Carduus hydrophilus Greene, Proc. Acad. Nat. Sci. Philadelphia 44: 358. 1893; Cirsium vaseyi (A. Gray) Jepson var. hydrophilum (Greene) Petrak

Biennials or monocarpic perennials, 100–220 cm; taprooted. Stems 1–several, erect, (hollow), openly branched distally or throughout, thinly arachnoid with fine, non-septate trichomes, glabrate. Leaves: blades elliptic to broadly oblanceolate, 10–40+ cm, pinnatifid ¹/2–²/3 distance to midveins, larger usually with broad sinuses, lobes broad, few lobed or dentate, main spines 2–9 mm, abaxial faces ± gray-tomentose, sometimes ± glabrate, adaxial thinly arachnoid-tomentose, soon glabrescent; basal present or withered at flowering, winged-petiolate; principal cauline sessile, progressively reduced distally, bases auriculate-clasping or shortly decurrent; distal cauline reduced, bractlike, often spinier than proximal. Heads borne singly or few at branch tips, sometimes subtended by clustered, ± leafy bracts, collectively forming ± open, many-headed paniculiform arrays. Peduncles 0–10+ cm. Involucres ovoid to campanulate, 1.5–2.5 × 1.5–3 cm, thinly arachnoid, glabrate. Phyllaries in 6–9 series, imbricate, dark green to brownish, lanceolate (outer) to linear (inner), abaxial faces with narrow glutinous ridge; outer and middle appressed, apices spreading, finely serrulate, spines slender, 1–2 mm; apices of inner erect, ± flexuous. Corollas pale rose-purple, 18–23 mm, tubes 8–10 mm, throats 5–6 mm, lobes 5–7 mm; style tips 3.5–4.5 mm. Cypselae dark brown to black, 4–5 mm, collars very narrow, stramineous; pappi ca. 15 mm. 2n = 32.

Varieties 2 (2 in the flora): California.