Centaurea solstitialis L. (Medit.) – A frequent but much decreasing, ephemeral alien. First collected in agricultural fields around in Liège in 1824. In the 19th century a rather regular weed in alfalfa and clover fields. Recorded as a wool alien in the Vesdre valley between 1891 and 1960. In the past decades much rarer and nearly always as a grain alien in port areas, on dumps, etc.
Centaurea solstitialis is a rather variable species (see Dostál 1976). The usual plant has appendages with 3-7 yellow spines, the apical much longer and stouter than the others. A plant with more or less equal brownish spines has been recorded as a grain alien in 1927 and 2012; it belongs to subsp. adamii (Willd.) Nyman (syn.: Centaurea adamii Willd.).
Selected literature:
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]
Dostál J. (1976) Centaurea. In: Tutin T.G. & al. (eds.), Flora Europaea, vol. 4. Cambridge University Press, Cambridge: 254-301.
Maddox D.M. (1981) Introduction, phenology and density of yellow starthistle in coastal, intercoastal and central valley situations in California. U.S. Dep. Agric.: Agric. Res. Serv., Agric. Res. Results, West. 20: 33 pp.
Maddox D.M., Joley D.B., Supkoff D.M. & Mayfield A. (1996) Pollination biology of yellow starthistle (Centaurea solstitialis) in California. Canad. J. Bot. 74(2): 262-267.
Roche C.T., Thill D.C. & Shafii B. (1997) Reproductive phenology in yellow starthistle (Centaurea solstitialis). Weed Sci. 45(6): 763-770.
Sun M. (1997) Population genetic structure of yellow starthistle (Centaurea solstitialis), a colonizing weed in the western United States. Canad. J. Bot. 75(9): 1470-1478. [available online at: https://hub.hku.hk/bitstream/10722/42262/1/22575.pdf?accept=1]