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Species In Our Midst
Rhododendron weyrichii

by Ian Donovan


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


Rhododendron weyrichii

The botanists who prepared the Edinburgh revision of the Genus Rhododendron are the first to admit that it is still evolving for many poorly studied species. This is particularly true in the azaleas with "A revision of Rhododendron IV: Subgenus Tsutsusi" (Chamberlain and Rae, Edinb. J. Bot., 47/2, 1990) where the authors report that they were neither able to view the plants in the wild nor study them at herbaria in China and Japan. Peter and Kenneth Cox in their Encyclopedia (1997) consider the revised azalea Section Brachycalyx, where we find Rhododendron weyrichii, "only provisional and of limited value." They recognize the research by the Japanese botanist Hatusima (1969), which reduces three closely related species to one--R. weyrichii. They establish a Weyrichii Alliance that also includes R. amagianum and R. sanctum, differentiated by characters that are not taxonomically significant.

R. weyrichii is a low shrub in cultivation to almost 15 ft. (5m), often tree-like and more vigorous in nature. Its leaves are broadly rhombic (diamond-shaped), 1.5-3.25 in.(3.8-8 cm) x 0.8-2.3 in. (1.5-6 cm), not shiny, both surfaces brown pilose (covered with long soft hairs) becoming almost glabrous (hairless) with age. The deciduous leaves are held in pseudowhorls of 2-3, which is typical of many species in Section Brachycalyx. Inflorescence is 2-4 flowered, rich salmon pink to bright red, almost brick red, with darker spots, usually appearing before or with the leaves, rarely white. Corolla is open funnel-campanulate (bell-shaped), 1.5-2.3 in. (3.8-6.3 cm) across. Stamens usually 10, ovary densely pilose.

The three species in the Alliance require summer heat to perform well, but R. amagianum and R. sanctum are reported to make better garden specimens, with R. weyrichii having the largest flowers. Expect it to flower in May. Reported hardiness in cultivation varies from 5°F (-15°C) to -10°F (-24°C), obviously tied to provenance of the wild collected seed, any introduced hybridity, and local cultural conditions. My 2.5 ft. plant from seed blooms moderately in my Zone 6, sandy soil garden, probably because it doesn't get timely watering to match the sun and heat exposure during recent years. The plant is a native of southwestern Japan (Kyushu, Shikoku, S. E. Honshu) and the South Korean volcanic island of Cheju (formerly Quelpaert); found in thickets and open woods at altitudes of near sea level to 3,200 ft. (1000 m.).

The Weyrich Azalea is named for the Russian naval surgeon Heinrich Weyrich, 1828-1863, who discovered it in the Goto Islands in 1853. It was later reported by Makino in Shikoku as R. shikokianum, now a synonym, and by Pàre Faurie on Cheju Island. The Arnold Arboretum introduced it to our Western gardens when C. S. Sargent sent seeds from Japan in 1892. E. H. Wilson sent seeds from his Shikoku collections to the Arnold Arboretum in 1914, later sending some of this seed on to Britain. He also sent seed from his 1917 Cheju Island explorations. The USDA Plant Introduction Section reintroduced it in 1932,1940, and 1961.

Plants of the three species in the Weyrichii Alliance are scarce in New England gardens. They are certainly hardy enough for most collector's gardens and should be grown more. In future issues we'll address many of the other fine azaleas from Asia.

Ian Donovan, Pembroke, MA


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