I have been asked for advice about overwintering clematis in pots. Bert the resident expert has been consulted and this is what he suggests. He overwinters clematis both as liners and full-grown plants. They are kept in unheated polytunnels. He advises some frost protection if it is very cold and to plant out in spring. Don’t worry if they die back a bit, as they’ll sprout again in spring. Don’t overwater. Good luck!