I think the fernies will survive in zone 3 which would be much colder than -10. I have seen the frost line or freeze level as much as 24 inches underground when the temp hit -15 and the fernies in that area survive just fine. Apparently, freezing does not bother them. Probably better for them rather than keeping them above freezing. I plan to leave mine out all winter and let the strong ones survive. No need to coddle them. Same way with some seeds which I had harvested from my species P.tenuifolia (single). The cc containers just fit in the hollowed sections of the concrete block walls of the beds and will offer some shelter from any extreme cold but one could gather all the pots together and throw an old blanket or quilt over them for the winter.
I chose sand this time for a cover over the crowns instead of potting soil because last year the potting soil seemed to hold too much moisture. We shall see next Spring if that was a good choice.
