Cals aren't reliable hardy where I am but surviving winter isn't purely about night low temps, but about soil temp. It can get down to single digits on occasion here, but even after such a night, the next day would probably at least 45 and more like 55. Avg low here for the coldest nights is 31, avg high during those days is 55-60.
Several things can help keep the ground warmer, like a wall, path, or patio, (especially brick or other dark color,) organic matter - anything from a light layer to a several-foot-high pile of leaves, adjacency to a big rock. Access to sunshine is important, trying to zone-push/cheat on the north side of a structure (for those of us in the N hemisphere) isn't something I'd recommend if other options exist.
The other aspect that seems to be equally important is drainage. For many species, cold + soggy = death but cold + more dry or moist in a not-soggy way can = survival.
My Mom has planted Cals under her oak trees a few times but they never come back. Too dry & at the time, very infertile soil. (We've been improving that over the yrs by using the leaves as mulch in a bed under the trees after decades of mowing under there & the leaves having been removed from the property by the original owners. I might suggest she try some again this spring.) I put mine in a bit more sun in spots where the soil is much more rich & fertile (and thus, well-drained) and haven't had any NOT return. She only lives 2 miles away. Since I'm not also augmenting the cheat with a pile of leaves, I can only attribute the difference to the improved drainage & fertility/tilth that comes over time when soil has been continuously covered with some kind of OM.
All of this starts to get into soil microbiology, which is fascinating from the aspect that ones' plants can only be as great as the soil in which they're growing. If you haven't seen it yet, I recommend listening to this short (15 mins) lecture (which sounds nerdy but microbiologist Dr. Elaine Ingham speaks in normal person terms.)
http://permaculturenews.org/20...
