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How to Calculate Growing Degree Days

You can calculate Fahrenheit Growing Degree Days (F-GDD) from weather data on the internet.

  • Visit uspest.org/wea/. If a page will not load, try refreshing it. 
  • Click on Map Index. This will bring you to an interactive map of the U.S.
  • Click on your state. A pop-up box will appear showing interfaces available.
  • In individual networks, select METAR, a NOAA site.  Now you will see the Heading, “Degree-day and Phenology Model Calculator.”
  • The first task is “Select Model”.  Hit the down-arrow on this bar and go to the bottom of the drop-down list to “winter wheat Karow et al 1993” and select that.
  • Next set the lower threshold to 32 and the upper threshold to 130.  Ignore the box asking for Calc. method.
  • Now enter your start (planting) date or, if you dusted the crop in, enter the date of the first significant rain after planting.
  • Then enter end date, for example: Sept. 25, Nov. 11.  Depending on your date choices select same year or following year.
  • Ignore the boxes asking for Forecast zip code and Calc. selection.
  • Now go down into the table and find a weather station near you, then in that line, select the correct year and, if needed, hit “CALC.”
  • Cumulative F-GDD will be shown in tabular form with plant milestones.

For more information contact Tami Johlke, USDA-ARS Biological Research Technician.

Washington State University