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Large shrub or tree, to 12 m or more; multistemmed. Branchlets stout, glabrous, yellowish to greyish brown. Buds with two glabrous, membranous bracts. Leaves deciduous, 6–10 × 3–6 cm, ovate to deltoid, upper surface green, lower surface pale green with appressed pubescence on veins, 12–15 lateral veins on each side of the midvein, margins doubly serrated with mucronate teeth, apex acute to shortly acuminate; petiole glabrous, 1–1.5 cm long. Staminate inflorescences catkin-like, robust, 4.5–9 cm long; pistillate inflorescences solitary, pedunculate, ellipsoid, 2.4–2.8 × 0.6–0.8 cm. Cone woody, 2.5–3 × 1.5–2 cm, bracts 0.5–0.6 cm wide. Flowering March to June, fruiting September to October (Japan). Ohwi 1965. Distribution JAPAN: Honshu. Habitat Lowlands, foothills, especially coastal areas. USDA Hardiness Zone 6–7. Conservation status Not evaluated. Illustration NT26. Cross-reference B274 (as A. firma var. sieboldiana).
A unique feature of the plantings at Stone Lane Gardens is the way that several specimens from a single wild-origin collection are planted in a group together – a pattern that enables natural variation to be observed and maintained in a way that is only too rare. Among these groups is a planting of Alnus sieboldiana, grown from a collection made by Kenneth Ashburner at Katanuma Lake, near Sendai, Honshu in 1980, and now forming a wonderful grove of sinuous grey trunks. Although Ohwi (1965) allows the wild tree to reach only 3 m, these are 8–9 m in height, growing on multiple stems from the base that support spreading limbs and a rounded canopy. The leaves are large, lanceolate to ovate, with an acute tip and a very crisply toothed margin, and are a handsome feature in their own right. They are retained well into December (K. Ashburner, pers. comm. 2007). The male catkins are almost globular and are extremely showy in spring with yellow anthers, while the very large cones, changing from green to russet-brown and held upright on the twigs, are a further good point. At Howick, where there are three trees from a Motowo and Yoriko Kobayashi collection from Aichi Prefecture, Honshu, it has not been easy to get it established (C. Howick, pers. comm. 2005), but the survivors were about 5 m tall in 2005.