If you want to leave them in the garden, give them a 2-3 inch layer of compost or peat (if you can get it) after you have cut them down and remember to label their position. In pots, put them in the garage and don`t let them dry-out completely, only very slightly moist. Otherwise you can chance lifting them, cut down to 4 inches, clean of any soil from the roots and wrap them in newspaper and keep in a cool place like an indoor cupboard, re-pot when weather starts to warm up.