Calculating Growing Degree Days
Growing degree days have traditionally been used in order to have a common language in describing crop maturity. People from different areas of the nation, even the world, can compare how their crops mature, or choose the crop type and variety based on growing degree day (GDD) accumulation.
Calculations of GDD can be completed using a number of methods, two of which are the Averaging Method, and the Baskerville-Emin Method.
Using the Averaging Method, the following formula is applied:
(Maximum Temperature + Minimum Temperature) / 2 - Base Temperature
The base temperature for many growers is 50 degrees F, or 10 degrees if you are using a Celsius scale. This base tempearture can vary with the crop for which you are trying to calculate GDD. If a negative number results, then there is no accumulation of GDD for that day. A positive number results in GDD units that accumulate over the growing season, giving an indication of crop maturity.
The Baskerville-Emin method uses a mathematical calculation which proves to be more accurate but more difficult to calculate. There are a number of information networks that gather weather data and calculate GDD using this method. However, GDD have no units; thus the two methods can be used without translation.
One note of caution, be sure that the base temperature correlates with the temperature scale that you are using (i.e., Celsius vs. Fahrenheit). For a Celsius based system, the base temperature is 10.
For more information, check out:
http://www.maes.msu.edu/nwmihort/gdd_calc.html.
Dr. Mercy Olmstead, Extension Viticulturist
An example of growing degree day accumulation compiled by the WSU Viticulture Research. L. Mills