Understanding Growing Degree Days

Growing degree days (GDD), or heat units, are used to estimate the growth and development of certain crops and pests during the growing season.

Corn growth, for example, follows very closely the accumulation of average daily temperatures during its lifetime. The accumulation of average daily temperatures is calculated as 'growing degree days (GDD)' and includes a minimum development threshold that must be exceeded for growth to occur. We call this minimum development threshold a base temperature. Alfalfa that is relatively cool-adapted has a base temperature of 41 degrees Fahrenheit. Corn, sweet corn, sorghum, and soybeans have a base temperature of ~50 degrees Fahrenheit. Below a crop or pests' base temperature there is little growth occurring.

To calculate GDDs you first need to record the mean temperature; this can be done by adding together the high and low temperature for the day and dividing that value by two. If the mean temperature is at or below the base temperature for a crop or pest of interest, then the GDD value is zero. If the mean temperature is above the base temperature, then the GDD equals the value of the mean temperature minus the base temperature. If the low temperature of the day is below your crop or pests' base value, use the base temperature during your calculations.

Example #1: tracking corn development –

Recorded high temperature: 80°F

Recorded low temperature: 60°F

Corn base temperature: 50°F

Mean temperature = 80°F + 60°F/ 2 = 70°F

GDD = 70°F – 50°F = 20

In the 'example #2' below, the recorded low temperature (45°F) was below the crop's base value (50° F) so we utilized the base temperature during this calculation.

Example #2: tracking corn development –

Recorded high temperature: 75°F

Recorded low temperature: 45°F

Corn base temperature: 50°F

Mean temperature = 75°F + 50°F/ 2 = 62.5°F 

GDD = 62.5°F – 50°F = 12.5 

Additional modifications exist for high temperature cutoffs. The growth rate of many crops and pests does not increase as temperature increases above a certain point. For example, corn development levels off at 86 degrees Fahrenheit. If the high temperature for the day is above 86 degrees Fahrenheit, use this value instead of the actual high temperature.

Example #3: tracking corn development –

Recorded high temperature: 93°F

Recorded low temperature: 74°F

Corn base temperature: 50°F

Corn cutoff temperature: 86°F

Mean temperature = 86°F + 74°F/ 2= 80°F

GDD = 80°F – 50°F = 30

People often utilize a calendar to predict plant and insect development for management decisions. However, calendar days are often misleading, especially during early growth stages. Research has shown that utilizing GDD provides a more accurate physiological estimate than calendar days alone. Slight deviations in development can be expected if the crop or pest becomes limited by mechanisms other than heat, such as moisture or fertility. The ability to predict a crop's growth stage, relative to insect and weed life cycles, facilitates better management.

Resources

2020. Climate Smart Farming Growing Degree Calculator. Cornell Institute for Climate Smart Solutions. [Accessed April 20^th, 2020].