This is a large and difficult genus, with over 200 species recognized in Europe alone. Our showiest ones are presumably escapes from gardens and others are weeds. Hybridization helps to confuse the lines between some species, and the key cannot accommodate all intermediates. Sometimes split into smaller genera, the details of which, however, are not yet clear.
1. Involucres ca. 4.5–5.5 (–10) cm in diameter; flowers yellow.
1. Involucres smaller (less than 3 cm broad); flowers white to pink, blue, or purple (yellow in C. solstitialis, with very long-spined involucres).
2. Cauline leaves (except the uppermost and bracteal ones) deeply pinnatifid or bipinnatifid.
3. Phyllaries blackened at the tip, softly fringed with flattened projections (the terminal one obscure, no longer than the lateral ones); flowers usually pink-purple; pappus well developed (1–2.5 mm long).
3. Phyllaries light yellow-brown at the tip, spinose-fringed with firm rounded projections (the terminal stiff spine longer than at least the lateral ones immediately below it); flowers usually white; pappus none or vestigial.
4. Involucre ca. (7–) 9–12 mm broad.
4. Involucre less than 5 mm broad.
2. Cauline leaves (except sometimes the basal ones) entire or with a few teeth or small lobes.
5. Phyllaries (at least the middle ones) tipped with a firm spine much longer than the body of the phyllary (usually at least twice as long); flowers yellow.
5. Phyllaries tipped by no spine or one much shorter than the body; flowers white to pink, blue, or purple.
6. Middle (and all) phyllaries rounded or tapered to lacerate-toothed apex (without expanded appendage).
7. Enlarged marginal flowers exceeding involucre by ca. 0.9–1.5 cm; involucre ca. (1–) 1.2–1.5 cm long, the phyllaries (at least the outermost) with pale (translucent to pinkish) border; largest leaves less than 1 cm broad.
7. Enlarged marginal flowers exceeding involucre by ca. 2–3 cm; involucre ca. 2 cm (or more) long, the phyllaries with prominent blackish brown lacerate-toothed border; largest leaves ca. 2–3 cm broad.
6. Middle phyllaries with an abruptly expanded appendage at the apex (broader than the narrow neck below it).
8. Appendage on middle phyllaries with body (not fringe) ± triangular, usually ca. 1.5 times as long as the base is broad, bordered with a prominent ciliate fringe; shaft of middle and inner phyllaries green (or purple tinged), clearly visible between the tips and appendages of the overlying phyllaries.
8. Appendage on middle (not necessarily lower) phyllaries with body ± orbicular, the border various; shaft of middle and often inner phyllaries concealed by the large appendages of the overlying phyllaries.
9. Middle and outer phyllaries with appendages light brown, irregularly cleft, erose, or lacerate, or even entire, little if at all ciliate-fringed; pappus none; marginal flowers much larger than others in the head.
9. Middle and outer phyllaries with appendages blackened, all deeply pectinate-ciliate; pappus ca. 0.5–1.5 mm long; marginal flowers not larger than others in the head [See text for hybrids with C. jacea].