Growing Degree Day 278 is upon us

GDDs are calculated by taking the average of the daily high and low temperature each day compared to a baseline (usually 10 °C). As an equation; GDD=(((High+Low)/2)-Baseline) GDDs are typically measured from the winter low.
For example, a day with a high of 23 °C and a low of 12 °C would contribute 7.5 GDDs. Any temperature below the baseline is set to the baseline before calculating the average. For example, a day with a high of 13 °C and a low of 7 °C would still contribute 1.5 GDDs. Likewise, the maximum temperature is usually capped at 30 °C because most plants and insects do not grow any faster above that temperature. However, some warm temperate and tropical plants do have significant requirements for days above 30 °C to mature fruit or seeds. Well what's all this got to do with the price of fish.



The Catalpa tree (catalpa bignonioides) starts flowering after 275 GDDs; this one started flowering three days ago, so in this part of leafy England we are on GDD 278.

What is the significance of this ? Well GDDs provide a means of predicting when a particular plant will crop or flower in any country in any part of the world. So if a packet of seeds had a GDD value printed on it you would be able to plan when to sow to provide a successional crop rather than the familiar summer glut.

I am told that commercial growers rely on GDDs but as far as I am concerned the pleasure in gardening is it's capricious nature so I'm sorry agronomists you can keep your GDDs - it's back to guesswork & glut for me.