I can't help in a positive way, but here's a thought.
Isn't the hardiness of a cultivar difficult to express as a Hardiness Zone?
One unusually cold winter, and plants that are "hardy in your zone" will die in droves.
Also, the USDA Hardiness Zone doesn't capture problems like long springs that oscillate between thaws and frosts, or a late, mild fall that suddenly crashes right down to an average winter minimum.
I usually think of any quoted "hardiness range" as only a fuzzy guide - like Plus or Minus two or even three zones.
Some years, plants will survive far outside their "safe zone" and will even survive an unusally cold winter, if the cold developes gradually and steadily, and there is lots of snow cover.
Other years, "allegedly safe zones" will kill plants supposedly hardy 2-3 zones colder than your average.
I'm always reading comments like "they SAY this is only hardy to Zone 6, but I've had it come back for me in Zone 4 for many years".
But what do I know? I live in Zone 8!
