My beef with hardiness zones is that only say what temperature HALF of recent winters have gone down to. And the variation from year to year of the actual minimum temp can be 10, 20 or more degrees.
I want to know what temperature we stay above 90% of the time, or 95%.
Why would people plant things that will be killed in HALF of all winters? They only have a 1/4 chance of lasting two years.
Or is there some implied "fudge factor" used when quoting "winter hardiness zone" for a plant? Like listing a zone 20 degrees warmer than will REALLY kill it? So that each winter only has a 50% chance of getting within 20 degrees of killing it?
And all that still ignores the difference between a brief extra-cold dip in the middle of a cold winter, vs. a "sudden death cold snap" in late fall, where temps drop overnight from "balmy" to "average winter low". Lack of vernalization kills more things than some absolute "low temperature" being reached gradually over weeks.
I guess it is still useful as a guideline (don't grow bananas outdoors in the Michigan UP).
