Tree 25–30 m, 0.5–0.7 m dbh. Bark smooth to scabrous, greyish brown. Branchlets slender, purplish brown, densely covered with appressed grey hairs. Buds stipitate, black, densely covered with grey hairs. Leaves deciduous, 2–5 × 3 cm (larger in cultivation), ovate to orbicular, upper surface dark green with hairs on the veins, lower surface pale with yellowish brown hairs on the nerves when young, eight to nine lateral veins on each side of the midvein, margins double-serrate, apex acute; petiole purplish black, ~2 cm long, densely covered with grey hairs. Staminate inflorescences solitary or in groups of two to four (to five), ~8 cm long. Pistillate inflorescences usually in groups of two to three (to five), more numerous than in A. hirsuta, pedunculate. Flowers inconspicuous. Cone woody, oblong to ovoid, 1–1.3 × 0.8 cm; bracts ~0.3 cm wide. Fruit a samara with membranous wings. Flowering from March, fruiting in autumn (Japan). Murai 1962. Distribution JAPAN: Hokkaido, northern Honshu; KOREA (?). Habitat Mesic habitats, not confined to riparian areas. USDA Hardiness Zone 5–6. Conservation status Not evaluated. Illustration NT140. Taxonomic note Alnus inokumae was recently reduced to a form of A. hirsuta by Ohba (2006), but Hugh McAllister (pers. comm. 2007) dis agrees with this dramatic reduction in status, pointing out that it is ‘quite distinct in appearance from A. hirsuta mainly because of its smaller (2–5 cm long, vs. 4–9 cm long in A. hirsuta), glabrous leaves, and in being a taller growing tree with a thinner canopy. However, the best precise distinguishing characters appear to be that the tertiary veins are arched forwards, whereas the comparable veins in A. hirsuta are straight; and the more sharply triangular lobing of the leaves. Also, A. inokumae is diploid (2n = 28) whereas A. hirsuta is tetraploid (2n = 56).’ They are, however, clearly closely related, as can be seen from a comparison of the two taxa growing side by side at Stone Lane Gardens.
Putting aside discussions over the taxonomic status of Alnus inokumae, the trees themselves are notable for their extremely fast, straight growth, as demonstrated by specimens at Ness and at Stone Lane Gardens. Several trees in a group at Stone Lane Gardens, planted in 1986, from Honshu, are very uniform in height and size, the largest being about 12 m tall (19 cm dbh) when observed in May 2007. These individuals had leaves that were not noticeably smaller than those of the adjacent A. hirsuta, but were a very attractive dark green, with irregular margins in their upper halves. This is an impressive species and could be planted widely wherever fast-growing screening trees are needed.