Alnus incana (L.) Moench

Alnus incana subsp. incana growing in Valbonë National Park, northern Albania. 4 June 2023. Image Tom Christian.

The grey undersides of the leaves of Alnus incana Image T. Baxter.

Alnus incana foliage on a tree at the Provinciaal Domein Het Leen Eeklo, Belgium (15 August 2018). Image © Jan De Langhe - Arboretum Wespelaar.

Leaves on Alnus incana (Provinciaal Domein Het Leen Eeklo, Belgium – 15 August 2018). Image © Jan De Langhe - Arboretum Wespelaar.

Pubescence on lamina lower surface, petioles, branchlet and stipitate bud of Alnus incana (Provinciaal Domein Het Leen Eeklo, Belgium – 15 August 2018). Image © Jan De Langhe - Arboretum Wespelaar.

Lamina margin with double-serrate shallow lobes on Alnus incana (Provinciaal Domein Het Leen Eeklo, Belgium – 15 August 2018). Image © Jan De Langhe - Arboretum Wespelaar.

A cultivated A. incana subsp. incana of good form and showing little sign of suckering. (Cumbria, UK; August 2022) Image Tim Baxter.

'Angustissima'

A cut-leaf form of Grey Alder with leaves divided to the midrib or nearly so; the ‘most elegant laciniate form’ for Jablonski (2018). The very narrow lobes are often toothed at the base, with the apex narrowing very gradually into a thread-like point. This form has been cultivated in Sweden since before 1870, having been found growing wild in several parts of Scandinavia (Bean 1976).

'Argentata'

Grown on a very modest scale in Finland, this name covers forms with silver-grey leaves (de Jong 2017).

'Aurea'

A useful and widely planted amenity tree, slower and less vigorous tree than the type, growing to 12 m with a 6 m spread, identifiable by the leaves being yellowish, especially when young, and downy beneath. Young bark and shoots orange, remaining so through the winter. Catkins remain on trees all winter and retain attractive orange to pink tones (Jablonski 2018; Edwards & Marshall 2019).

The pinkish-orange glow of Alnus incana 'Aurea' in winter makes this a very attractive small tree; the colour derives from both the bark of the younger shoots and the over-wintering catkins (Yorkshire Arboretum, UK, December 2020). Image John Grimshaw.

Alnus incana 'Aurea' at the Surrey University Campus, Guildford, UK; 27th September 2018. From late summer, a warm golden colour suffuses the foliage. Image Owen Johnson.

Alnus incana 'Aurea' makes an attractive plant with yellowish foliage. Image T. Baxter.

Deje E

Plants marketed in Sweden as Alnus incana Deje E since 2002 are raised from seed gathered from a source near Deje, Sweden. Selected by the Uppsala University of Agriculture (SLU) for their suitability for planting in central and southern Sweden, these plants, probably best regarded as an ecotype or Cultivar Group, are fast-growing with pyramidal crowns to 15 m tall. It is generally more robust and uniform than the typical species, and is particularly tolerant of dry and exposed conditions, including salt exposure (de Jong 2017).

'Elegant'

A slightly weeping tree with laciniate leaves. One of several cultivars originated in an A. incana breeding programme by Arboretum Borova Hora, Zvolen, Slovakia to improve the range of ornamental alders (Jablonski 2018).

'Foliis Aureomarginatis'

Similar to ‘Razzmatazz’ but has glossier leaves with stable yellow variagation, and suffers less from reversion. Originating in Rożińsky, Poland, c. 1913, it was rediscovered in the Korník Arboretum after years of presumed extinction (Jablonski 2018).

'Fruticosa Aurea'

A shrubby plant with golden leaves and reddish catkins, from the breeding programme of Arboretum Borova Hora, Zvolen, Slovakia (Jablonski 2018). The latinized cultivar name is invalid as it was published after 1959; it is not clear whether a new name has been applied.

'Gold Cascade'

A small tree with golden leaves, prominent reddish buds and catkins, and pendulous habit; from the breeding programme of Arboretum Borova Hora, Zvolen, Slovakia (Jablonski 2018). ‘Gold Fountain’ is virtually identical and probably synonymous.

'Green Fountain'

Upright habit, pendulous branchlets, and a narrow crown; from the breeding programme of Arboretum Borova Hora, Zvolen, Slovakia (Jablonski 2018).

'Laciniata'

Bean (1976) called this ‘the handsomest of the cut-leaf alders, the blade being pinnately divided into six or eight pairs of narrow, lanceolate, toothed lobes, reaching two-thirds or more of the way to the midrib’. The name has become confused in horticulture and several authors have suggested ‘Pinnata’ is synonymous, but this has much more coarsely divided leaves.

'Monstrosa'

A shrubby selection with fasciated stems and yellow leaves in the spring, turning green later. It was distributed by Späth from 1892 but was considered lost until rediscovered in the Korník Arboretum, Poland (Jablonski 2018). Jablonski notes that it must be grafted deeply on the rootstock.

'Pendula'

Piet de Jong calls this the only weeping form of Alnus incana (as opposed to those forms with merely pendulous branches e.g. ‘Green Fountain’) (de Jong 2017). The main branches spread quite widely in graceful arcs, with secondary branches and branchlets hanging somewhat stiffly from these; given plenty of space it makes a graceful landscape tree. It was raised either in Germany or what is now Poland in the late 19th century (Jablonski 2018).

A single-stemmed Alnus pendula 'Incana' at the Yorkshire Arboretum, UK, December 2020. Image John Grimshaw.

A mature Alnus incana 'Pendula' at Marwood Hill, Devon, UK. Image Owen Johnson.

'Pinnata'

Largely restricted to Scandinavian collections, and rare in cultivation even here, Alnus incana ‘Pinnata’ was first found in or near Lesjöfors in Värmland, Sweden. It was propagated in the Uppsala Botanic Garden around 1800. ‘Pinnata’ has small leaves with several pairs of blunt, toothed lobes (Bean 1976).

'Ramulis Coccineis'

Very similar to ‘Aurea’, but its leaves are almost glabrous beneath and branchlets and buds a good red. This cultivar comes true from seed (Jablonski 2020). This cultivar and ‘Aurea’ make fine additions to any garden or park.

Alnus incana 'Ramulis Coccineis' in a well chosen spot at Ness, UK, with Metasequoia to the right. Image T. Baxter.

'Razzmatazz'

Leaves with irregular bright yellow-green margins in spring; not a spectacular cultivar as the variegation often fades later in the season and trees suffer from reversion; the rediscovered ‘Foliis Aureomarginatis’ is similar and more stable. Originated in the USA some time before 2000; ​​often erroneously listed under A. glutinosa (Jablonski 2018).

'Rubra'

Widely grown in Finland where it probably originated, Alnus incana ‘Rubra’ has leaves strongly flushed purple in the spring, almost resembling the best purple-leaved forms of Corylus avellana. After several weeks these turn dark green (Jablonski 2018; de Jong 2017).

subsp. kolaensis (N.I. Orlova) Á.Löve & D.Löve

Alnus incana subsp. kolaensis is a tree of northernmost Europe, where it is sympatric with the nominate subspecies but restricted to subalpine and arctic regions. Rare in cultivation outside Scandinavia.

Alnus incana subsp. kolaensis from Norway. Image T. Baxter.

subsp. rugosa (Du Roi) R.T. Clausen

Speckled Alder is variable throughout its range; it is most easily recognised by its thick short-shoot leaves with coarsely double-serrate acute teeth and which are more or less lobulate only on some leaves, unlike other Alnus incana variants that are reliably lobed (n.b. the long-shoot leaves of all A. incana are lobed). The thicker leaf texture distinguishes this taxon from its western cousin, A. incana subsp. tenuifolia. Typical subsp. incana is often a small tree with conspicuously tomentose leaves and shoots which persist until late in the season, while subsp. rugosa becomes more or less glabrous late in the season. Subsp. rugosa intergrades with subsp. tenuifolia in the west of its range and with A. serrulata in the southern part (Furlow 1982). Hybrids with A. serrulata have been treated under Alnus × fallacina Callier; Bean described such material as ‘Clearly intermediate between the two [parents]. Largely distinct and initially close to A. serrulata but distinguished by serrulate leaves slightly lobed and sparsely pubescent, with cones larger’ (Bean 1976).

Speckled Alder is a plant of some ornamental merit and very useful in wet conditions. The largest plant ever recorded in the UK was 15 m tall (1967); it grew at the Royal Botanic Garden Edinburgh (Tree Register 2021).

Alnus incana subsp. rugosa is a pioneer as seen here from the mass seedlings in the wild near Saint Johns, Ontario, Canada. Image Andrew Bodrug.

Young leaves of Alnus incana subsp. rugosa Image Andrew Bodrug.

Old cones of Alnus incana subsp. rugosa. Image Andrew Bodrug.

Alnus incana subsp. rugosa growing near St Johns, Ontario, Canada. Image Andrew Bodrug.

subsp. tenuifolia (Nutt.) Breitung

Thin-leaved Alder is a large shrub or small tree common throughout the western part of North America, from Alaska in the north to California and New Mexico in the south (possibly extending into northern Mexico). It is primarily a mountain species of stream sides, wet fields and meadows, lake edges and moist slopes from about near sea level in the northern part of its range to over 3000 m in Colorado and Arizona (Furlow 1979).

It is distinguished by its thin-textured leaves with larger and more rounded teeth than other A. incana. Its leaves and twigs are more sparsely hairy than the type. It could also be confused in the wild with the sympatric A. oblongifolia and A. rhombifolia, but the buds are more rounded and completely covered in bud scales in A. incana which has stamens and perianth parts in fours and the stamens are shorter than the perianth.

It makes an attractive small tree or large shrub. It is not uncommon in cultivation but remains less frequently planted than the type. It is suitable for wet ground, especially at altitude. In Britain, two trees measured at 12 m in 2013 grow at Blagdon Hall, Northumberland, while a 4.5 m × 94 cm specimen at Borde Hill, W Sussex, blew down around 1945 (The Tree Register 2024).

Alnus incana subsp. tenuifolia has thin textured leaves with larger, more rounded teeth than other forms of the species (Wallowa Lake, OR; 16th June 2022). Image Julian Sutton.

Alnus incana subsp. tenuifolia: bark (Wallowa Lake, OR; 16th June 2022). Image Julian Sutton.

Alnus incana subsp. tenuifolia: vigorous coppice regeneration from a damaged tree (Wallowa Lake, OR; 16th June 2022). This ability to coppice naturally is retained in cultivation Image Julian Sutton.

Alnus incana subsp. tenuifolia is a plant of lakeshores, streamsides and similar moist habitats across the American West. It tends to be more tree-like than the eastern subsp. rugosa (1340 m asl, Wallowa Lake, OR; 16th June 2022). Image Julian Sutton.

'Varjo'

An unusual selection growing much broader than tall; established plants can reach 3–4 × 8–10 m. Raised in Finland, it is barely cultivated outside that country (Jablonski 2018).

'Wonderful Fountain'

Resembling ‘Green Fountain’ in habit but with deeply cut leaves with narrow, toothed lobes; from the breeding programme of Arboretum Borova Hora, Zvolen, Slovakia (Jablonski 2018).