Can a small rhododendron for zones 7-10 be wintered indoors in a colder zone? Do they still have a dormant period in warmer zones?
Z7-10 rhododendrons can be grown in containers, but indoor house temperatures and dryness would likely be fatal eventually. Dormancy or at least a winter rest period is also important for long term health. The ideal would be a cool greenhouse or other place kept just above freezing.
One thing some people do is build a cold frame attached to their home just outside a north facing basement window. It gets some heat from the basement, but hopefully not the dry air. It would be protected from sun and wind and have a more temperate temperature. It would need to be watched and watered when there was any sign of dryness.
I have built a shelf using a sink cutout in my unheated, unattached garage inside a north-facing window. I keep my new plants that haven't been planted yet there. It does freeze but is protected from sun and wind. An attached garage would be better, but would need closer watching.
" but indoor house temperatures and dryness would likely be fatal eventually. "
I keep my house in the mid 60s in winter, and some rhododendrons I had inside because I was attempting to graft onto them (yes, the _wrong_ time of the year for BLEs, but the right time for conifers) did just fine with plastic bags over their tops - kept off the foliage of course.
I just wanted to be sure it could _not_ be done in winter. But I'm not sure what I've proven because I think my technique was really bad. I have read somewhere that people have sometimes grafted in winter. The correct time is supposed to be summer after the new growth has slightly hardened. The rootstock plants were not harmed and I'll try again; one of the 3 grafts still looks like it might have taken. I'll try again. I did a little better with my conifer grafts, with 2 out of 3 I'm sure having taken.
Indoors in zone 5 is much drier and hence much more severe than indoors in zone 7.