Years back when I used to order all my Asian pears from Fowlers nursery, a salesman told me that their betula had been obtained from much farther north than most of what is in this country and that it is quite hardy. I haven't lost trees to winter kill from them during winters that have gotten down to -18F or so, if that's any indication. Haven't tested them in a real cold winter to see if it matches the toughest betch.
[NAFEX] hardy betula stock,
Alan Haigh, 09/04/2009