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 charts, the southern locations of Fairfield and Red Oak have accumulated more growing degree days so far this year than the state average and the towns in the northern part of the state. The bottom graph shows a comparison of GDD for the years 2012 to 2015 at the various locations. As you can see, the state average is slightly above 2013 and 2014, but other locations such as Des Moines are actually below the previous years.
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