Sorbus Explained

Sorbus is a genus of over 100 species of trees and shrubs in the rose family, Rosaceae. Species of Sorbus (s.str.) are commonly known as rowan or mountain-ash. The genus used to include species commonly known as whitebeam, chequer tree and service tree that are now classified in other genera (see below). The genus Sorbus, as currently circumscribed, includes only the pinnate leaved species of former subgenus Sorbus.^[1]

Sorbus is not closely related to the true ash trees which belong to the genus Fraxinus, although the leaves are superficially similar.

Genus

As treated in its broad sense, the genus was traditionally divided into several subgenera, but this treatment was found to be paraphyletic, comprising two disparate lineages within the Malinae subtribe. Consequently, each of the former subgenera have since been elevated into genera in their own right, with the genus name Sorbus retained only for the rowans. Additionally, it was recognised that despite a comparably large genetic distance between them, the genera Sorbus (sensu stricto), Aria, Chamaemespilus and Torminalis have interbred extensively, in various combinations, which lead to the recognition of an additional five intergeneric hybrid genera.^[2]

Sorbus (Sorbus)

• now genus Sorbus s.s., are commonly known as the rowan (primarily in the UK) or mountain-ash (in Ireland, North America and the UK), with compound leaves usually hairless or thinly hairy below; fruit carpels not fused; the type is Sorbus aucuparia (European rowan). Distribution: cool-temperate Northern Hemisphere.

Sorbus (Aria)

• now genus Aria and its intergeneric hybrids (see below), the whitebeams, with simple leaves usually strongly white-hairy below (hence the name, from German Weissbaum, 'white tree'); fruit carpels not fused; the type is Aria edulis (common whitebeam). Distribution: Europe, Maghreb and western Asia.

Sorbus (Micromeles)

• now genus Micromeles, a group of east Asian species with narrow leaves; sometimes previously included in subgenus Aria. Distribution: temperate and tropical east Asia.

Sorbus (Cormus)

• now genus Cormus, with compound leaves similar to subgenus Sorbus, but with distinct fused carpels in the fruit; just one species, Cormus domestica (true service tree). Distribution: North Africa, warm-temperate Europe, western Asia. Used to flavour some apple wines, see Apfelwein.

Sorbus (Torminaria)

• now genus Torminalis, with rather maple-like lobed leaves with pointed lobes; fruit carpels not fused; just one species (wild service tree). Distribution: warm-temperate Europe, south to the mountains of North Africa and east to the Caucasus and Lebanon ranges.

Sorbus (Chamaemespilus)

• now genus Chamaemespilus, monotypic, with a single shrubby species, false medlar. Has simple, glabrous leaves and pink flowers with erect sepals and petals. Distribution: mountains of southern Europe.

Intergeneric hybrid genera

• Hedlundia - Aria × Sorbus hybrids
• Karpatiosorbus - Aria × Torminalis hybrids
• Majovskya - Aria × Chamaemespilus hybrids
• Scandosorbus - syn. Borkhausenia; Aria × Sorbus × Torminalis hybrids
• Normeyera - Aria × Chamaemespilus × Sorbus hybrids

Uses

Ornamental trees

Sorbus species are cultivated as ornamental trees for parks and gardens and as avenue trees, and have given rise to several cultivars. The following, of mixed or uncertain parentage, have gained the Royal Horticultural Society's Award of Garden Merit:^[3]

• 'Eastern Promise'^[4] (purple autumn colour, pink berries)
• 'Leonard Messel' named for Leonard Messel of Nymans in Sussex.^[5] (small tree to 4m, pink berries)
• 'Wisley Gold' ^[6] (yellow berries)

Further reading

• Price, D.T. 2007. One-way introgressive hybridisation between Sorbus aria and S. torminalis (Rosaceae) in southern Britain. Watsonia. 26: 419–431.

Notes and References

1. Web site: Sorbus L. Plants of the World Online Kew Science . 2025-01-31 . Plants of the World Online . en.
2. Rushforth, Keith (2018). The Whitebeam problem, and a solution. Phytologia 100(4): 222-247. 21 December 2018.
3. Web site: AGM Plants - Ornamental . July 2017. 98 . Royal Horticultural Society . 12 November 2018.
4. Web site: RHS Plantfinder - Sorbus 'Eastern Promise' . 12 November 2018.
5. Web site: RHS Plantfinder - Sorbus 'Leonard Messel' . 12 November 2018.
6. Web site: RHS Plantfinder - Sorbus 'Wisley Gold' . 12 November 2018.