predict emergence, flowering and maturity, to choose hybrids and to time pest control.
The formula
For each day:
GDD = (daily high + daily low) ÷ 2 − base temperature
If the result is negative, the day counts as 0. Many crops also use an upper cutoff, because growth stops increasing above a certain temperature.
For corn in the United States, the standard modified method uses a base of 50 °F and a cutoff of 86 °F: any high above 86 °F is set to 86, and any low below 50 °F is set to 50 before averaging.
Example
A summer day reaches 85 °F with a low of 60 °F:
GDD = (85 + 60) ÷ 2 − 50 = 22.5
A corn hybrid needing 2,700 GDD from planting to physiological maturity would take about 2,700 ÷ 22.5 = 120 days if every day were like this one.
Typical requirements for corn
| Stage | Approximate GDD (°F) from planting |
|---|---|
| Emergence | 100 to 120 |
| Silking | 1,200 to 1,400 |
| Black layer (maturity) | 2,200 to 3,000, depending on the hybrid |
Seed companies list the GDD rating of each hybrid. Choosing one that matches the GDD available in your area between planting and the first fall frost avoids an immature crop.
Other base temperatures
| Crop or organism | Base |
|---|---|
| Corn, soybeans, sorghum | 50 °F |
| Wheat, barley, oats | 32 °F |
| Alfalfa weevil | 48 °F |
| Many insect pests | 50 °F |
Degree-day models are widely used in integrated pest management: extension services publish GDD thresholds for pests such as the codling moth or the European corn borer.
Using the calculator
Enter one day’s temperatures to get its GDD, or average temperatures for a period to estimate a typical day. Enter the degree days already accumulated since planting to see the share of the requirement reached and the days left. For accurate tracking, add each day’s value from local weather station data.