Eurybia (Cass.) Cass.
References with an asterisk treat Aster in a broad way, including Eurybia and other currently accepted aggregates.
Eurybia is a genus of 23 species, all native to North America. Eurybia sibirica (L.) Nesom (syn.: Aster sibiricus L.) also occurs in northern Eurasia. It is a segregate of the genus Aster. From Symphyotrichum it is distinguished by outer involucral bracts that are less than 2,5x as long as wide, rounded to obtuse at apex and a corymb-like inflorescence. The only Belgian representative of the genus has leaves that are deeply cordate at base.
Several members of the genus Eurybia (all with basally cordate leaves) are cultivated as ornamentals in Europe (Yeo 2000, Jäger & al. 2008). Eurybia schreberi (Nees) Nees (syn.: Aster schreberi Nees) appears to be the usual garden escape, at least in the British Isles (Sell & Murrell 2006, Stace 2010). It is closely related to Eurybia divaricata but produces sterile rosettes on short rhizomes and has leaves with 15-30 teeth per side (versus 6-15 teeth in E. divaricata). A third taxon with cordate leafbases is now accommodated in Symphyotrichum (S. cordifolium (L.) Nesom; syn.: Aster cordifolius L.). It has linear-lanceolate involucral bracts that are more or less acute at apex and a paniculiform inflorescence with many heads. As a rule, species of Aster s.l. (Eurybia, Symphyotrichum) with cordate leafbases are only rarely cultivated in Belgium.
References
Brouillet L. (2006) Eurybia. In: Flora of North America Editorial Committee (eds.), Flora of North America, vol. 20. Oxford University Press, New York-Oxford: 365-382.
Hoffman M.H. (1996) Die in Zentraleuropa verwilderten und kultivierten nordamerikanischen Astern. Feddes Repert. 107(3-4): 163-188.
Jäger E.J., Ebel F., Hanelt P. & Müller G. (eds.) (2008) Rothmaler Band 5. Exkursionsflora von Deutschland. Krautige Zier- und Nutzpflanzen. Springer Verlag, Berlin: 880 p.
Sell P. & Murrell G. (2006) Flora of Great Britain and Ireland. Vol. 4 Campanulaceae – Asteraceae. Cambridge University Press, Cambridge: XXVIII + 624 p.
Stace C. (2010) New flora of the British Isles, 3th ed.: XXXII + 1232 p. Cambridge University Press.
Thellung A. (1913) Die in Mitteleuropa kultivierten und verwilderten Aster- und Helianthusarten nebst einem Schlüssel zur Bestimmung derselben. Allg. Bot. Zeitschr. 19: 87-89, 101-112, 132-140.
Yeo P.F. (2000) Aster. In: Cullen J. & al. (eds.), The European Garden Flora, vol. 6. Cambridge University Press, Cambridge: 570-578.