Centaurea diluta Ait. (syn.: C. raphanifolia Salzm. ex DC.; incl. C. algeriensis Coss. et Durieu, syn.: C. diluta subsp. algeriensis (Coss. et Durieu) Maire) (W-Medit.) – A rather rare and decreasing, ephemeral alien. First recorded in 1918 between Veurne and Koksijde (as C. algeriensis). Remarkably often seen between the 1950’s and 1980’s, usually on dumps or near grain mills (for instance in Durnal, Harelbeke, Heppignies, Hoboken, Jemappes, Koksijde, Mechelen, Nieuwpoort, Overmere, Ravels, Turnhout, Zaventem, etc.). Since 2009 also repeatedly recorded on an unloading quay for grains at the Amerikadok in the port of Antwerp and occasionally elsewhere, e.g. in the port of Roeselare. Most records of Centaurea diluta were obviously associated with cereals (including birdseed).
Some Belgian records were claimed to be Centaurea algeriensis. This species has bracts with a long apical spine that is ca. 5-6x as long as the lateral ones (only twice as long in C. diluta s.str.). The taxonomic value of this species is debated and it is nowadays mostly included in Centaurea diluta (e.g. Devesa Alcaraz & López Nieto 2013). Moreover, only a single collection from 1918 could be ascribed with certainty to C. algeriensis (det. Arènes).
Selected literature:
Conti F. & Manzi A. (1997) Centaurea diluta Aiton, new to the Italian flora. Flora Medit. 7: 51-53. [available online at: http://147.163.105.223/flora/7-051.pdf]
Devesa Alcaraz J.A & López Nieto E. (2013) Estudio taxonómico de Centaurea sect. Mesocentron (Cass.) DC. y sect. Hymenocentron (Cass.) DC. (Asteraceae) en la Península Ibérica y Baleares. Fl. Montiberica 55: 3-25. [available online at: http://www.floramontiberica.org/entrada.htm]
Duvigneaud J. (1979) La basse vallée du Bocq (province de Namur, Belgique). Nouvelles notes floristiques. Nat. Mosana 32: 175-184.
Giardina G. & Lucchese F. (2002) The native-alien status of Centaurea diluta Aiton in Italy. Additional points and clarification. Arch. Geobot. 6(2): 183-188.
Havrenne A. & Lebeau J. (1978) Trouvailles floristiques effectuées en 1977 dans la partie septentrionale de la région de Charleroi. Nat. Mosana 31(3): 123-124.
Henrard J.Th. (1918) Bijdrage tot de kennis der Nederlandsche adventiefflora. Nederl. Kruidk. Arch. 1918: 181-205.
Méndez Pérez B. & Wildpret W. (1978) Centaurea diluta Ait. nueva cita para la flora canaria. Vieraea 7(2): 133-136.
Rodriguez Oubiña J. & Ortiz S. (1987) Centaurea diluta Aiton y Soleirolia soleirolii (Req.) Dandy, dos adventicias herborizadas en Santiago de Compostela (Galicia). An. Jard. Bot. Madrid 44(1): 175. [available online at: http://dialnet.unirioja.es/servlet/articulo?codigo=2962815]