I think that there may be drought induced death in many plants if the roots freeze in a pot, whereas in the ground though the bulk of a tuber or bulb etc, including most of it's roots, may be frozen solid, the few roots which have their tips deep enough may keep the plant going. (While frozen there is very little moisture of a liquid form available to the plant.) I particularly feel that this may be the case with Cyclamen and rhizomatous Iris. This could equally well aply to the rootstocks of woody plants. Peter (UK)