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Tree to 15 m; usually smaller in cultivation. Bark grey and longitudinally fissured, becoming rather shaggy. Branchlets reddish brown to grey-brown, glabrous. Leaves deciduous, papery or leathery, 6–8(–12) × 2.5–7(–11) cm, palmately three-lobed, lobing very shallow, upper surface shiny green and glabrous, lower surface glaucous and largely glabrous, margins irregularly serrate, apex acute; petiole 3–9 cm long, reddish; autumn colour yellow to orange-red. Inflorescence terminal and lateral, umbellate with 4–10 flowers. Flowers 5-merous, dioecious; sepals oblong, red, petals none to five, smaller than sepals, stamens five to six, inserted outside the nectar disc. Samaras 2–3 cm long, wings spreading acutely. Flowering April, fruiting May to June (Japan). Van Gelderen et al. 1994, van Gelderen & van Gelderen 1999. Distribution JAPAN: central Honshu. Habitat Wet, lowland sites. USDA Hardiness Zone 5. Conservation status Not evaluated (but restricted distribution). Illustration NTiii, NT104. Cross-reference K100. Taxonomic note This species is very similar to the closely related A. rubrum, though it has slightly smaller leaves and stature.
Acer pycnanthum was said by Bean (1976a) to be probably not in cultivation in the United Kingdom, but over 25 specimens are currently on the TROBI list of notable individuals, the tallest in England being two male trees of over 14 m at the Sir Harold Hillier Gardens. An important source of seed in the early 1970s was the Japanese nurseryman Seyo Yamaguchi, who gave material to Peter Barnes and Roy Lancaster (R. Lancaster, pers. comm. 2007), from which some of the largest specimens today are derived (including the Irish champion at Dunloe Castle Hotel, Co. Kerry, 15 m tall when measured in 2000, TROI); but the first introduction to the West is credited to James Harris in 1971 by van Gelderen et al. (1994). The species can make shapely pyramidal specimens with considerable presence, several being designated National Monuments in Japan, and in cultivation many look set to exceed the 15 m quoted for wild trees. A smaller stature is said to be a distinguishing feature from its close relative, the North American A. rubrum, but the difference may become blurred in cultivation if these trees continue to grow strongly. Other horticultural distinctions are its slightly glossier leaves and less reliable autumn colour – at Hergest Croft it is ‘sometimes a good red’ (L. Banks, pers. comm. 2006), although in the wild it can be a brilliant scarlet. The young foliage can also be tinted red, and as in A. rubrum the flowers and reddish young fruit can be spectacular. Several variegated cultivars are known (van Gelderen & van Gelderen 1999).