Yes, Lilium rosthornii is perfectly hardy for me. I live at the western edge of Hennepin County. Similar to Lilium henryi, but not so much tendency for the stem to turn to the horizontal in the upper half. A more refined flower, typical of most species compared to hybrids, IMO. I have never done anything special for it - 5 years now. As long as it has drainage, there should never be a problem.

Rick