The generic limits of Rhaponticum have long been obscure. However, recent molecular phylogenetic research (Hidalgo & al. 2006) demonstrated that, among others, the related genera Leuzea DC. and Acroptilon Cass. are better included in Rhaponticum. In Belgium, only the latter is of importance. It was long accepted as a monospecific genus, related to (and sometimes included) in Centaurea. Susanna & al. (1995) already proved that it differs from Centaurea in many respects, not only cytologically and morphologically but also genetically. It is best distinguished by its bisexual outer florets (sterile in Centaurea s.str.).
Hidalgo O., Garcia-Jacas N., Garnatje T. & Susanna A. (2006) Phylogeny of Rhaponticum (Asteraceae, Cardueae–Centaureinae) and related genera inferred from nuclear and chloroplast DNA sequence data: taxonomic and biogeographic implications. Annals of Botany 97(5): 705-714.
Susanna A., Garcia-Jacas N., Soltis D.E. & Soltis P.S. (1995) Phylogenetic relationships in tribe Cardueae (Asteraceae) based on ITS sequences. Amer. J. Bot. 82: 1056-1068.