I’ve come across the concept of Heating Degree Days or Cooling Degree Days in work I’ve done (basically the number of days where the temperatures has crossed some specific threshold where heating or cooling of a building is required) and have used it for some years, but only in the last couple of days I’ve also come across the concept of “Growing Degree Days”, which is a similar measure where the temperature exceeds a given value (often 5°C, it would appear) that allows plants to grow.
As an example, if the threshold is 5°C, then four days where the average temperature was 8°C would constitute twelve degree days (the sum of the actual temperature less the threshold for each day).
It seems that research may support the idea that to within a relatively small margin, specific varieties of plant reach the point of flowering once a given number of degree days have occurred, I think from the starting point of the plant having developed two true leaves.
Whilst flowering isn’t an issue for quite a few vegetables because we eat them before they get that far, I think it might be interesting to track the numbers for those that I do want to flower, so this year I’m going to try to record the necessary data. If it proves practical then I can continue to do so and see how the figures compare year on year using the data from my weather station to calculate the “GDD” figures.