Growing degree days (GDD), also known as heat units, are calculated by taking the average of the high and low temperature in a day, minus the base temperature of 50°F. The optimal number of growing degree days varies depending on the crop and maturity. As you can see in the graphs below, the Des Moines and Red Oak locations have accumulated more growing degree days so far this year than the northern locations. The bottom graph shows a comparison of GDD for the years 2012 to 2016 at the various locations. As you can see, the 2016 Iowa average GDD are currently ahead of 2013 and 2014, but are behind the pace of 2012 and 2015.
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