Calculating Growing Degree Days

Published online: Jun 13, 2023 Articles Nora Olsen & Mike Thornton, University of Idaho
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The talk of this spring is how cold the weather has been. Calculating growing degree days (GDD) or heat units is one method to look at the effect of temperature on the potato crop.

Plant growth and development is greatly influenced by temperature, and we use GDD to get an idea of how the plant is developing throughout the season. It also gives a great snapshot on how the current season compares to previous seasons.

Daily GDD is calculated by adding the maximum temperature and the minimum temperature, dividing by 2 and subtracting a base temperature. The base temperature is used to define the temperature at which potato growth is diminished.

To quantify this period of cold temperatures, we collated the cumulative GDDs for Parma, Kimberly and Shelley, all located in Idaho. The source of the GDD was AgriMet ( weather stations located at the three sites. The AgriMet calculation of GDD uses 50 degrees F as a minimum and 86 degrees F as a maximum. There are limitations to the use of these minimum and maximums since potatoes will grow outside of this range, but these calculations can still provide a reasonable prediction of the temperatures impacting potato growth. The GDD these past two weeks is quite a contrast to the GDD from previous years and the 10-year average.

To help provide a picture of the relationship between temperature and plant development, we will continue to update the GDD graphs weekly for the three locations and post at

The 2023 dotted green line shows a slower start in accumulating GDD units.