Sorbaria (Seringe ex DC.) A. Braun
Sorbaria is a small genus native in eastern Asia. As currently (i.e. narrowly) circumscribed (Rahn 1989) it counts four species and all are cultivated for ornament in Europe (e.g. Cullen 1995, De Koning & al. 2000, Roloff & Bärtels 2006, etc.). Many more species have been described but these are now given infraspecific rank or reduced to synonymy. All species have been recorded in the wild in Europe (e.g. Stace 2010), three of them also in Belgium.
In general appearance all species of Sorbaria are much alike and the genuine distribution and frequency of the species encountered in Belgium should be critically (re-) assessed (although S. sorbifolia seems to be, by far, the commonest species in the wild as well as in cultivation). Most species also seem to hybridize. Hybrids have a reduced fertility and fruits are not produced (Rahn 1989).
All species of Sorbaria are freely suckering and may eventually form dense thickets. They are therefore often considered undesirable environmental weeds.
For an accurate identification mature fruits are helpful although all species may be told apart on floral characters as well.
1 Follicle (fruits) covered by short simple hairs, 4,5-5,6 mm long. Sepals acute, longer than broad, with glandular margins. Stamens often more than 30. Inflorescence usually erect === Sorbaria sorbifolia
Follicle glabrous (very rarely with a few simple hairs), 2,7-5,3 mm long. Sepals obtuse, broader than long, glabrous. Stamens usually 20. Inflorescence usually pendulous === 2
2 Follicle with style attached well below apex. Longest stamens ca. twice as long as petals. Lower leaf surface often with stellate hairs === S. kirilowii
Follicle with style attached at or near apex. Longest stamens ca. as long as petals. Lower leaf surface exceptionally with stellate hairs === S. tomentosa
Literature:
Cullen J. (1995) Sorbaria. In: Cullen J. & al. (eds.), The European Garden Flora, vol. 4. Cambridge University Press, Cambridge: 328-329.
De Koning J., Van den Broek J.W., Van de Laar H.J. & Fortgens G. (2000) Nederlandse dendrologie (13e druk). H. Veenman & zonen, Ede: 585 p.
Duvigneaud J. (1975) Les spiréoïdées (Rosaceae subfam. Spiraeoideae) en Belgique et dans les régions voisines: premier essai de traitement. Nat. Mosana 28(2): 33-55.
Gu C. & Crinan A. (2003) Sorbaria. In: Zhengyi W. & Raven P.H. (eds.), Flora of China, vol. 9, Missouri Botanical Garden Press: 75-76. [available online at: http://flora.huh.harvard.edu/china/PDF/PDF09/Sorbaria.PDF]
Henrickson J. (2014) Sorbaria. In: Flora of North America Editorial Committee (eds.), Flora of North America, vol. 9. Oxford University Press, New York-Oxford: 396-398. [available online at: http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=130714]
Rahn K. (1989) A survey of the genus Sorbaria (Rosaceae). Nord. J. Bot. 8(6): 557-563.
Roloff A. & Bärtels A. (2006) Flora der Gehölze (2e Auflage). Ulmer, Stuttgart: 844 p.
Stace C. (2010) New flora of the British Isles, 3th ed.: XXXII + 1232 p. Cambridge University Press.