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Shrub or small tree, 8–10 m. Bark grey and smooth. Branchlets greenish brown, pubescent to tomentose, somewhat sticky when young. Leaves deciduous, papery, 5–7 × 6–9 cm, palmately lobed with (7–)9(–11) lobes, divided to half of the length, upper surface dark green, lower surface pale green with dense white hairs that persist along the veins, margins double-serrate, apex acuminate; petiole 3–6 cm long, pubescent; autumn colour coppery red. Inflorescence terminal, corymbose, 3–5 cm long with 15–20 flowers. Flowers 5-merous, staminate or hermaphrodite; sepals narrowly ovate, pale purple, petals ovate, greenish yellow, shorter than sepals, stamens eight, inserted inside the nectar disc. Samaras 1.5–2 cm long, greenish yellow, wings spreading at an angle of 130º–160º. Flowering April to June (Japan). Van Gelderen et al. 1994, van Gelderen & van Gelderen 1999. Distribution JAPAN: Honshu, Shikoku, Kyushu. Habitat Montane forest in southern Japan; in northern Japan, between 0 and 900 m asl. USDA Hardiness Zone 4–5. Conservation status Not evaluated. Illustration NT73, NT108. Cross-reference B206, K105.
Acer sieboldianum is anything but a ‘new tree’, but given its curious treatment in a very short paragraph by Bean (1976a), and minimal coverage elsewhere (for example, Hillier & Coombes 2002), a fuller account is desirable here. Introduced by Philipp von Siebold in the nineteenth century, it has remained overshadowed by its allies A. japonicum, A. palmatum and A. shirasawanum but is deserving of greater attention, not least for its exceptional and reliable autumn colour and much greater hardiness (although it can show some damage after a severe Polish winter: P. Banaszczak, pers. comm. 2007). According to van Gelderen et al. (1994) it flourishes in Scandinavia where A. palmatum is not hardy, and this would suggest a wide applicability in North America, where it is freely available. There are good specimens in many European collections, mostly making small trees of up to 10 m or so and, where there is room, forming wide-spreading rounded crowns, although sometimes the branches are more upright. The leaves are a fresh green in summer, with late shoots emerging bright red (L. Banks, pers. comm. 2006) before the tree becomes clad in superb autumn colour of scarlet and gold.