Annuals or biennials, (60-)100-300 cm; taprooted. Stems usually 1, erect, stout, (hollow, 2-10 cm diam. at base), openly branched distally, thinly arachnoid, villous with jointed trichomes, at least proximally. Leaves: blades elliptic to broadly oblanceolate, 15-70 × 30-150+ cm, flat, pinnatifid ¹/2-²/3 distance to midvein, larger usually with broad sinuses, lobes broad, few lobed or dentate, main spines 3-8 mm, abaxial faces gray-tomentose, adaxial thinly arachnoid-tomentose, sometimes midveins with jointed trichomes; basal present or withered at flowering, winged-petiolate; principal cauline sessile, progressively reduced distally, bases clasping or short-decurrent 1-2 cm; distal cauline reduced, becoming bractlike, sometimes spinier than proximal. Heads 1-several at branch tips, closely subtended by clustered leafy bracts or not, collectively forming open, corymbiform or paniculiform arrays. Peduncles 0-15 cm. Involucres ovoid to campanulate, 1.5-3 × 1.5-3 cm, ± glabrous. Phyllaries in 5-7 series, weakly unequal, dark green to brownish, lanceolate (outer) to linear (inner), abaxial faces without (or with very obscure) glutinous ridge; outer and middle appressed or apices spreading, at least outer irregularly spiny-fringed, finely serrulate, spines slender, 3-5 mm; apices of inner erect, abaxial faces gray-tomentose, ± twisted. Corollas pale rose-purple (white), 19-26 mm, tubes 9-12 mm, throats 4-6 mm, lobes 5-9 mm; style tips 3.5-4.5 mm. Cypselae dark brown, 5-5.5 mm, collars narrow, ± stramineous; pappi 15-20 mm. 2n = 32.

Flowering spring (Apr-Jun). Freshwater marshes, canal banks; 5-100 m; Calif.

Cirsium crassicaule is known only from a few sites in the San Joaquin Valley. Some populations are threatened by habitat modification and development pressures.