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Species In Our Midst
Rhododendron arborescens

by Susan Clark

Rhododendron arborescens
Picture by S & J Perkins
Rhododendron arborescens in Andover, MA

Rhododendron arborescens

Rhododendron arborescens, 'The Sweet Azalea', is one of this country's lovely native deciduous azaleas. It is in Section Pentanthera (the section has 16 deciduous azalea species) and has been classified in R. subsection Pentanthera, which has all the azaleas in the section except R. molle. 14 of the 15 species in the subsection are from North America; only R. luteum is not indigenous, but is from the Caucasus region. The 13 eastern azaleas are a somewhat closely related group. The taxonomy of these azaleas is still being debated and rearranged, using the newer DNA and cladistic techniques. And, of course, a new species was just added to their ranks, with the discovery of R. eastmanii in South Carolina

Rhododendron arborescens grows in parts of Pennsylvania, West Virginia to Tennessee, North Carolina, Georgia and part of Alabama. It is found along streams in the mountains, on treeless balds, and in moist woods; it grows from almost sea level to 4500'. It is usually a late bloomer, but is variable in flowering time, in some places blooming as early as April and as late as September. Greer lists the species as a very late bloomer. It seems reasonably hardy; Greer gives it a -10°F rating.

As its specific name suggests, this plant becomes 'tree-like' with age. It grows as a shrub or small tree up to 18' tall, but usually tops out as a bush-like 10'. Its young twigs are yellow-brown and glabrous (lacking hairs), a good distinguishing characteristic. Its leaves are ovate to obovate (shaped like an egg or like an upside down egg, wider near the tip) or elliptic and usually glabrous; they have rounded tips, lacking a point. The midvein of the leaf may have a few hairs or be quite densely hairy. The leaf margins are entire (without notching or teeth) and they are fringed with fine hairs (ciliate). Leaves are shiny green above, waxy below and 1 3/4 -3" long. Plants at high altitudes tend to have noticeably smaller leaves and shorter heights. Occasional plants will have rather glaucous (bluish) leaves.

Rhododendron arborescens has fragrant, terminal inflorescences, of 3 to 7 flowers; these appear as or after the leaves have opened and are white or have a blush tint. Each flower is 1-1 3/4" across and 1 1/2 -2" long and is tubular-funnel shaped. The 5 stamens and the stigma are bright red and protrude well beyond the petals (about 2 times the length of the corolla tube) and are quite showy.

This species crosses readily with other deciduous azaleas and many plants labeled as the true species are in fact natural hybrids. Any open pollinated plant should be considered a hybrid until proved otherwise. R. arborescens is most closely related to R. viscosum, The Swamp Azalea, but can be told from it by the glabrous young twigs, the red stamens and stigma, which stand out against the white petals, the larger flower size, and the rounded leaf tips. They also have different fragrances, although fragrance is always difficult to describe. Bower says R. arborescens smells like heliotrope; Kron says it smells like cinnamon. Bower thinks R. viscosum smells of clove. Since the two species bloom at the same time and share habitats, natural hybrids occur, which can be difficult to distinguish. R. arborescens also hybridizes with R. cumberlandense to produce a great variety of hybrids, with color ranges from white to reddish blooms.

R. arborescens was first discovered by John Bartram, the famous American plant explorer, and it was first legitimately described as Azalea arborescens by Pursh, from plants growing on Blue Mountain in Pennsylvania and from plants growing in Bartram's garden in Philadelphia. The plant was actually first observed by Michaux in 1795 in North Carolina, but no specimen was found in his collection, so the formal credit goes to Bartram and Pursh.

This is a lovely garden plant, justifiably prized for its late, showy white flowers with their red stamens and stigma, and their marvelous fragrance. It is included in the chapter's 'Proven Performers' list. R. arborescens has been used in hybridizing, including in some of the Exbury azaleas and Weston Nurseries' summer-blooming azaleas. Joe Parks has used it in chilly New Hampshire to produce the beautiful and fragrant 'Cherokee Frosty Morn' and 'Cherokee Lemon Chiffon', both of which can be seen on our chapter web site. I have three unnamed crosses from Fred Knippel of R. arborescens X R. cumberlandense; they are planted under huge pines with high deep shade and bloom in June with somewhat fragrant, frilly apricot and cream flowers. They couldn't be nicer.

Susan Clark, Concord, MA

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