Growing-Degree Days – Spring is Coming

We are starting to see early signs of flowers in Virginia. It has been abnormally warm this year, and the bulbs are starting to peek up from the soil. There a science called phenology that is the study of periodic cycles in biology. One concept that has come from this field of study is growing degree days, or GDD. The idea is that plants and insects require a certain level of warmth before they sprout and bloom, or emerge in the case of insects. You can calculate the GDD for your area as an estimate of when different plants might bloom for spring flowers or agricultural management

If you search for phenology or growing degree days online you will find a lot of references. There are some complex ways to calculate GDD, though the easiest is to just take the low plus high for the day and divide by 2. Typically GDD is calculated relative to 50 F, though depending on the plant 40 F may be used as well. For example, if a day has a low of 50 F and high of 60 F, then they sum to 110. Divided by 2 is 55, so relative to 50 F this is 5 GDD.

A group of daffodil bulbs emerging by our front walk.

You can also find growing degree calculators to determine the GDD in your region. If you select the near-to-me Manassas, Virginia location at that linkand set the time from the equinox on December 21 to January 12, you get a GDD of about 21 thus far. There are plenty of GDD resources online, this one from the University of Maryland indicates that one of our early lawn weeds Veronica persica (Persian Speedwell) may start blooming soon after 16 GDD. I will have to keep an eye out for it the next sunny day we have. That would very early, as last year I didn’t see Speedwell blooming until February 21.

May you prosper and find honey.

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