Alnus firma Sieb. & Zucc.

Alnus firma belongs to a distinct Section within Subgenus Alnobetula, Section Bifurcatus (Murai 1964), which is identified by higher leaf vein number, ovate-lanceolate glossy leaves, and fruit scales having more recurved apices than other members of Subgenus Alnobetula. In the current treatment this Section includes A. firma, A. sieboldiana, A. pendula and A. betulifolia (see below). These taxa have been variously treated as distinct species or as varieties of one another, and more work is needed to present a stable taxonomy. A. pendula is the most distinct in the group but often misidentified.

A. firma is distinguished as being a small tree or upright shrub with ovate to ovate-lanceolate leaves with 9–17 veins either side of midrib, and fruit 18–22 mm long and typically held in erect pairs (but can be in groups of 1–6). It is a species of high mountains in Japan. It is readily distinguished from A. pendula as this has ovate leaves with 18–27 vein pairs, smaller female inflorescences and fruit held in pendulous racemes. A. firma (tetraploid or octoploid – Rice et al. 2015), is morphologically most easily confused with A. sieboldiana, but that species occurs in lowland areas, has larger fruit held singly (rarely in groups), and is hexaploid (Rice et al. 2015; Oginuma et al. 2000).

There is an unresolved question regarding diversity within A. firma. There appear to exist two rather distinct, disjunct entities within the species. These are currently ascribed to two varieties, the glossy-leaved and glabrous var. firma is found only in the mountains of Kyushu (easily mistaken for A. sieboldiana), and the var. hirtella with matt, pubescent leaves, found in the mountains of northern Honshu. These vegetative differences appear to be the only recorded distinctions, but floral positions would be worth investigating, especially in line with the recently described A. betulifolia. There is no data to suggest this distinction is linked to ploidy level. The relationship between these entities and A. sieboldiana remains unclear. The main morphological distinctions are the hairiness of twigs and number of fruit, in which there is overlap and confusion; for example A. sieboldiana KFBX 125 has fruit in clusters of 1–8. Ploidy level appears to distinguish the taxa, but they could also be thought of as ecological types within a much broader species which would include all entities, from coastal A. sieboldiana to mountainous var. hirtella, with var. firma intermediate.

Introduced to western cultivation in 1892 (Jacobson 1996) A. firma has proven itself a versatile and attractive species, nevertheless it remains far rarer than it deserves to be. It grows into a large upright shrub or small tree to 10 m (rarely to 15 m) in height, usually developing a narrow, graceful habit, often somewhat pendulous with highly attractive, glossy to somewhat matt ovate leaves with conspicuous fine venation that is pleated when the leaves emerge. It grows best in full sun or dappled shade in wetter soils but will tolerate some drought. A. firma is also attractive in flower, although the yellow-green male catkins are not as showy as those of A. sieboldiana, being thinner, shorter and less abundant. It is relatively fast-growing and is best propagated from seed or softwood cuttings under mist or in a hot pipe system. Plants should take 5–10 years to flower from seed. It is highly resistant to Alder Leaf Beetle. Notable specimens exist in several UK collections, for example at Hollycombe, West Sussex (14.5 m in 2018), Sheffield Park, East Sussex (12 m in 2012), Stone Lane Gardens, Devon (11 m in 2017) and at the Royal Botanic Garden Edinburgh (10 m in 2004 but since lost) (Tree Register 2021). A collection at Ness (TM 11128) has grown into polygamous form, with one plant monoecious, and the other two with hermaphrodite flowers which produce viable seed (T. Baxter pers. obs.). Jacobson (1996) calls it ‘exceedingly rare’ in North America, but recent introductions by the RBG Edinburgh, Howick, Quarryhill and Polly Hill expeditions of 2005 and 2007 (BBJMT and BCJMM), which between them targeted a broad range of Japanese alders, have gone some way to addressing this.