What are growing degree days?

The Growing Degree Day, or GDD, is a heat index that can be used to predict when a crop will reach maturity. Each day’s GDD is calculated by subtracting a reference temperature, which varies with plant species, from the daily mean temperature (we ignore values less than zero).

The reference temperature for a given plant is the temperature below which its development slows or stops. For example, cool season plants, like peas, have a reference temperature of 40 degrees F while warm season plants, like sweet corn and soybeans, have a reference temperature of 50 degrees F.

The total GDDs over a growing season is related to plant development. The development of plants depends on the accumulation of heat. Since cool season plants have a lower reference temperature, they accumulate GDDs faster than warm season plants.

Unless plants are overly stressed by drought or pests, the total GDDs can be used to predict when a crop will reach maturity. Corn, for example, requires 1360 GDD to mature.

GDDs can be computed using climatic information for any location. That computation, along with data on soil, water, and minimum and maximum temperatures, helps suggest which crops will grow best in a given region.