I'm amazed that anyone gets much use out of the zones. They say how cold an "average" winter got, over the last 20-30 years.
Do we buy expensive plants based on their ability to survive HALF of our winters?
I might care if I knew what temperature only one winter in the last 20 years went down to.
We could look at the "rating" for each zone, like "8b gets down to 15-20F in half of all winters". I assume then that a plant rated for "Zone 8b" would be likely to have problems WHEN the air temp actually goes below 15-20F ... ignoring factors like how rapid the drop was, whether plants had had time to vernalize, how much wind, snow cover, exposure, slope, etc.
Or do plant recommendations for zone hardiness try to take the 50% into account and allow another 10 or 20 degrees to cover slightly unusual winters? I don't know that, but "trial and error" sound good to me.
