I keep potted young trees in an unheated garage until March. The required dormancy time is species dependant, but for sub tropicals sustained cold temps for 250 - 500 hours, the more woody the tree, the longer the time. The coupling of the rest with the winter equinox and the length of day increase and corresponding sun elevation for a gradual wake-up seems to make mine happy. I move 1 year trees into compost banked tall cold frames, with special bait treats for voles and rats to protect the young trees.