Hi Caroline! You can overwinter pretty much any dormant perennial in a container. If it's a hardy perennial, the root can freeze solid when dormant with no ill effects (although continual freezing and thawing should be avoided). If it's not frost-hardy, then you'd need a storage area that doesn't freeze, but is cool and dry so that the root doesn't start into growth. I've successfully overwintered in my minimally-heated greenhouse various containerized perennials that I didn't have time to plant in the garden before freeze-up, and also some potted sub-shrubs not hardy in my zone, such as lavender, culinary sage, rosemary, and fuchsia. The hardy plants were on the greenhouse floor and froze solid, while the others were raised up on shelving and they got a bit of warmth from the heater (but no more than a couple of degrees above freezing).
