The hard to find Rhododendron 'Cinnkeys' has tubular blossoms that look more like a Cyrtanthus falcata from South Africa ( a bulb in the Amaryllis Family) than it does a Rhody. It can be awesome in a container, if you can find a plant. Image from the Quarterly of the ARS.
I am addicted to the Rhododendron species that are more tender, particularly those which require protection from the cold winter temperatures we get here in New England. If I lived in Seattle, Oregon, New Zealand or the United Kingdom where the climate is both cool in the summer, and winters are wet and temperate, growing these rarely seen Rhododendrons would not be a problem. But most of us live in areas where the temperature shift is too wide, where extreme heat or extreme cold makes any success with tender Rhody's nearly impossible.
The Rhododendron nuttallii form named 'John Paul Evans' is large, but magnificent when in bloom. At five feet tall when mature, it would require a redwood tub. ©singingtree gardens.
With a cold greenhouse like mine, where winter temperatures remain near 40 deg.F in the winter, I've discovered that tropical and tender Rhododendrons thrive. Most of them perform excellent in pots, where they can be hauled out in the summer and kept under the shade of trees. They must never dry out, but with proper care, many of these plants ( or small trees) can make spectacular specimen plants. There was a time when estate greenhouses and botanic gardens often had displays of tender Rhododendrons, but today, only a few people grow them as container plants, since modern homes have atmospheres that are too hot and dry with central heating, and modern luxuries like insulation and reliable thermostats.
If you happen to own an old home, you may have some luck growing these plants indoors, with the best success coming from an unheated room, where there are large windows, and perhaps a window seat which has been converted into a plant window with gravel in trays for added moisture. Ideally, success will come easily with a glass greenhouse, or a conservatory, where the air is kept cool and buoyant. If you are interested in more info, check out the American Rhododendron Society, and the Rhododendron Species Foundation for details and links to nurseries. For Vireya's, check out the Vireya net site.
The Vireya type Rhody named Rhododendron macgregoriae from the Vireya net site.
The classic Rhododendron species for conservatory culture in containers are the Asian species, particularly R. fragrantissimum, R. Fragrant, bell shaped white blossoms, which include R. edgeworthii, R. nutallii and others. Perhaps the most tropical provide the brightest colors, and those would be found in the group known as the Vireya's ( most are native to the cool misty mountain slopes on the island of Borneo). Few nurseries sell Vireyas, but Bovees Nursery in Oregon is the number one choice, and this is the best time of year to order them.
Vireya R. crassifolium has unusual shaped flowers. ( Bovees Nursery)
R. macgregoriae ( from the Vireya Net)
R. sainpauli, a new species with very interesting flower. It would be great in a hanging wooden slat basket, as many Vireya like.