Clethra barbinervis - Clethra du Japon, Clèthre du Japon

Clethra barbinervis, the Japanese clethra, is a large deciduous shrub for acid, moist soil valued for its summer flowering, autumn colours, tiered branching and decorative bark. Its clusters of white, trailing, fragrant flowers appear in late summer, and its foliage then turns yellow, orange and red. On older specimens, winter reveals a remarkable exfoliating bark, mottled with grey, brown and cinnamon. It is a fine botanical species, but it is rather demanding in terms of soil and climate.

Clethra barbinervis belongs to the Clethraceae family. It is a shrub whose deciduous foliage falls in autumn. It is native to a large part of eastern Asia, from China to Japan via Korea. It grows in open montane forests, on woodland edges and slopes, up to 1,800 m in altitude in China. Introduced into European gardens around 1870, this species is far less common than Clethra alnifolia, the North American sweet pepperbush. The shrub develops several upright stems and horizontal branching, distributed in tiers, which botanists and dendrologists compare to a candelabra or a pagoda. Its growth is moderate, fairly slow during the establishment period. In a garden, it reaches 3 m in height with a 2.50 to 3 m spread at maturity. Old specimens can exceed 5 m in height in a mild climate and form a small tree of 10 m in the wild. This species can produce offshoots from its stump and slowly widen. The leaves, ovate to obovate, measure 5 to 12 cm long. They are dark green, slightly glossy, with a finely toothed margin and prominent, hairy veins on their undersides. This characteristic explains the epithet barbinervis, meaning 'with bearded veins'. In autumn, the foliage takes on yellow, orange and red hues. From late July to September, the tips of the current year's shoots bear several arching or pendulous clusters, 5 to 17 cm long. They bring together numerous small white flowers with five petals, with a sweet fragrance, visited by pollinating insects. The fruits are small brown capsules with three valves, which persist on the branches in winter. With age, the grey-brown to reddish-brown bark peels off in fine flakes, revealing lighter, greyish or cinnamon-toned areas. This shrub received the Award of Garden Merit from the Royal Horticultural Society in 1993. It withstands temperatures of around -15 °C in a sheltered position; young shoots and branch tips are more sensitive to late frosts and cold winds.

In a border with acid, humus-rich soil, at the edge of a cool woodland or near a water feature, this Japanese clethra can be given the space of a small tree; its trunks can be gradually cleared to better enjoy the beauty of its bark. In a Japanese-inspired garden, you can pair it with Clethra alnifolia 'Ruby Spice', lower-growing and flowering pink, with Enkianthus campanulatus 'Red Bells' with its spring flowering, and Acer japonicum 'Aconitifolium' with its red autumn foliage. A few clumps of Hakonechloa macra 'Aureola' will create a broad, golden and luminous border at the foot of the shrubs, in partial shade.