A heat index that relates the development of plants, insects, and disease organisms to environmental air temperature. GDD is calculated by subtracting a base temperature from the daily mean temperature and GDD values less than zero are set to zero. The summation over time is related to development of plants, insects, and disease organisms. The reference temperature (base temperature) below which development either slows or stops is species dependent. For example, cool season plants (canning pea, spring wheat, etc. ): base temperature is 40°F (5°C); warm season plants (sweet corn, green bean, etc. ): base temperature is 50°F (10°C); and very warm season plants (cotton, okra, etc. ): base temperature is 60°F (15°C). See degree day, heat unit.