Alnus glutinosa (L.) Gaertn.

TSO logo

Sponsor

Kindly sponsored by
a member of the International Dendrology Society

Credits

Tim Baxter & Hugh A. McAllister (2024)

Recommended citation
Baxter, T. & McAllister, H.A. (2024), 'Alnus glutinosa' from the website Trees and Shrubs Online (treesandshrubsonline.org/articles/alnus/alnus-glutinosa/). Accessed 2024-05-27.

Genus

  • Alnus
  • Subgen. Alnus, Sect. Glutinosae

Common Names

  • Common Alder
  • Black Alder

Synonyms

  • Alnus vulgaris Hill
  • Betula alnus var. glutinosa L.
  • Betula alnus subsp. glutinosa (L.) Ehrh.
  • Betula glutinosa (L.) Lam.

Glossary

acuminate
Narrowing gradually to a point.
acute
Sharply pointed.
apex
(pl. apices) Tip. apical At the apex.
emarginate
Notched at the apex.
entire
With an unbroken margin.
glabrous
Lacking hairs smooth. glabrescent Becoming hairless.
pedunculate
With a peduncle.
pendent
Hanging.
ploidy
Number of chromosomes.
retuse
Slightly notched at apex.
sessile
Lacking a stem or stalk.
subspecies
(subsp.) Taxonomic rank for a group of organisms showing the principal characters of a species but with significant definable morphological differentiation. A subspecies occurs in populations that can occupy a distinct geographical range or habitat.

Credits

Tim Baxter & Hugh A. McAllister (2024)

Recommended citation
Baxter, T. & McAllister, H.A. (2024), 'Alnus glutinosa' from the website Trees and Shrubs Online (treesandshrubsonline.org/articles/alnus/alnus-glutinosa/). Accessed 2024-05-27.

Tree to 35 m, crown pyramidal or oval, occasionally multi-stemmed. Bark dark grey-brown when young, darkening with age and becoming fissured. Young twigs usually glabrous, viscid, terete. Buds ovate with 2–3 scales, green, viscid, 2–3 mm, stipe 2 mm. Leaves obovate, elliptic, suborbicular or broadly ovate, 4–11 × 3.5–11 cm, apex rounded, obtuse or emarginate, base cuneate, truncate to rounded, margins lobulate and double toothed with 1–6 teeth between primary veins, blade glabrous adaxially, with hairy domatia abaxially in vein axils, 7–8 veins either side of midrib, craspedodromous. Petiole 20–50 mm, thin, ± glabrous. Male catkins in a lax raceme on a 7 mm peduncle, formed in autumn and exposed over winter, , 30–80 × 2–6 mm extending to, 80–120(–160) mm at anthesis, reddish or brownish, anthers red, loose. Female catkins formed in autumn and opening in spring, pendulous to semi-erect lax raceme, 4–6 × 1–2 mm, peduncle 5–15 mm long, angled and grey with warty appearance at maturity. Fruit borne semi-erect, dry-resinous, 8–12 × 6–20 mm, ovoid, apex and base truncate, green at first, dark brown or black at maturity. Seed very narrowly winged. Diploid, rarely triploid, 2n=28, 42. (Tutin et al. 1972; Vit et al. 2017; T. Baxter, pers. obs.).

Distribution  AlbaniaAlgeriaArmeniaAustriaAzerbaijanBelarusBelgiumBosnia and HerzegovinaBulgariaCroatiaCzechiaDenmarkEstoniaFinlandFrance including Corsica GeorgiaGermanyHungaryIranIrelandItalyKazakhstanLatviaLithuaniaLuxembourgNorth MacedoniaMoldovaMonacoMoroccoNetherlandsNorwayPolandPortugalRomaniaRussiaSerbiaSlovakiaSloveniaSpainSwedenSwitzerlandTunisiaTurkeyUkraineUnited Kingdom

Habitat Wet soils, stream and lake sides, wet flushes, fens and damp woodlands. Common throughout most of its range to 1800 m asl.

USDA Hardiness Zone 3-7

RHS Hardiness Rating H7

Conservation status Least concern (LC)

Alnus glutinosa is the common alder found throughout Europe and Russia as far east as the Ural Mountains, and south into the Caucasus and northern Iran (Tutin et al. 1964). It is variable across its extensive range and the latest taxonomic understanding is that there are four subspecies: subsp. glutinosa is the typical form found throughout much of its range; subsp. barbata occurs in the Caucasus mountains, north-east Turkey and northern Iran; subsp. antitaurica occurs in northern Turkey and south-eastern Europe; subsp. betuloides occurs in a small area of eastern Turkey (Plants of the World Online 2021).

The typical form in Europe is a large tree with dark brown fissured bark, hairless twigs that are sticky when young, grey-brown obovate buds on long (1–3 mm) stalks; its leaves are flat, matt, broader than long with an apical notch (retuse to occasionally emarginate), irregularly lobed at the margins; female catkins are pedunculate and somewhat pendent. It is easily distinguished from A. incana which has hairy twigs, leaves that are acuminate to acute at the apex and often evenly lobulate, grey or yellow bark, and relatively smooth, sessile female catkins. Alnus matsumurae of Japan is vegetatively similar but its leaves are always emarginate and glabrous, its buds elliptic with an acute apex, and its erect fruit are larger.

Within what has been treated as Alnus glutinosa, two new polyploid taxa have been named recently as A. rohlenae from south-east Europe and A. lusitanica from Spain (Vit et al. 2017). Tetraploid ‘glutinosa’ has been known since the 1980s with an anomalous count for plants originating from the Sperkios Valley and grown at Ness (H. McAllister pers. obs.). This modern investigation and discovery of a clear pattern in distribution of ploidy levels does appear to support the separation of two new species.

In cultivation all forms of Alnus glutinosa grow into medium-large trees or upright multi-stemmed shrubs. It is a rather plain species apart from some of the cultivars with more attractive foliage; despite this it is still one of the most widely planted of all alders in parks, gardens and urban environments, not only within its native range but in North America too (at least in the guise of its many cultivars) (Jacobson 1996). In the wild it grows in wet soils, especially in riparian habitats. In cultivation it is moderately sensitive to drought and makes a useful tree for planting in wet places where few other trees would thrive. In drought-prone areas, provenances from SE Europe, Turkey and Iran (subspp. barbata, antitaurica and betuloides) should be more widely grown. A. glutinosa is commonly used in agroforestry for its nitrogen fixing ability, and it has been widely introduced outside its natural range for land stabilisation and amenity. However, it is also a moderate to serious non-native invasive species of wet sites in parts of North America, Australia and New Zealand (CABI 2018).

The tallest recorded tree in Britain grows at Audley End, Saffron Walden (32 m), with the largest a coppice stool at Furze Hills, Essex with a stool size of 14.75 m (Tree Register 2021). Plants are easy to raise from seed but hybrids do occur in cultivation, as noted above. Alder seed germinates in water (P. Bartlett pers. comm. 2015) but needs potting on as soon as the first true leaves appear. Seed tolerates immersion in salt water for at least two weeks and can be found germinating on drift lines. A. glutinosa is especially susceptible to Alder Leaf Beetle, Agelastica alni, and this beetle can strip the foliage from entire stands of trees. Trees appear to survive relatively unharmed but are likely to become more susceptible to other stress factors such as seasonal drought and Phytopthora alni.

The timber of Alnus glutinosa has many contemporary and historic uses including for charcoal, gunpowder, and turning. Perhaps its best-known traditional use in Britain was for making clogs, especially in mill towns in the north of England: H.J. Elwes gives a fascinating account of the process and economics of clog-making in The Trees of Great Britain and Ireland (vol. IV) (Elwes & Henry 1906–1913). Its timber, which turns bright rusty-orange when freshly cut and exposed to air, is not especially rot-resistant, except when submerged in water when it has a high degree of water resistance. It was frequently used for underwater piles and supports, most notably throughout Venice, and for mill wheels, troughs, and lock gates on canals. It has also been used for dye making. The bark is astringent and has been used to treat sore throats and inflammation (Meier 2021).


'Angustifolia'

Very similar to the widely grown ‘Laciniata’ but with more deeply cut leaves with narrower lobes; it arose in Russia in 1957 but appears never to have been much planted elsewhere (de Jong 2017).


'Aurea'

A tree with golden yellow leaves and yellow winter twigs, not so vigorous as the type, first raised in Vervaene’s nursery, Ledeberg-les-Gand, Belgium, about 1860 (de Jong 2017). ‘Thillie Trompenburg’ is similar (Jablonski 2018).


'Carelica'

Synonyms / alternative names
Alnus glutinosa 'Muhkura'
Alnus glutinosa 'Mukura'
Alnus incana 'Gibberosa'

A very unusual plant from Finland with stems and branches covered in horrid, subglobose, wart-like swellings, giving the wood a curled grain, called “visa” formation in Finnish. Propagation by in vitro only (Jablonski 2018). It has occasionally been treated as a Grey Alder cultivar (Alnus incana ‘Gibberosa’).


'Cascade'

A vigorous and more or less pendulous form, the only such aberrance known among A. glutinosa. It originated at the Arboretum Borova Hora, Zvolen, Slovakia, sometime before 1998,a product of a breeding program to improve ornamental Alnus (Jablonski 2020).


'Charles Howlett'

Alnus glutinosa ‘Charles Howlett’ was first registered in 1986 by D.M. Howlett of Winchester, UK. The foliage is variously shaped and variegated, and the bark is streaked yellow to orange. Slightly less vigorous than ordinary A. glutinosa, it is believed to be rare in cultivation but is mentioned in catalogues in Europe and North America (Jablonski 2020).


'Drucourt'

A selection raised in France before 2015 with the leaves irregularly variegated with large sections of white. Although named, it isn’t clear if this selection has been propagated and distributed, nor how stable the vareigation is (Jablonski 2018).


f. minutifolia H.Lindb. ex Hiitonen

Synonyms
'Minutifolia'

A plant occurring naturally in the Åland archipelago between Sweden and Finland. A short-growing, small-leaved form probably not in cultivation (de Jong 2017).


'Foggy Bottom Sunshine'

A gold-leaved selection made by Adrian Bloom of Bresingham, Norfolk, UK named for his garden, Foggy Bottom. It has had limited distribution in the UK.


frøkilde Sauherad E

An elite seed strain offered and quite widely planted in Norway, recommended for the country’s southern lowlands. The name translates simply as ‘seed source Sauherad’ (a former municipality in Telemark), the ‘E’ suffix denoting suitability for Nordic conditions (Elite Planter 2024).


Fyris E

Plants marketed in Sweden as Alnus glutinosa Fyris E since 2002 are raised from seed gathered from a source near the Fyris River, northwest of Uppsala, 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, develop attractive conical crowns and grow rapidly even on poor soils (de Jong 2017).


'Greenwood'

A seedling from the species selected by J. Mauritz of Opheusden, the Netherlands, before 2001. It is an upright tree with twisting branches. In older age the main stem grows straight upwards forming an upright tree. ‘Greenwood’ is marketed by Van Aart Boomkwekerijen from Oudenbosch (Jablonski 2020).


Hybrids

Wild Alnus glutinosa hybrids are not uncommon and can be almost impossible to determine accurately. These are generally always intermediate between the two parents. For example in 1964 P.W. Ball described the hybrid A. × pubescens (A. glutinosa × A. incana) in the first volume of Flora Europaea (Tutin et al. 1964) which has young twigs and leaves pubescent (at least abaxially on veins) with obtuse to shortly acuminate apices, and female catkins shortly stalked. Such hybrids are variable and frequently occur wherever the parents grow together (Banaev & Bazant 2007). Hybrids are also known to exist with A. cordata (A. × elliptica), A. incana subsp. rugosa (A. × silesiaca), and A. rubra (A. × ljungeri) but the status of the latter is uncertain. Only A. × elliptica is at all widely grown and this is discussed separately.

This situation gets considerably more complex in southern Europe, the Caucasus and Iran. Zare & Amini (2012) name several new species in this region, but these should be regarded with caution pending further work. There is some reluctance among botanists to name new taxa in regions where genetically compatible species overlap, especially in wind pollinated genera, unless there is clear evidence of biological separation rather than purely morphological differences. It is here suggested that hybridisation and introgression with the diploid species Alnus incana, A. orientalis, and A. subcordata may be involved. Polyploid forms of some of these taxa are already known to exist. The situation in Alnus taxa across this region is likely to be similar to that found in Betula (e.g. B. utilis subsp. occidentalis) (McAllister & Ashburner 2013), where there is considerable mixing of taxa where numerous compatible species intermingle as species spread out (mostly northwards) from refugia found throughout these regions. The infraspecific taxonomy adopted here – accepting A. glutinosa subspp. antitaurica, barbata and betuloides – remains the best reflection of current understanding but this position may change in the future.

  • Alnus × ellipticaA. glutinosa × A. cordata (the only hybrid discussed here at all widely grown, it is discussed further under A. × elliptica)
  • Alnus × ljungeriA. glutinosa × A. rubra (?)
  • Alnus × pubescensA. glutinosa × A. incana
  • Alnus × silesiacaA. glutinosa × A. incana subsp. rugosa

'Imperialis'

Awards
AGM

A thin, rather ungainly but upright slender small tree of slow growth, prized for its filigree-like, deeply cut and pinnately lobed sea green leaves (Bean 1976). The lobes, reaching more than half-way to the midrib, are lanceolate, slender and pointed; they are not toothed. First selected as a seedling by Brossart of Alençon, France c. 1855, it was noted as being in wider cultivation in Orléans in 1858, and was probably introduced elsewhere at about the same time. It is one of the most commonly planted of the A. glutinosa cultivars and arguably one of the most attractive, at least in foliage (Jablonski 2018).


'Incisa'

Synonyms / alternative names
Alnus glutinosa var. incisa Willd
Alnus glutinosa f. incisa (Willd.) Koehne
Alnus glutinosa oxyacanthifolia Hort.

Leaves small, usually less than 2.5 cm long, rounded or ovate in outline, deeply cut into broad, toothed lobes, or even right to the midrib. The plant once grown at Kew made a dwarf, compact bush and was probably of the clone distributed by Loddiges’ Hackney nurseries in 1836, and by Booth’s nurseries, Hamburg, Germany, as A. glutinosa oxyacanthifolia – the thorn-leaved alder.

The name Alnus glutinosa var. incisa has also been used, wrongly, for the clone ‘Laciniata’ (q.v.). The tallest currently known in cultivation is 20 m tall, in Alexandra Park, Hastings, UK (Tree Register 2021).


'Laciniata'

Alnus glutinosa ‘Laciniata’ was originally found growing in a garden near St Germain, Paris, in 1819. It is often likened to ‘Imperialis’ but they are actually quite different: ‘Laciniata’ is a sturdier tree of stronger growth with the leaves neither so deeply cut nor narrowly lobed; it differs from ‘Incisa’ in the leaves oblong in outline with the lobes pointed and untoothed, cf. rounded in outline with the lobes toothed in ‘Incisa’ (Bean 1976; de Jong 2017).


'Luszyn'

A tree found in a park in Luszyn in central Poland by Włodzimierz Seneta and erroneously named ‘Pyramidalis’ at first. As a young tree it has a rounded crown which later developts an upright, very regular, columnar habit, though much wider than ‘Pyramidalis’ (Jablonski 2020). See also discussion under ‘Pyramidalis’.


'Pyramidalis'

Synonyms / alternative names
Alnus glutinosa 'Fastigiata'
Alnus glutinosa fastigiata hort.
Alnus glutinosa f. pyramidalis (Dipp.) Winkler
Alnus glutinosa 'Birkiana'
Alnus glutinosa 'Pyramidalis Birkiana'

Alnus glutinosa ‘Pyramidalis’ is generally considered the original name for a beautiful, narrow, fastigiate Black Alder distributed by Späth of Berlin from 1880 (Jablonski 2020). In many Alnus cultivars a proportion of seed-raised plants will come true to type; those conforming closely (or even approximately) to their parent will often be labelled and distributed under the same name, despite not being clonal reproductions. This problematic practice is especially common when the parent is difficult to propagate vegetatively, as seems to have originally been the case with ‘Pyramidalis’ (de Jong 2017). Consequently, Piet de Jong found at least two clones cultivated in the Netherlands under this name, and evidence for others in Finland, Sweden and the United States (de Jong 2017). This is not surprising given that even in the early 20th century Späth was also marketing plants under the alternative name ‘Pyramidalis Birkiana’, which de Jong hypothesises represent a distinct origin, probably from the Pirkkala district south of Tampere, Finland, while a tree received by Arboretum Trompenburg from the Hillier Nurseries in 1965 (with an accompanying note from Harold Hillier explaining that they had grafted it especially from a tree at Kew) was labelled Alnus glutinosa fastigiata (de Jong 2017).

More recently, a tree found in Poland was originally identified as ‘Pyramidalis’ but has since been renamed ‘Luszyn’ on account of its distinctive origin and habit, while ‘Sakari’, from Finland, is different again (see separate entries above and below). Clearly there is some merit in establishing a formal Cultivar Group (Pyramidalis Group) as a catch-all for fastigiate A. glutinosa of uncertain origin, whilst maintaining named clones within it.


'Rubrinervia'

Leaves with red veins and stalks.

'Sääksmäki'

A narrow-crowned cultivar from Sääksmäki, western Finland, selected about 2004; similar to ‘Sakari’ (Jablonski 2020).


'Sakari'

A fastigiate alder with attractive foliage. It was selected in Finland c. 1990 by Prof. Sakari Saarnijoki of the Finnish Forest Research Institute (Metla) in Kotka. Growing to 15 m high and 2–4 m wide, it is now one of the most popular street trees in Scandinavia. It has excellent cold tolerance and is good for heavy clay soils (Jablonski 2020). ‘Sääksmäki’ is similar; see also discussion under ‘Pyramidalis’.


'Sorbifolia'

Synonyms / alternative names
Alnus glutinosa 'Lacera'
Alnus glutinosa f. lacera (Mela) Mela

Described by Dippel in 1892, Alnus glutinosa ‘Sorbifolia’ has leaves oblong-oval in outline, deeply cut into about six pairs of broad lobes, resembling the leaves of Scandosorbus (Sorbus) intermedia hence ‘Sorbifolia’ (de Jong 2017). Similar forms occur wild in Scandinavia and have been treated as f. lacera (Mela) Mela (Bean 1976); ‘Sorbifolia’ may have been lost to cultivation (Jablonski 2018).


subsp. antitaurica Yalt.

Common Names
Toros Alder

Differing from the type in twigs and petiole more or less tomentose; leaves elliptic and more shallowly lobed, apex blunt, green-greyish and pubescent especially when young, at least densely so along the veins; fewer secondary leaf veins (4–8 pairs) than subsp. barbata (cf. 8–11 pairs). (Davis 1984; Colagar et al. 2016).

Distribution

  • Iran – northern Hyrcanian forests only
  • Turkey – southern Anatolia

Alnus glutinosa subsp. antitaurica has a restricted distribution to the west of Lake Van Golu in south central Turkey (Ansin & Ozder 1993) and in Hyrcanian forests of northern Iran (Zare & Amini 2012). Colagar et al. (2016) found that, at least in northern Iran, subsp. antitaurica is genetically separated from both subsp. glutinosa and subsp. barbata. Its ploidy level is not known but is likely to be diploid. This taxon is very rare in cultivation, indeed it seems to only be grown within its native countries. Nevertheless, it might prove to be a drought and heat tolerant plant of ornamental merit with furry leaves, potentially of use in horticulture elsewhere.


subsp. barbata (C.A.Mey.) Yalt.

Synonyms
Alnus denticulata C.A. Mey

Distinguished from the type by: downy shoots; ovate-elliptic leaves that are rounded or acute at the apex (lacking the typical apical notch), with a higher number of secondary leaf veins (8–11), dark glossy green above and abaxially downy especially along the veins. Diploid, 2n=28. (Davis 1984; Colagar et al. 2016).

Distribution

  • Azerbaijan
  • Georgia
  • Russia – Caucasus only
  • Turkey

RHS Hardiness Rating: H7

Alnus glutinosa subsp. barbata is found in north-eastern Turkey and from the Caucasus to northern Iran. This taxon is rather similar to plants much further north and west and intermediates may exist, especially in light of recent work into the southern European complex within A. glutinosa (Vit et al. 2017). This work did not extend into Turkey, but Ansin & Ozder (1993) provide an accurate distribution map and Colagar et al. (2016) found it to be genetically distinct from subsp. glutinosa, with subsp. antitaurica somewhat intermediate between the two. It is not uncommon in cultivation and has so far proven to be a more drought tolerant plant than the typical form. The largest plant in UK cultivation is at Hergest Croft, Herefordshire, which stands 25 m tall (Tree Register 2021). Many larger arboreta grow this plant including Kew, Edinburgh, Ness, and Stone Lane Gardens in the UK, as well as in the National Plant Collection of Alnus at St Aubin, Jersey.


subsp. betuloides Ansin

Distinguished from the type by: bark white on stems and older branches, trunks of mature trees appearing white to dark grey and deeply fissured; leaf base broadly cuneate; fruit held in dense clusters of 3–5. (Ansin & Ozder 1993).

Distribution

  • Turkey – Bitlis

Alnus glutinosa subsp. betuloides was first described by Ansin & Ozder (1993). It is most easily distinguished by the unusual white bark on its stems and branches. It is restricted to eastern parts of Turkey to the west of Lake Van. It is uncertain whether this taxon is worthy of its status as a separate subspecies; its close proximity, overlapping range and similarities to subsp. glutinosa could justify treatment as a local variant or hybrid rather than a subspecies. Sadly, Colagar et al. (2016) did not include this taxon in their useful analysis. Ploidy level is unknown but it is likely to be diploid. It is uncommon, but far from absent in cultivation; the largest tree is 8 m tall (2019) at Howick Hall, Northumberland, with other plants present at the Hillier Gardens and Kew in the UK (The Tree Register 2024).


'Thillie Trompenburg'

A yellow-leaved form which arose as a chance seedling at Arboretum Trompenburg, Rotterdam, in 2001, one of many that germinated between trees of Alnus glutinosa ‘Aurea’ and ‘Pyramidalis’ (the latter being the tree grafted from Kew and sent by Hilliers as ‘A. glutinosa fastigiata’ – see discussion under ‘Pyramidalis’); a proportion of these were yellow but this seedling was unique in combining yellow foliage with an upright branching structure, presumably the result of cross pollination between the two extant trees. Spotted by Gert Fortgens, it was removed to the nursery and then planted out in 2004; by 2017 it was 12 m tall with a canopy spread of 3 m. It was introduced to the trade in 2018, and while still rare it shows promise. The cultivar name commemorates the garden volunteer who tended the young seedling in its early years, and the arboretum where it was found (Jablonski 2018; G. Fortgens pers. comm. 2023).