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Species In Our Midst
Rhododendron hyperythrum

by Susan Clark

Rhododendron hyperythrum
Picture by S & J Perkins
Rhododendron hyperythrum in Westport Point, MA

Rhododendron hyperythrum

This elepidote (unscaly) rhododendron is a member of the hardy subsection Pontica, but it seems only distantly allied to the other members of the subsection (such as R. brachycarpum, catawbiense, maximum, smirnowii and yakushimanum). The specific epithet ‘hyperythrum’ means ‘reddish below’ and refers to the reddish pits, which dot the underside of the leaves. This distinguishing characteristic is less noticeable to the casual observer than the unmistakable recurved, shiny dark green leaves. These leathery 5-6 inches long, narrow leaves have their edges rolled sharply under; their undersides are pale green and glabrous (hairless). Interestingly, the plants in the wild usually have flat leaves and only those in cultivation display the unusual curled edges. The leaf petioles (stems) are bright yellow turning reddish brown near the stem (the same color as the flower buds in winter).

This species tends to form a compact and rounded plant, reaching 6 feet in height and diameter at maturity. In mid-May in the Boston area it produces an abundance of white, open funnel-form flowers up to 2 inches in length in trusses of 5 to 12 blooms. Occasionally the flowers are spotted with purple. R. hyperythrum seems quite hardy, taking -15°F, a surprising cold tolerance given the species' origins in the subtropical lower slopes of the mountains of Northern Taiwan.

R. hyperythrum's distinctive, curled foliage offers an excellent contrast with the flat, larger leaves of many of our favorite plants and with the indumented foliage of R. yakushimanum, smirnowii and bureavii. It is a free-flowering, lovely white bloomer. It is surprising that this plant is seen so rarely in cultivation in this country and that so few nurseries and catalogs offer it. The only reliable source seems to be the chapter’s Van Veen order, where it is offered most years. Little hybridizing has been done with it, except for a substantial breeding program by John Thornton in Louisiana (see ARS Journal Vol. 44 #2 1990) and a few other southern hybridizers interested in its tolerance to heat and wet feet. Many of Thornton's named crosses sound quite suitable to New England (in spite of his breeding goals) and should be looked into. Roslyn Nursery in Long Island is offering 'Woodcock', a rosy-flowered cross of 'Elizabeth' x hyperythrum from Wisley Gardens.

I find it a most satisfactory garden plant, interesting and attractive all year. It ought to be grown more widely in our area.

Susan Clark, Concord, MA

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