I hate to say it , but I'd guess perhaps an even thicker mulch might be required in those cold zones, but it of course depends on how cold the winter turns out to be, and some years in my Michigan garden I barely have the ground freeze more than an inch or less, but any freezing of those rhizomes should the ground freeze, which is possible even using a very heavy mulch, and those won't survive the freeze. I'd guess however, some may take freezing better than others, and of course it might depend on having ground that isn't overly soggy besides cold!! I've not had any of my costus barbatus survive after a cold winter outdoors, though they didn't get extremely heavy mulching, but the ground was a bit wet as welll.