Idaho's Growing Degree Days

Published online: Jun 08, 2021 Articles Nora Olsen & Mike Thornton
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Source: University of Idaho Extension

"Weather whiplash" is one term being used to describe Idaho's weather this spring. Temperature fluctuations seem unpredictable: one day cold, the next hot, then back to cold, and just last week the region experienced some unseasonably hot temperatures. Add in the winds that come with temperature fluctuations and lack of soil moisture from sub-optimal precipitation, and this spring becomes even more unsettling.

To focus on the temperatures so far, one method is to look the effect of temperature on the potato crop 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 lower temperature that potato growth is diminished.

Experiencing this weather whiplash and the need to quantify it, we collated the cumulative GDDs for Parma, Kimberly and Aberdeen. The source of the GDD were collected from AgriMet (usbr.gov.pn/agrimet/) weather stations located at the three sites. The AgriMet calculation of GDD uses 50F as a minimum and 86F 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. Interestingly, even with this crazy spring weather, the GDD calculations for Parma and Kimberly put the current 2021 year comparable to the 9-year average and Aberdeen above the 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 www.uidaho.edu/cals/potatoes/storage.