15. Centaurea iberica Treviranus ex Sprengel, Syst. Veg. 3: 406. 1826.
Iberian star thistle or knapweed
Annuals, biennials, or short-lived perennials, 20–200 cm. Stems 1–several, divaricately much branched, often forming rounded mound, puberulent to loosely tomentose. Leaves hispidulous to loosely tomentose, ± glabrate, minutely resin-gland-dotted; proximal leaves petiolate, blades 10–20 cm, margins 1–2 times pinnately lobed or dissected, rosette with central cluster of spines; mid sessile, not decurrent, blades ± lanceolate, shorter; distal blades linear to oblong, entire to coarsely dentate or shallowly lobed. Heads disciform, borne singly or in leafy cymiform arrays, sessile or short-pedunculate. Involucres ovoid to hemispheric, (10–)13–18 mm. Principal phyllaries: bodies greenish or stramineous, ovate, scarious–margined, appendages stramineous, spiny–fringed at base, each tipped by stout spreading spine (0.5–)1–3 cm. Inner phyllaries: appendages truncate, spineless. Florets many; corollas white, pink, or pale purple, those of sterile florets slender, 15–20 mm, those of fertile florets 15–20 mm. Cypselae white- or brown-streaked, 3–4 mm, glabrous; pappi of white bristles 1–2.5(–3) mm. 2n = 16, 20.
Flowering summer (Jun–Sep). Roadsides, pastures, fields; 0–1500 m; introduced; Calif., Kans., Oreg., Wash., Wyo.; Europe; Asia.
Centaurea iberica is native to southeastern Europe through central Asia.
Iberian star thistle is considered to be a noxious weed in several states of the western United States. Weed control measures in Oregon and Washington have apparently eradicated the species in those states. Centaurea iberica is very similar to C. calcitrapa, from which it differs by its pappose cypselae and often more robust habit. The Kansas and Wyoming plants were originally reported as C. calcitrapa (R. L. McGregor 1986).