Reprinted from the Vancouver ARS Chapter newsletter
Subsection Arborea (Arboreum series under the Balfourian classification) consists of seven subspecies of R. arboreum together with R. lanigerum and R. niveum. The name "Arboreum" is derived from the Latin adjective "arboreus" meaning "tree-like". While not the largest rhododendron (R. giganteum is said to grow twice as tall) its eventual height of 40 feet in the wild and its tree-like growth habit fully justify the name.
The seven subspecies, ssp. arboreum, ssp. cinnamomeum, ssp. cinnamomeum var. roseum, ssp. delavayi, ssp. delavayi var. peramoenum, ssp. nilagiricum and ssp. zeylanicum are, in general, differentiated by the colour of the flowers and the colour and density of the indumentum. R. arboreum is widely distributed throughout the Indian sub-continent, the southern provinces of China (including Xizang or Tibet), Burma and Indo-China. As is to be expected the sub-species from the more northern areas and in particular from the higher elevations are hardier than those from the southern and lowland areas.
R. arboreum ssp. arboreum is, as has been mentioned, usually tree-like in habit but may grow more in the form of a large shrub. Its leaves are up to 10 inches in length (far short of R. sinogrande's 36 inches). Shiny green above the leaves show a thin indumentum varying in colour from white to cinnamon. The flowers are in a very compact truss and number from 15 to 20. Typically the flowers are blood-red but frequently are of much lighter colour. It has long been considered that the higher the elevation the lighter the colour of the flower. To some extent this is being disproven as deep red forms have been found at high elevations in Sikkim, Bhutan and Nepal. These are of particular interest to rhododendron enthusiasts in the Northwest, as the low elevation forms cannot be relied upon to survive our occasional spells of exceptionally low temperatures.
Mention should be made of R. arboreum ssp. cinnamomeum. Its dark rusty brown indumentum is particularly attractive. For most of the year, in common with all of the subsection, the leaves stand up at an angle of 45 degrees. The evening sun shining on the cinnamon coloured underside of the leaves presents an impressive picture. 'Sir Charles Lemon', which is probably a form of R. arboreum ssp. cinnamomeum (but which Tony Schilling considers to be a natural hybrid with R. campanulatum), is even more impressive than the subspecies usually grown. The newly opening leaves are an unusual silver-white and the indumentum and petioles (the leaf stalks) a striking rust colour. The two remaining members of the subsection, R. lanigerum and R. niveum, are similar in many respects. Usually making a smaller plant than R. arboreum their leaves on first opening are covered with a dense white tomentum. In both cases their names describe their appearance - "lanigerum" means "woolly" and "niveum" means "snow-like". R. lanigerum's flowers varying from rosy purple to magenta form ball-like trusses. R. niveum's flower is a smokey blue, a colour rarely met with in rhododendrons.
R. arboreum and its relatives are all somewhat tender. Most are rated 5°F (approx. -15°C). Some - in particular the subspecies nilagiricum and zeylanicum - will not withstand as low a temperature (one is from central India and the other from Ceylon). Their relative tenderness and their eventual size mean that R. arboreum's usefulness in private gardens is very limited. Understandably they are more often seen in botanical gardens where shelter can be provided and their size is an asset. (Both VanDusen Gardens and University of British Columbia's Asian Garden have good specimens.) To complicate matters plants are hard to find. They are not easy to propagate vegetatively and are more often grown from seed. Seedlings are always a gamble in that you may wait ten years or more only to find the flower is a muddy, washed out pink.
Probably the arboreum (or arboreum hybrid) most suited to the average garden is 'Sir Charles Lemon'. It makes a very fine plant attractive the year round. Its white flower deserves a higher rating than the "3" given in Greer's Guide. Equally important, 'Sir Charles Lemon' is becoming available in the trade. As a second choice consider R. niveum although admittedly the colour is not to everyone's taste.