Alnus inokumae Murai & Kusaka

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Tim Baxter & Hugh A. McAllister (2024)

Recommended citation
Baxter, T. & McAllister, H.A. (2024), 'Alnus inokumae' from the website Trees and Shrubs Online ( Accessed 2024-04-21.


  • Alnus
  • Subgen. Alnus, Sect. Glutinosae


  • Alnus hirsuta f. inokumae (Murai & Kusaka) H.Ohba
  • Alnus hirsuta var. microphylla Tatewaki


Egg-shaped; broadest towards the stem.
Number of chromosomes.
Covered in hairs.
Arranged in a net-like manner.
(pl. taxa) Group of organisms sharing the same taxonomic rank (family genus species infraspecific variety).


Tim Baxter & Hugh A. McAllister (2024)

Recommended citation
Baxter, T. & McAllister, H.A. (2024), 'Alnus inokumae' from the website Trees and Shrubs Online ( Accessed 2024-04-21.

Upright sometimes pendulous tree 25–30 m tall, 0.5–0.7 m dbh. Bark smooth to scabrous, greyish to silvery brown. Branchlets slender, purplish brown, densely covered with appressed woolly grey hairs. Buds with stipe 1.5–2 cm, bud black, densely covered with grey hairs. Leaves deciduous, 2–6 × 2.5–4 cm (twice as long as wide), 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 and prominent abaxially, tertiary venation chevron-shaped, craspedodromous, margins double-serrate with lobes ± triangular, apex acute; petiole purplish black, c. 20 mm long, densely covered with grey hairs. Stipule 10–15 mm, sides concave with acuminate apex. Staminate inflorescences in pendulous clusters of 2–4(–5), ~80–150 mm long at anthesis, often terminal on branches. Pistillate inflorescences pendulous, held below staminate in clusters of 2–5(–8), pedunculate. Fruit oblong to ovoid, 10–16 × 6–10 mm; bracts ~3 mm wide. Seed with thick, narrow to wide membranous wings. Flowering from March; fruiting in autumn (Japan), often earlier in cultivation. (Murai 1962; H. McAllister, pers. obs.).

Distribution  Japan northern Honshu, Hokkaido

Habitat Distributed in temperate forests, mountains and hills from 700–1,800 m asl, from dry uplands to wet ground along river banks, in grassy bogs and wet flushes.

USDA Hardiness Zone 3-9

RHS Hardiness Rating H7

Conservation status Least concern (LC)

Alnus inokumae is an uncommon tree of Japan that is often described as a form of Grey Alder (A. hirsuta f. inokumae) or sunk into A. hirsuta entirely (e.g. Plants of the World Online 2024), but it can be reliably distinguished from it by several characters. They are, however, clearly closely related, as can be seen from a comparison of the two taxa growing side by side, as they do at Stone Lane Gardens and Ness Botanic Gardens in the UK. A. inokumae grows into an upright tree with a somewhat sparse crown, sometimes of pendulous habit, with thinner branches and twigs, smaller fruit (10–16 mm long), and longer, thinner and more conspicuous male catkins. It also has pubescent young shoots and smaller leaves than A. hirsuta (2–6 cf. 4–9 cm long) that are distinctly triangular-lobed (cf. rounded toward the base), more commonly ovate and longer than broad, and the tertiary veins form a chevron pattern (cf. more or less straight), especially towards the blade margin (McAllister, pers. obs.). It is thought to occur in mountainous regions of central Honshu and Hokkaido, but remains unconfirmed from the Korean Peninsula (Iwatsuki 1993).

In cultivation Alnus inokumae is a fine upright tree, often of weeping habit and especially conspicuous in catkin in spring. In many respects it is a more refined form of A. hirsuta. A. inokumae appears to come true from seed and be at least partially self-compatible: several seedlings from the single tree growing in the Arnold Arboretum – close to trees of A. hirsuta – were uniform, diploid, and clearly true to the parent tree. The same is usually true at Ness, with most seedlings clearly coming true. A notable exception was a self-sown seedling, which had clearly crossed with A. glutinosa subsp. barbata, but which has ornamental merit (T. Baxter, pers. obs.).

A major question remains regarding the taxonomic status of Alnus inokumae. It is clearly a distinct entity, quickly recognisable from typical A. hirsuta. It is accepted as being diploid (2n=28) while A. hirsuta is reported as diploid, triploid and tetraploid (Rice et al. 2015; Goldblatt & Johnson 1970), albeit with identification of much of this material not entirely reliable. Both are part of the A. incana species complex alongside A. rubra, A. hirsuta and A. glutinosa (Chen & Li 2004). Ren, Xiang & Chen (2010) found it more closely related to A. incana than to A. hirsuta, possibly an indicator of a (tetraploid) A. hirsuta containing a genome from a more distantly related taxon. This is a complex picture and suggests not enough work has been carried out to confirm any particular hypothesis. It is currently unknown whether the situation is simlar to that of A. glutinosa in southern Europe, with ploidy level varying based on locality with a reticulate evolutionary history. Hpwever, any pattern of ploidy differences within the A. hirsuta complex (including A. inokumae) is not so easily discerned.