I doubt you have anything to worry about. Here in Illinois, the foliage almost always freezes, and turns brown, then drying up like any "spent" leaf would do. Trim off the brown, dead parts of the leaves, and you will be good to go ! We just got home yesterday from "wintering" at Dauphin Island, Al., and you're right, it got colder than usual, even there! In "heavy" soils (clay, etc.) you need to make sure that you DO have adequate drainage ( no standing water) and that the rhizome ,itself, is not "buried' (covered up with soil), just the roots should be in the soil, and possibly the bottom half of the rhizome. The top of the rhizome should be sticking out of the soil, so the sun and airflow can "dry" it....Arlyn
