Calculating Growing Degree Days

Learn how to calculate growing degree days for a more accurate prediction of when things bloom and more successful honey production.

September 17, 2012

By Kim Flottum

There’s a simple technique for starting your own growing degree days (GDD) database, but first a ground rule. Plants essentially don’t grow if the ambient high temperature for the day is more than 86°F (30°C), or the low temperature for the day is below 50°F (10°C), so these two extremes must be noted in all calculations.

The basics of calculating growing degree days is as such:

Using various resources or media (such as weather websites or networks), find the high temperature for the day, add it to the low temperature for the day, and calculate the average by dividing by two. Subtract that number from the base temperature you are using (which is 50°F [10°C]).

If the high temperature for the day was 84°F (29°C) and the low was 60°F (16°C), the calculation would be:

Remember the extremes. If the high for the day was 90°F (32°C), the low 40°F (4°C), the calculations go like this:

The first question should always be, “When do I begin calculating this figure each season?” The simple answer is: a few days before the last frost date in your area. This annual event is available from gardeners, Extension offices, most university Web pages, and governmental information sources that feature agricultural information.

Alternatively, any local program (check local university or grower organization Web pages for this information) can provide the information, then refer to your records. The correlations are easy: When did maples bloom? When the local growing degree days reached a certain point. And they’ll reach bloom when the growing degree days reach that point every year, rain or shine, sun, snow, or sleet, no matter what the calendar says.

Even though information for, say, apple growers, is being tapped, the data can be used to predict all manner of honey plant bloom dates. If you have a good history of those dates and access to past dates for agricultural growing degree days, the rest is easy. If not, a year or two of careful record keeping will provide the basics.

Read more: Learn how plan for your best honey crop yet in Producing and Selling Honey for the Backyard Beekeeper.

This excerpt has been reprinted with permission from The Backyard Beekeeper's Honey Handbook, published by Quarry Books, 2009.

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