I feel one of the biggest problems, Karyn, is that plants cannot read zone labels. I have been moved to zone 8, but believe me, there are many plants that are hardy to zone 8 that WILL NOT survive where I live. I think it purely just experimentation. All of my brugs inground returned, and are more vigorous this year than last. That is probably because of the cool, wet spring we have had. Now, if I get a winter where we have prolonged days of freezing weather, who's to say whether they will survive or not. This is just my opinion, for what it is worth.
USDA Zone 7/8
Copper Canyon
North Central Texas