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Species In Our Midst
Rhododendron pachysanthum

by Susan Clark

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


Rhododendron pachysanthum is an elepidote (without scales) in subgenus Hymenanthes, Section Ponticum, subsection Maculifera. Subsection Maculifera is a diverse group of species, related to subsection Selensia. This subsection includes some favorite foliage species, like R. strigillosum, morii, pachytrichum, pseudochrysanthum, and the eponymous maculiferum. All nine members of this subsection come from SW China or Taiwan, and probably evolved together when Taiwan was connected to the mainland 18,000 years ago during an ice age. It is likely that the Taiwanese non-vireya rhododendrons are relict populations now separated by a relatively recent geologic movement of the island.

R. pachysanthum grows above the tree line in the mountains of Taiwan at Nan-fu-ta-shan, at elevations of 9,800-10,500 feet (3,000 -3,200 meters) in grassy meadows. It grows mixed in with R. morii, with which it forms natural hybrids. Hayata described the plant in 1913 and John Patrick, the American collector in Taiwan, introduced it in 1972. Its specific name means 'with thick flowers', not very helpful since its flowers don't seem observably 'thick'.

R. pachysanthum is a compact, globose (ball-shaped) shrub to 4 feet (1.2 m). New shoots of mature plants are covered with a dense fawn-colored tomentum (felty covering), which also covers the upper surfaces of the leaves. R. pachysanthum retains this tomentum longer than other members of this subsection, but it eventually wears off, leaving the upper leaf surface shiny, dark green and somewhat leathery. The leaves are oblong or oblong-lanceolate up to 3 1/2" long (4 -10cm) and 5/8 -2" wide (1.5 -5 cm), with pointed tips and a rounded base. The undersides of the leaves have a thick whitish-brown to rufous indumentum. The petioles (leaf stems) are similarly covered with tomentum. The veins on the underside of the leaf are deeply impressed and the edges of the leaves are recurved. The bark is rough and grayish brown.

The pretty trusses are an inflorescence with 10-20 flowers, 1 1/2" (4 cm), held on pedicels (flower stalks) about 1" long (2.5 cm). The flowers are widely campanulate (open bell-shaped), white to pale pink, sometimes with purple or crimson flecks. Many of the flower parts are glandular.

R. pachysanthum has excited several disputes among taxonomists, first when it was lumped in with R. morii, with which it grows and crosses. Peter Cox, in his The Larger Rhododendron Species, labels this bluntly as 'ridiculous'. H.H. Davidian, in his Rhododendron Species, Vol. III, emphasizes R. pachysanthum's similarity to R. wasonii, and includes it in his Subseries Wasonii, Taliense Series.

D.F. Chamberlain, in his "Revision of Rhododendron", Notes From the Royal Botanic Garden Edinburgh, 1982 (the official taxonomy of the Species Study Group), rejects all of this and puts R. pachysanthum in subsection Maculifera and keeps it separate from R. morii and R. pachysanthum because of the different structure of its indumentum. Even he admits that further work on the relationship of these 3 species is needed!

We must patiently wait while the taxonomists continue their arguments, but meanwhile we have a spectacular plant for our gardens. With its thick tomentum and indumentum and its pointed dark green leaves, with its dense growth habit, and with its pretty flowers, it is a major asset to any garden. Greer thinks it features the best features of R. pseudochrysanthum and R. yakushimanum! Cox thinks it "could be one of finest foliage plants of the genus". Gardeners and species fanatics are quick to add this species to their gardens, where it always attracts attention. Its hardiness in New England is still somewhat uncertain. Greer, safe on the West Coast, rates its flowers as hardy to -5°F (-20°C). Only 4 members of Chapter are listed in our Species Booklet as having this plant in their gardens, so we need much more information on how it fares in New England. The Perkins have never flowered it in their garden in Salem, New Hampshire, thriving but never setting flower buds by Canobie Lake. We know of plants in Concord and Belmont. Joe Coleman from Cohasset just won Best in Show in this year's Fall Foliage Show with R. pachysanthum, beating out a number of other gorgeous indumented entries. That's a pretty good testimonial to this plant's great appeal!

Susan Clark

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