Rhododendron kiusianum: An Ideal Rhododendron For The Rock Garden
Frank Dorsey, North Vancouver, Canada
Reprinted from the Vancouver Chapter newsletter.
All but the most rabid rhododendron enthusiast will admit that the list of rhododendrons suitable for growing in a rock garden is quite limited. The two obvious restricting factors are size and hardiness. Most rhododendrons are simply too big. Some, mainly hybrids, are small of stature but their leaves and flowers are of a size that is out of place among alpines. Others, while of an acceptable size "in all their parts", demand conditions which are difficult to meet in a rock garden. Most true alpines thrive in full sun - most rhododendrons demand some shade.
It is said that alpine rhododendrons grow on Chinese mountains as calluna, the purple flowering heather, clothes the hills of Scotland and as cassiope and phyllodoce cover the slopes of the mountains of Western North America. It would appear then that some rhododendrons must be suited for rock gardens. There is, in fact, a fair number. Many of us grow them. The greatest number is to be found in the Lapponica subsection: impeditum, russatum, intricatum and fastigiatum are but a few. The Saluense subsection provides us with the various forms of calostrotum, with keleticum and with saluense itself. In the Uniflora subsection we find the choice but difficult ludlowii, and the easy pemakoense, pumilum and uniflorum var. imperator. Rhododendron ferrugineum, the "Alpenrose" of Western Europe is yet another good rock garden subject and has the advantage of being late blooming.
Possibly the best of all low growing, sun tolerant rhododendrons is kiusianum. Its name is derived from Kyushu, the southernmost of Japan's four main islands, where it is to be found at heights of from 1200 to 1700 metres (4000 to 5500 feet).
The Rhododendron Handbook, The Royal Horticultural Society, (1980) states that kiusianum is an evergreen or semi-evergreen shrub up to 4 feet. Presumably under some conditions it will eventually reach this height. Grown in full (or near full) sun in our area it is unlikely to exceed 18" in ten years. It has the happy capacity of conforming to the slope of the ground, a trait which makes it particularly suitable for the rock garden. Its leaves are described as "dimorphic". (I had to look that up — it means having two shapes.) The first leaves to appear do so immediately following the blooms. They are larger than the second set which come in about July and surround the new flower bud. The larger leaves usually drop off in October although in mild years they will hang on until spring. The smaller leaves provide colour throughout the winter.
The flowers, in trusses of two or three, vary from a light pink to purple. (The commercial growers' catalogues rarely use the word "purple" preferring the more poetic "lavender" or "lilac".) There are white forms which are usually given the name 'Mount Fuji'. I have a white form which I know is the true 'Mount Fuji'. Garth Wedemire has one which he knows is the true 'Mount Fuji'. I have seen at least six other true 'Mount Fuji'. They're all quite different. Rhododendron kiusianum is a prolific and reliable bloomer and covers itself with flowers year after year. In our area they bloom about mid May, consequently they are unaffected by April and even the occasional early May frost.
Some work has been done in the Northwest on developing or isolating superior forms. One of the most unusual is the bi-colour, 'Komo Kulshan', with its pale pink centre and purplish red margin. It is unusually strong growing and I suspect may have some kaempferi blood in its veins. The ARS Seed Exchange usually lists a few lots of kiusianum seed (or crosses). Seedlings can be expected to bloom in about four years. Cuttings should be taken by the end of September and can be classed under "rootability - high."
As is to be expected most of the named forms come from Japan. Fred Galle in his book Azaleas lists some sixty forms, most of which were brought to the United States from nurseries in Japan. For some reason the plants from Japan have Japanese names. It really would be much better if they gave them simple, descriptive names as we do; names like 'Souvenir of W.C. Slocock' (sounds like a memorial to the president of the plumbers' union).
I have seven or eight forms but my favourite is one I grew from a cutting Margaret Charlton gave me some ten or twelve years ago. It's my recollection her plant won the award for the best in the show. Only recently I learned that her plant, a luminous pink, came from the late Ed Lohbrunner who, in turn, got it from Koichiro Wada (from whom all good things flow).
One last point, although I have stressed the suitability of kiusianum as a rock garden plant, it is equally suitable for the front of a border and, equally important, makes the perfect bonsai.