One of the weaknesses of the growing degree day concept, and similar climatic indices


I remember back in my old job, when we had a scientific conference on the nature of intelligence. It’s a topic you approach with caution. Nature versus nurture? Genetics versus environment? Which wins?

One of the concepts that got me was that we assume that two kids living side by side would be exposed to a common environment, and then any difference you see would be as a result of genetics.

But there’s a flaw in this thinking of common environments: the same environment will be experienced differently by different kids. So, take for example a library. The same library will be a very different environment to a smart, bookish kid than to one less academically inclined. A sports field will be a different environment for a sporty kid compared with a lanky uncoordinated one.

So it is with vines.

Take a similar growing season in terms of temperatures, experienced by a vine that starts off fast versus one that starts off slowly. The increased canopy size of the former will mean that, subsequently, even the same GDDs or light hours will be experienced differently by the two vines.

A vine with more leaves will get more from the same heat/light environment than a vine with fewer. Even an identical number of GDDs can have very different effects on two vines. Early season GDDs count a lot more than later ones, because the benefit of later GDDs will be incrementally more, because the vine will make more of those later GDDs with its bigger canopy.

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