Symphyotrichum xsalignum (Willd.) Nesom (= S. lanceolatum var. lanceolatum x S. novi-belgii var. novi-belgii; syn.: Aster xsalignus Willd.) (hort.) – A very rare but poorly understood species. Possibly locally naturalized but widely confused with its putative parents.
Symphyotrichum xsalignum is a critical taxon. Its hybrid nature is sometimes contested but Jones & Hiepko (1981) found out that the type does not correspond with any of the North American taxa of Symphyotrichum. Hence, a hybrid origin (formed naturally in Europe or artificially in cultivation) is inevitable. It is a very variable taxon and some plants are close to either parents. Most typically, Symphyotrichum xsalignum has the violet-blue ligules and slightly clasping-auriculate leaf bases of S. novi-belgii and the narrowly lanceolate leaves and unequal involucral bracts of S. lanceolatum.
The interpretation of Symphyotrichum xsalignum in Europe is still very controversial. Adema (1983) proved that all Dutch records in fact belonged with either Symphyotrichum lanceolatum or S. xversicolor (incl. S. laeve). However, in the British Isles Symphyotrichum xsalignum is said to be the commonest taxon (Sell & Murrell 2006, Stace 2010) and this also applies to France (Tison & de Foucault 2014).
The similar Aster concinnus Willd. (a synonym of Symphyotrichum laeve according to present-day American authors) might have been overlooked. Its leaves are clasping and its involucral bracts are more unequal (the outer at most ½ as long as the inner) (Sell & Murrell 2006).
Selected literature:
Adema F. (1983) Drie kleinbloemige Aster-soorten in Nederland. Gorteria 8(11): 194-195.
Jones A.G. & Hiepko P. (1981) The genus Aster s.l. (Asteraceae) in the Willdenow Herbarium at Berlin. Willdenowia 11: 343-360.
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.
Tison J.-M. & de Foucault B. (coord.) 2014) Flora Gallica. Flore de France. Editions Biotope, Mèze : xx + 1196 p.