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A deciduous shrub or tree to 10 (-18) m. Bark green, soon turning grey-brown. Branchlets glabrous, red to purplish brown, becoming faintly striped white. Buds stipitate, oblong to oblong-lanceolate, with two pairs of valvate scales. Leaves triangular ovate, base cordate, lobes ovate, apex long caudate, margins sharply double-serrate, upper surface dark green, lower surface paler, with rusty pubescence in vein axils and along secondary veins at first, sometimes persisting in vein axils; petiole 3–8 cm long, reddish, glabrous and grooved; autumn colours yellow to red. Inflorescence axillary or terminal, racemose, spreading, becoming pendulous, 20–40 flowered, 4–10 cm long. Flowers yellowish green, 5-merous, usually androdioecious, pedicels 0.2–0.8 cm long, sepals oblong to elliptic, ~0.1 cm long, petals obovate, ~0.1 cm long, stamens eight, inserted outside the nectar disc. Samaras 1.5–2 cm long, wings spreading broadly. Flowering May to July (Japan), fruiting October. (Ogata 1999; van Gelderen and van Gelderen 1999; Rix 2016).
Distribution Japan Honshu; Shikoku; Kyushu
Habitat Cool temperate forests, often found on higher mountain ridges.
USDA Hardiness Zone 6-7
RHS Hardiness Rating H6
Awards Award of Garden Merit
Conservation status Least concern (LC)
Small but charismatic, Acer micranthum is an excellent ornamental, with limited stature that renders it suitable for confined spaces as well as an accompaniment for other plants in larger landscapes. Though it has been described as an upright tree (Rix 2016), recent introductions possess a spreading habit and though specimens can be heavily branched, judicious pruning in youth alleviate any potential for overcrowding in the crown later on. With red shoots and sometimes red foliage at first, specimens are at their finest in autumn, when foliage turns shades of yellow to fiery red.
Introduced to cultivation in 1879 by Charles Maries (van Gelderen and van Gelderen 1999), the species is most likely to be confused with Acer tschonoskii, which also grows in the mountains of Japan. Flower size, the smallest in the section and hence the epithet, micranthum, is the easiest character to distinguish it from this species, though these are also borne in more abundance than those of A. tschonoskii and A. komarovii. Its leaves are also smaller, with often finer, deeper serrations. Its fruits are also more diminutive than those of its closest relatives.
Recent introductions include WJAP 014, collected in 2011 from Nakakawane Forest in Shizuoka Prefecture, Honshu and BBJMT 291, collected along the Nakatsuga River, Gifu Prefecture, Honshu (as A. tschonoskii var. australe) (BBJMT 291). Plants of both at Westonbirt are growing strongly in youth, with spreading forms and strong autumn colour. The British and Irish Champion grows at Crarae Garden, Argyll, measuring 9 m tall in 2012 (The Tree Register 2019). This is one of a number of sizeable specimens there, though some confusion between this species and A. tschonoskii has been reflected on plant labels.