For what it's worth I have three that I left outside in pots all winter (Kitsap county). I just mulched them with leaves before the handful of frosts we had, and have them against a light colored south facing wall. I had intended to bring them into the shed if it got and stayed cold, but it never did. They're starting to bud out now and so I've removed the mulch, and am just keeping an eye on the weather in case I need to cover them. (Just to clarify I have two that are real small tissue cultures I got late in the season which were totally mulched over, and one of them is a tissue culture that I got last spring which is about two foot high from the top of the pot to the top of the tree and is only mulched around the roots.)