> really amazing you can leave it with no protection there <

When I bought it, it had a tag from the production nursery where it began its life that rated it to 5a or 5b (I forget which). But I'm sure they meant a dry Zone 5. The kind of Zone 5 you'd find west of the Mississippi. As far as the east coast is concerned, I've never seen them in places colder than 7a, but we had that Yucca guy in Massachusetts (Tom, I think) who was growing them, so I don't know. Maybe 6b is where they peter out in the east.