Distylium is a genus of approximately 16 species of evergreen shrubs and trees belonging to the witch hazel family, Hamamelidaceae, and placed within the order Saxifragales. The genus is native to eastern and southeastern Asia, with its greatest diversity in China; species also occur in Korea, Japan (including the Ryukyu and Ogasawara Islands), Taiwan, and extend south through Indochina, Peninsular Malaysia, Sumatra, Java, and the Lesser Sunda Islands.
Members of the genus are slow-growing woody plants that remain in leaf year-round. The best-known species, Distylium racemosum — commonly called the isu tree — can reach tree height in the wild (up to 20 m) but typically grows as a large shrub in cultivation, rarely exceeding 2 m. It flowers in early spring (March–April) with inconspicuous, apetalous flowers, and produces fruits that ripen by midsummer. Other notable species include D. buxifolium, D. indicum (ranging from Meghalaya to Indochina), D. stellare (Peninsular Malaysia and the Sunda Islands), and D. lepidotum, endemic to the Ogasawara Islands.
In the wild, Distylium species typically inhabit evergreen broadleaved forests, often at elevations of 1000–1300 m. The wood of D. racemosum is hard, fine-grained, and dark brown, historically valued for small wooden articles, musical instruments, and — in Japan — the ash was used in glazing porcelain.
Etymology
The genus name Distylium derives from the Greek di- (two) and stylos (pillar or style), referring to the two styles characteristic of the flowers.
Distribution
Distylium is native to eastern and southeastern Asia. The majority of species occur in China; the range extends to Korea, Japan (Ryukyu and Ogasawara Islands), Taiwan, Meghalaya, Indochina, eastern Thailand, southern Vietnam, Peninsular Malaysia, Sumatra, Java, and the Lesser Sunda Islands.
Ecology
Distylium species typically grow in evergreen broadleaved and oak forests, often at elevations of 1000–1300 m. They favour moist, humus-rich soils and are adapted to dappled or partial shade under a woodland canopy, consistent with their natural understorey or forest-edge habitat.
Cultivation
Distylium species prefer moist, well-drained, humus-rich soils with a neutral to mildly acid pH, though they tolerate lime. A semi-shaded position under light woodland is ideal. Growth is slow; established plants are fairly cold-hardy, but tender new growth in spring is vulnerable to late frosts. In temperate gardens D. racemosum is hardy to approximately UK zone 8.
Propagation
Propagation is by seed, cuttings, or layering. Seed is best sown fresh in a cold frame; stored seed may require 2–3 months cold stratification. Germination typically takes 2–3 months at 15 °C. Semi-ripe cuttings are taken in July–August; hardwood cuttings of the current year's growth can also be struck in a frame. Layering in autumn takes approximately 18 months but gives a good success rate.
Cultural Uses
The wood of Distylium racemosum is hard, fine-grained, and dark brown, and has been used in Japan for small wooden articles and musical instruments. The ash produced by burning the wood was traditionally used as a flux in glazing porcelain.
