Calculating Growing Degree Days

Published online: May 09, 2022 Articles Nora Olsen & Mike Thornton, University of Idaho
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Snow, hail, rain, sunshine and wind appear to be interchangeable within a given day this past week. We welcome the moisture, but overlay these conditions with prolonged cold temperatures and potatoes already planted are sitting in the ground waiting for warmer weather to start to grow.

To focus on the temperatures so far this month, one method is to look the effect of temperature on the potato crop is by calculating growing degree days (GDD) or heat units. 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 Aberdeen (all 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. As expected, the GDD calculations for Parma and Kimberly stalled on April 8, indicating not much sprout development has occurred since then. The GDD this past week is a stark contrast to the GDD from 2021 and the 9-year average, indicating a cooler than normal spring so far.

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

Aberdeen GDD was not available at the time of this article, but will be posted on the website when available.