Clethra barbinervis (in Japanese, ryobu) has several common names: Japanese Clethra, Tree Clethra, and Japanese Summersweet. We have one plant, easily overlooked, in Area F of the Japanese Garden. Native to Japan, it grows in southern Hokkaido, on hills and mountains. . Traditionally, its wood was used for construction and tool-making.
Dan Hinkley, the noted plant explorer and former long-time owner of Heronswood Nursery, describes it as:
“A wonderful and rarely planted shrub with late summer panicles of drooping white flowers and orange-red autumn color, in fact, one of the best autumn-coloring shrubs in the garden. My observations of mature stands of this species in its native haunts of Japan have brought to my realization the profound beauty of this species’ exfoliating bark.”
This is a deciduous tree or large shrub that grows quickly when young, and reaches 30 feet tall at maturity in its native woodlands. In cultivation in the U.S., mature height is about 20 feet. Small, white, fragrant flowers are held in racemes (stalks including many small flowers) at the tips of branches, usually in July, and are followed by persistent spikes of seed capsules. Dark green, obovate (egg-shaped & narrower at the base), sharply toothed leaves are also clustered at the tips of branches. Fall color is variable, and likely to be best with some sun.
The most spectacular aspect of Japanese Clethra is its smooth, polished, gray and brown exfoliating bark. According to some growers, its beauty rivals that of Stewartia pseudocamellia (Japanese Stewartia).
Among the plant attributes most valued in Japanese Gardens, attractive bark ranks along with leaf shapes and colors, plant growth habits (natural and achieved by pruning), and nature’s many shades of green. In Japan flowers are valued as much for their transience as for their beauty – unlike in the United States, where gardening traditions value flowers above all else.
Clethra barbinervis grows best in light shade, in acid, moist, well-drained and fertile soil. In Japan, it’s one of the first trees to recolonize cleared woodlands.
Although still a rare plant in American gardens, Japanese Clethra is available for purchase from some mail-order and retail nurseries.