Cirsium Mill.
Cirsium is a genus of ca. 200 species native in the northern hemisphere. Eight species are native in Belgium (see Lambinon & al. 2004). Rather few are cultivated as ornamentals in Europe (see Knees 2000 and Jäger & al. 2008 for an overview). Some species (for instance Cirsium arvense and C. vulgare) have become noxious, nearly cosmopolitan weeds.
1. Flowers yellowish white. Heads subtended by large leaf-like bracts (native) === Cirsium oleraceum
1. Flowers purplish or pinkish (rarely white) but never yellowish. Heads not subtended by large leaf-like bracts === 2
2. Plant stemless (native) === C. acaule
2. Plant with distinct stem === 3
3. Leaves with rigid setae on upper surface (native) === C. eriophorum and C. vulgare
3. Leaves without rigid setae on upper surface === 4
4. Heads (sub) sessile or shortly stalked, in clusters of 2-8 === 5
4. Heads distinctly stalked, not in clusters === 6
5. Lower leaves long decurrent in spiny wings. Stem more or less leafy throughout. Biennial (native) === C. palustre
5. Leaves not decurrent, almost all basal. Perennial === 2. Cirsium rivulare
6. Heads usually numerous in a corymb-like panicle (native) === C. arvense
6. Heads usually solitary or more rarely in pairs === 7
7. Cauline leaves widened at base, amplexicaul, white tomentose beneath. Heads 35-50 mm across === 1. C. heterophyllum
7. Cauline leaves narrowed at base, not amplexicaul (at most slightly auriculate), green or white tomentose beneath. Heads 20-30 mm across (native) === C. dissectum and C. tuberosum
Literature:
Arènes J. (1954) Les composées – Cynarocéphales de Belgique. Bull. Jard. Bot. Etat 24: 241-327.
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.
Knees S.G. (2000) Cirsium. In: Cullen J. & al. (eds.), The European Garden Flora, vol. 6. Cambridge University Press, Cambridge: 527.
Lambinon J., Delvosalle L., Duvigneaud J. (avec coll. Geerinck D., Lebeau J., Schumacker R. & Vannerom H. (2004) Nouvelle Flore de la Belgique, du Grand-Duché de Luxembourg, du Nord de la France et des Régions voisines (Ptéridophytes et Spermatophytes). Cinquième édition. Jardin botanique national de Belgique, Meise: CXXX + 1167 p.
Werner K. (1976) Cirsium. In: Tutin T.G. & al. (eds.), Flora Europaea, vol. 4. Cambridge University Press, Cambridge: 232-242.