Globe centaurea, big-head knapweed, yellow bachelor’s button or cornflower, centaurée à gros capitules

Grossheimia macrocephala (Puschkarew ex Willdenow) Sosnowsky & Takhtajan

Perennials, 50–170 cm. Stems usually several, erect, unbranched or sparingly branched distally, villous with septate hairs, thinly arachnoid-tomentose, fistulose proximal to heads. Leaves short-villous and thinly arachnoid, ± glabrate, resin-gland-dotted; basal and proximal cauline petiolate, blades oblanceolate to narrowly ovate, 10–30 cm, margins entire or shallowly dentate; cauline sessile, shortly decurrent, not much smaller except those crowded proximal to heads, blades lanceolate to ovate, 5–10 cm, entire, often ± undulate, apices acute. Heads disciform or weakly radiant, borne singly, sessile, closely subtended by clusters of reduced leaves. Involucres ovoid to hemispheric, 25–35 mm. Phyllaries: bodies pale green or stramineous, ovate or broadly lanceolate, glabrous, appendages erect to spreading, brown, scarious, abruptly expanded, 1–2 cm wide, ± covering phyllary bodies, lacerate fringed, sometimes tipped by weak spines 1–2 mm, glabrous. Florets many. corollas yellow; corollas of sterile florets slightly expanded, ca. 4 mm; corollas of disc florets ca. 3.5 mm. Cypselae 7–8 mm; pappi of many setiform scales ("flattened bristles"), 5–8 mm. 2n = 18 (Russia).

Flowering summer (Jun–Sep). Garden escape in meadows, grassy clearings; 400–2000 m; introduced; Ont., Que.; Colo., Mich., Wash., Wis.; e Europe; w Asia.

Although Centaurea macrocephala is cultivated as an ornamental and for cut flowers in many areas, it has been declared a noxious weed by the state of Washington because of its potential status as an invader.