JARS v51n4 - Watts With the Species: Rhododendron calostrotum

Watts With the Species: Rhododendron calostrotum
Lynn Watts
Bellevue, Washington

Rhododendron calostrotum* was first discovered by Frank Kingdon-Ward in July 1914 on Mwai divide, northeast Upper Burma. In later years this species was discovered by other collectors in east Upper Burma, northwest Yunnan, southeast Tibet and in Assam. In these locations it is generally found in open locations on rocky hillsides and meadows and at times in moorlands and along mountain streams at altitudes ranging from 10,000 feet to 16,000 feet.
Rhododendron calostrotum has flat saucer-shaped flowers of a rich pink to rosy-purple with crimson spotting in the throat. The leaves are small and rounded, bluish green on the upper surface and densely scaly below of a cinnamon color. The foliage is delightfully aromatic. It blooms in the Pacific Northwest in April to May. The plant is hardy to at least -5°F.

In his book Rhododendrons of the World , David Leach describes R. calostrotum as "a 15-inch, free blooming under shrub in various shades of pink...occasional robust forms may grow to 4 feet." Peter Cox, in his book The Smaller Rhododendrons asserts, "One of the finest of all dwarfs, especially in 'Gigha' FCC 1971 which has masses of rose-crimson flowers over compact glaucous foliage." Quoting from H. H. Davidian's The Rhododendron Species Volume 1, Lepidotes : "A cushion or compact rounded or compact spreading, broadly upright, prostrate or matted shrub, rarely epiphytic..."

On a later page Davidian states, "Six distinct forms are in cultivation: Form 1. A low compact spreading shrub with pale silvery-bluish leaves and rose-crimson flowers, known as the 'Gigha' form. Form 2. A broadly upright shrub up to 1 foot high with claret flowers, sometimes known as the "claret" form. Form 3. A compact rounded shrub 2 feet high and as much across with pale grayish leaves. Form 4. A broadly upright shrub 3 feet high with fairly large leaves and rosy-purple flowers. Form 5. An upright lax somewhat straggly shrub, 2 feet high with small flowers, which was known as R. riparium Form 6. A large somewhat rounded shrub 4-5 feet high, with large leaves, upper surface bluish-green, and with crimson-purple flowers, known as 'Rock's 'form."
A unique variety of R. calostrotum having smaller leaves and pink to rosy-purple flowers known as R. calostrotum ssp. riparium was first collected by Frank Kingdon-Ward in Upper Burma in 1926. This form tends to be very open growing particularly if placed in a somewhat shady location. Both Cox and Davidian agree that this is an attractive plant and worthy of inclusion in any rock garden. As most rhododendron species collectors are aware, there exists considerable difference of opinion among the taxonomists as to the proper placement of many rhododendron species. The common terms which have been applied to two of the positions are the "lumpers" and the "splitters." Each position has its merit and, quite honestly, the lumpers do some splitting and the splitters do some lumping; it's all just a matter of degree.

Whichever position you prefer, it is apparent that R. calostrotum is a somewhat variable dwarf rhododendron. In any case, the beautiful gray-green foliage and the bright rosy-purple flowers combine to produce a charming addition to any rhododendron collection. In selecting "the" R. calostrotum for your garden I agree with Peter Cox, the best form is 'Gigha' (pronounced "Ghia" as in the Karman Ghia).

* The revised classification recognizes four subspecies of R. calostrotum : R. calostrotum ssp. calostrotum , R. calostrotum ssp. keleticum , R. calostrotum ssp. riparioides and R. calostrotum ssp. riparium .

Lynn Watts is the ARS Western Vice-president.