This species can grow quite tall in its native Tibet, China, or Vietnam, where it is found both in the open and in heavy forest areas at great heights. In Yunnan we encountered them at over 100 feet tall and 8 feet in diameter!
Here in Western Washington it is a slow grower, but over time could become a large domed shrub/tree. Flowers may range from pale rose to crimson-purple. Very old R. protistum plants in Scotland have bloomed bright red. Tortured in a small pot, ours, grown from seed Bob collected in Yunnan, bloomed in a mere ten years and now has reached six feet while living in our greenhouse.
Best if you grow it in a pot, as the early and lovely growth is subject to damage by frost and the plant itself will not survive a hard freeze. Bring the pot into the garage on sub-freezing days.
This rhododendron will like: filtered sun or shade, small amounts of time-release fertilizer once a year in the spring, gentle watering, well aerated soil with good drainage, and, of course, protection from both the cold and strong winds.