I hold mine over in a similar zone in my unheated attached garage in plastic bags full of barely moist peat, then placed in a cardboard box, wrapped the outside with beach towels. This keeps them away from freezing and thawing extremes but yet insulated enough to get the cold period they need without the risk of rotting that trying to winter in pots through weather extremes risks. I have kept potted lilies safe for repotting for many years this way. I check them in the box late February or early March. I pot them up and leave them in the garage as spring warms, then ease them out to their summer location late April.
