I'm Watching this thread to find out the right answer.
My guess is that if a plant needs some chilling to thrive or bloom, the "upper zone limit" might caution us that that plant needs the coldest cold each year to go below a certain temperature.
My beef with Hardiness Zone zone info is that it shows an AVERAGE of the last 20 years' coldest moments.
So it is a statistic that tells you that 50% of all your winters will kill a given plant (ignoring micro-climates and heroic measures).
I would be more interested in a statistic that told me that 90%, or 95% of the last 20 (or 50) winters stayed below X degrees. Then I could pick plants that would only be killed by cold every 10-20 years, where I live.
How many gardeners are such gamblers that they would DELIBERATELY pick plants with a 50% chance of dieing EVERY year?!?
"Dead AGAIN? Seems like this happens every other year!! The tag SAYS Zone 8 and I AM Zone 8!!!"
