I am reflecting on the nature of cold tolerance and hardiness zones as I survey the winter damage to my plants, both outdoors and in my unheated greenhouse. I gambled heavily last year and collected tons of stuff that I thought MIGHT survive here based on accounts read on this and other forums. As I was mourning the loss of my Seashore Mangosteens and Achachairu that were in the greenhouse, it really hit home just how worthless hardiness zones are.
If I read on this forum that someone in Florida zone 9b had success with a plant, I felt it was worth trying, because I am in zone 9b in California. That has turned out to be TOTALLY WRONG. My message to future California zone pushers reading this: ignore every bit advice you get from Florida growers.
It really doesn't matter how cold it gets - it matters how cold it stays. Correct me if I'm wrong, but my perception is that if it gets down to 28F in Florida it might stay that low for 30 minutes, while in California it could hold for hours. The Florida humidity makes the effective temperature much higher, and that afternoon you're back into shorts and a t-shirt. In California after a frost when it "warms up" you are still going to want a sweater. So, injured plants in Florida get the warm conditions to promote healing almost immediately, but in California the ambulance won't arrive for months.
Someone should really develop a model for plant hardiness that accounts for humidity and total chill hours in addition to absolute lows.