Symphyotrichum pilosum (Willd.) Nesom var. pilosum (syn.: Aster pilosus Willd., A. ericoides L. var. pilosus (Willd.) Porter) (N-Am.) – A very rare, locally naturalized alien. Formerly sometimes cultivated as an ornamental but, in view of its caespitose (not colonial) habitat, rather unsuitable for this purposes. First recorded on a dump in Ensival in 1947 and subsequently on a dump near Mechelen (Galgenberg) in 1963. These records are probably associated with garden waste. A single individual was observed along a railway track in the port of Gent in 1996 (vector of introduction obscure, possibly as a grain alien; see also Chmielewski & Semple 2001). Discovered in abundance on waste land near a timber storage in the port of Antwerpen (angle Amerikadok – Albertdok) in 2007 and obviously naturalized since quite some time there (location destroyed soon afterwards). Likewise, several specimens have been recorded at a disused timber storage in Heinsch (Stockem) in 2008 where it has been introduced between 1986 and 2000 with timber from Pennsylvania and adjacent states (along with, among others, Euphorbia vermiculata and Symphyotrichum lateriflorum). In Belgium Symphyotrichum pilosum is confined to man-made habitats. It never forms dense stands (not colonial); future invasive behavior therefore is rather unlikely.
Symphyotrichum pilosum is easily distinguished from the other Belgian representatives of the genus: stems densely hairy throughout (not in lines), involucral bracts involute at apex (appearing spine-like) and plants caespitose (not colonial). However, several additional species share this particular combination of characters, especially Symphyotrichum ericoides (L.) G.L. Nesom. The latter has smaller heads and spine-tipped involucral bracts and is sometimes claimed as an escape from cultivation (probably always in error).
All Belgian populations seen so far are distinctly hirsute hairy and belong with var. pilosum. Glabrous plants are in fact more often cultivated and are ascribed to var. pringlei (A. Gray) Nesom (syn.: Aster pringlei (A. Gray) Britton). They have been found as escapes from cultivation in other parts of Europe (e.g. Italy, Spain).
Selected literature:
Chmielewski J.G. & Semple J.C. (2001) The biology of Canadian weeds. 114. Symphyotrichum pilosum (Willd.) Nesom (Aster pilosus Willd.). Canad. J. Pl. Sc. 81: 851-865. [available online at: http://pubs.aic.ca/doi/pdf/10.4141/P00-074]
Harriman N.A. (1978) Chromosome numbers of Aster ericoides L. and Aster pilosus Willd. (Compositae). Sida 7(4): 396.
Kaul M.K. (1975) New plant records for India from Kashmir: 2. J. Bombay Nat. Hist. Soc. 72(2): 609-612.
Semple J.C. (1978) The cytogeography of Aster pilosus (Compositae): Ontario and the adjacent United States. Canad. J. Bot. 56(10): 1274-1279.
Semple J.C. & Chmielewski J.G. (1985) The cytogeography of Aster pilosus (Compositae-Astereae): 2. Survey of the range, with notes on A. depauperatus, A. parviceps and A. porteri. Rhodora 87 (851): 367-379.