Thanks for the ID, guys - you're so great!
Mj, the tree canopy does give quite a bit of protection. Remember the P. gloriosums I sent you? I have two of them in the ground under my biggest oak, and even after temps down into the 30's last week they're still holding up leaves. So is my little Polly which is really a wimp usually.
If they're lusty enough to climb to the tops of the trees then even if the tops get nipped by the frost they should survive. Mulched heavily with oak leaves, of course.