Applying growing degree days to beekeeping

A few years ago Dave Barr introduced me to the concept of Growing degree days(GDD), a calculation of accumulated heat that helps you to predict the blossoming of plants and trees with pretty good accuracy. It's no subsititute for physically looking around you and taking note of what is in bloom, but following the GDD might help predict the timing of flows and corresponding seasonal bee behaviour.

See wikipedia for a detailed explanation on how the calculation is made and flowering time of various plants.

Use this calculator to check the GDD for your area. All GDD references are given using base 10 throughout the rest of this post.

The spring has been excessively warm this year in Toronto. We jumped up to 25 C GDD in the last week. The average for this time of year is 0 C GDD and in 2010, which seemed like a warm early spring at the time, we were only at 3.5 C GDD. Normally this might be a bit early for silver/freeman maples, but just as you would expect from our current GDD, they are just finishing their bloom

freeman maple and native bee

and the native collettes have emerged in time to profit from them and mate:

What I have noticed over the past few years is that bees will start returning to the hive with full pollen baskets as the growing degree days rise just above 0 C.

full pollen basket on honey bee

One of the more detailed theories on bee behaviour in early spring I've been able to find is on Michael Bush's site. It attempts to relate certain changes in hive behaviour to bloom times. You may be in a different region than Michael, and things may progress at an unusual rate this year, but I think it will be interesting to keep an eye out for any of the correlations he mentions in the next month or two.


- If the brood nest starts contracting before the peak of the apple blossoms - they may be switching from buildup to swarm preparation. Look for backfilling, or patches of nectar surrounded by brood like in the photo below.

backfilling the broodnest

- When the Black locust blooms (140 C-160 C GDD) should be the start of the main flow and the interest in initiating a swarming should be reduced.

- Established colonies start making white wax shortly afterwards.

My hypothesis is, that for here in Ontario, around 70-100 C GDD might be a good time to look for the first signs of swarming preperation, with Queen eggs most likely appearing around 115-160 C GDD. However, I imagine with the large amount of stores the bees still have after what was a mild winter, they could run out of space even before this.

Update 2012: Short mild winter, I saw the first queen egg in one of my hives at 64.5 GDD. You can get a quick peek of it behind the bees:

The first swarm in my area was a few days later at 65.5 GDD or about three weeks after the dandelions first bloom, two weeks after the apples and at about the time the lilac trees were in bloom.

forsythia swarm

2013: I observed eggs and larvae in queen cups at 88.5 GDD. A little under a week after the apple blooms, two weeks after the dandelions and about the time the lillacs bloomed, but it's possible they were supercedure cells. It was a cool and wet spring, and I'm not sure there was very many reproductive swarms. I did find a feral colony that had moved in sometime before 95 GDD, but the first report of an actual swarm in the area did not come till around 265 GDD.

2014: Very, very cold winter, with a long cool spring. First report of a queen cell came from a friend around 50 GDD at the time the apple bloom started and two weeks after the dandelions. The first swarm report I heard reported came four weeks later at 252 GDD, about a week after the black locust bloom.

3 thoughts on “Applying growing degree days to beekeeping

    1. David Vierig

      I am interested in your work in GDD. I live in GA just south of the airport 35 miles.
      D Vierig

    2. Beth Tillman


      I am interested in seeing some of your work. I live in the Appalachian Mountains, where the weather is a tad finicky and hard to predict.



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