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Species In Our Midst
Rhododendron calostrotum

by Dick Brooks


Rhododendron calostrotum
Picture by S & J Perkins
Rhododendron calostrotum growing in Federal Way, WA


Rhododendron calostrotum

This lepidote species is a member of Subsection Saluenensia, a group consisting of two species and several subspecies, which were formerly classified as distinct species. R. calostrotum itself encompasses four such subspecies: ssp. calostrotum, ssp. riparium, ssp. riparioides, and ssp. keleticum, the last including the taxon formerly known as R. radicans. The species is native to the mountainous areas of northeast India (Arunachal Pradesh), northern Burma, southern Tibet, and northwest Yunnan province in China. It occurs on rocky slopes, alpine meadows and cliffs, at elevations of 10,000 to 15,000 feet. Its discovery is credited to Frank Kingdon-Ward in 1914. The specific epithet 'calostrotum' translates as "with a beautiful covering", but it is not clear whether the description refers to the typically very free-flowering plant covered with flowers, or to some finer botanical characteristic such as the dense covering of scales on the undersurface of the leaf.

R. calostrotum is variable in plant habit, ranging in height from a few inches up to 4 feet. The dwarfest forms, with a prostrate habit, are found in ssp. keleticum. The branchlets are smooth and the rounded leaves, up to 1 1/2 inches long, are clad underneath with dense overlapping scales. The flowers are borne in clusters of one to three, on pedicels that hold them up above the foliage. They are large in relation to the diminutive scale of the other plant parts, and widely funnel-shaped or rotate (saucer-shaped). Their color ranges from light pink to magenta or purple. An especially fine form with rosy-crimson flowers has been named 'Gigha'. Blooming period in our area is usually early May.

This is a fine species for the rock garden or similarly confined spaces. It is reputed to be of easy culture, but I suspect this is true only in more temperate climates than inland New England. Greer rates its hardiness at -5°F, and like so many alpines it seems to have an aversion to our hot humid summers. Our neighbors in Maine and Nova Scotia who live near the coast have a better chance of success.

With few exceptions hybridizers have overlooked R. calostrotum. Bulgin lists only three hybrids with this species as a parent: 'Cindy', 'Grouse' and 'Wigeon'. The last of these has profuse lavender-pink flowers with some of the open-faced charm of R. calostrotum, and a more adaptable constitution.

Dick Brooks, Concord, MA


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