Hardy in Salem, New
John and Sally Perkins
Native to the mountains of southern Japan, Rhododendron semibarbatum is usually listed as hardy to zone 7 although reportedly it varies in hardiness. This deciduous species is unique in having 5 unequal, dimorphic stamens where the 2 shorter stamens are densely pilose with globose-ovoid anthers. In fact its name 'semibarbatum' refers to these partially bearded stamens. The small (1/2 - 3/4 inch), white, rotate flowers are borne in clusters of 1-3 flowers in the axils after the leaves are fully expanded. Seed was collected by Wilson in 1914 and sent to the Arnold Arboretum and later on to Britain. It was first introduced by Tschonoski to the Botanical Gardens of St. Petersburg where it flowered in the greenhouse in 1870. An excellent botanical illustration of this species is found in The Book of Rhododendrons by Marianna Kneller.
In the fall of 1991 we purchased a seedling of R. semibarbatum at the Arnold Arboretum Case Estate's sale against our better judgment and the advice of more knowledgeable members of the Rhododendron Society. Our little plant was not given winter shelter in a coldframe but instead planted on a northern slope with dense shade. We thought it would die and were surprised to notice it pushing little green buds late the following spring. The first week of July in 1994 we noticed small white flowers had fallen to the ground near R. semibarbatum and we wondered from where. Amazingly, hidden in the foliage were two more flowers. We quickly consulted Davidian and were pleased to find that the label fit the description. It flowered again the end of June in 1995, 1996, 1997, 1998, and 1999. It has again set flower buds for next year.
R. semibarbatum has never shown any significant winter damage (minimum -14° F). In 4 years, the plant's habit created a pleasing architectural form at 12" high by 18" wide. The paper thin, wavy edged leaves are especially attractive when in its yellowish orange fall color. Although more of a curiosity than a horticultural gem, R. semibarbatum appears hardy enough to be tried by adventurous New England Rhododendron collectors.
Davidian, H. H. The Rhododendron Species Vol. III. Timber Press, Inc. 1992 pp.303-304.
Kneller, Marianna The Book of Rhododendrons. Timber Press, Inc. 1995 pp.120-121.
This article first appeared in the Rosebay, a publication of the American Rhododendron Society, Massachusetts Chapter
To learn about Rhododendron species hardy in New England see Species in Our Midst.