Centaurea diffusa Lam. (SE-Eur., SW-As.) – A very rare and ephemeral alien. Formerly associated with the wool processing industry. In 2018 a single individual was recorded in a roadside in the Ghent port area, probably as grain alien. In the same year C. diffusa was also discovered on a coal mining spoil heap in Hénin-Beaumont in northern France, without obvious vector of introduction (Lemoine 2018). In 2019 the species was recorded again, with several individuals, in the Ghent port area.
Centaurea diffusa is one of the parent species of C. xpsammogena (C. diffusa x stoebe). The hybrid plant is very variable often within a single population: flowers can range in color from white to purple and from disciform to radiating. Compared with the latter, C. diffusa has always non-radiating white flowers, the pappus is absent or very inconspicuous and the terminal spine of the appendage is always well-developed (ca. 3 mm long).
Lemoine G. (2018) Brèves botaniques de comptoir - Plantes de friches urbaines. Bull. Soc. Bot. N. Fr. 71: 29-30.
Lorca M.P. & Chueca F.E. (1979) Centaurea diffusa Lam., nueva para la Peninsula Iberica. An. Inst Bot. A.J. Cavanilles 36: 139-142.
Ochsmann J. (2000) Morphologische und molekularsystematische Untersuchungen an der Centaurea stoebe L.-Gruppe (Asteraceae-Cardueae) in Europa. Dissert. Bot. 324, J. Cramer, Berlin-Stuttgart: IX + 242 p.
Savelsbergh E. (1988) Centaurea diffusa Lam. im Aachener Stadtgebiet (TK 5202/23). Flor. Rundbr. 21(2): 89-91.
Sheley R.L., Jacobs J.S. & Carpinelli M.F. (1998) Distribution, biology and management of diffuse knapweed (Centaurea diffusa) and spotted knapweed. Weed Technol. 12(2): 353-362.
Watson A.K. & Renney A.J. (1974) The biology of Canadian weeds: 6. Centaurea diffusa and C. maculosa. Canad. J. Pl. Sci. 54(4): 687-701.