Centaurea nigrescens Willd. (syn.: C. jacea subsp. nigrescens (Willd.) Čelak.) (Eur.) – A very rare, locally naturalized alien. In the 19th century it was recorded on several occasions, the first time probably in 1826 (Durand 1899, Verloove 2006). Since the 1990’s known from a railway siding in Bellem near Aalter. In a similar habitat it was also discovered in 2009 near Oudenaarde (Hoste & al. 2015). It might have been overlooked elsewhere.
Centaurea nigrescens closely resembles species of the C. jacea Group. It is a very variable species itself that encompasses several infraspecific taxa. In North America several different names have been applied to refer to Tyrol Knapweed (Keil 2006). In its most typical expression C. nigrescens is differentiated from members of the C. jacea Group by its narrow heads, usually radiate flowers and the absence of a pappus on the achene. Most striking, however, is the checkered pattern of the involucre: the black phyllary appendages are small, leaving much of the green phyllary uncovered.
Although very similar to the C. jacea Group, the systematic position of this species is controversial. Dostál (1976) accommodated C. nigrescens is a separate section Nigrescentes (Hayek) Dostál. C. nigrescens was not considered as belonging to the C. jacea–C. nigra complex by Sommer (1990). Molecular data and morphological traits, however, refute this option (López-Alvarado 2011). This species and members of the C. jacea Group readily seem to hybridize when occurring in close proximity. For instance in the locality in Bellem the common native taxon C. jacea subsp. grandiflora (Gaudin) Schübl. & Martens seems to intergrade with C. nigrescens (Hoste & al. 2015). Similar behavior has been observed in France (Tison & de Foucault 2014).
Selected literature:
Dostál J. (1976) Centaurea. In: Tutin T.G. & al. (eds.), Flora Europaea, vol. 4. Cambridge University Press, Cambridge: 254-301.
Durand T. (1899) Phanérogames. In: De Wildeman E. & Durand T., Prodrome de la flore belge. A. Castaigne Editeur, Bruxelles: 1112 p.
Hoste I., Verloove F. & Van Heddegem W. (2015) Twee standhoudende populaties van Centaurea jacea subsp. nigrescens in Vlaanderen. Dumortiera 106: 43-47. [available online at: http://www.plantentuinmeise.be/DUMORTIERA/DUM_106/Dum_106_43-47_Centaure...
Keil D.J. (2006) Centaurea. In: Flora of North America Editorial Committee (eds.), Flora of North America, vol. 19. Oxford University Press, New York-Oxford: 181-194. [available online at: http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=106012]
Korneck D. (1955) Centaurea nigrescens Willd.: ssp. eunigrescens Gugl. und Centaurea jacea ssp. jacea Gremli var. pectinata Neilr. im Rheingau-Taunus. Hessische Floristische Briefe 4: 216-217.
López-Alvarado A. (2011) Centaurea L. section Phrygia Pers.: Phylogeny and Biogeography. PhD, Università degli Studi di Sassari: 126 p. [available online at: http://eprints.uniss.it/6614/1/L%C3%B3pez-Alvarado_J_Centaurea_L_section...
Malende B. (1952) Die Zweifelhafte Flockenblume, Centaurea nigrescens Willd. ssp. eudubia (Sister) Gugler, bei Hanau. Hessische Floristische Briefe 1: 8.
Sommer S. (1990) Isozymanalyse zur Ermittlung genetischer Variabilität und mikroevolutiver Prozesse bei Centaurea sect. Jacea (Asteraceae). Dissertation zur Erlangung des Doktorgrades der Naturwissenschaften, Universität Bayreuth, Bayreuth, Germany.
Tison J.-M. & de Foucault B. (coord.) (2014) Flora Gallica. Flore de France. Editions Biotope, Mèze : xx + 1196 p.
Verloove F. (2006) Catalogue of neophytes in Belgium (1800-2005). Scripta Botanica Belgica 39: 89 p. [available online at: http://alienplantsbelgium.be/sites/alienplantsbelgium.be/files/tabel_2.pdf]