Year old Hollier and Improved Celeste (larger caliper, 3/4 inch) came through the winter with minimal die back (less than 6 inches at 4 feet above soil line), after being exposed to 20 deg F and dips to 15 deg F., before being stored under cover. The O'Rourkes were killed to the soil line, they were all in 5 gallon buckets. I didn't have White Triana last year, but will be trialing it this season. They will all be planted in ground eventually, with some protection.
<edit> O'Rourke was cold hardy... the tree that had dieback to soil line was LSU Tiger, it was mislabled from the nursery...