Slough Thistle

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

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.

Carduus crassicaulis Greene, Proc. Acad. Nat. Sci. Philadelphia 44: 357. 1893

Cirsium crassicaule is a species of thistle known by the common name slough thistle. It is endemic to the San Joaquin Valley of California, where it is known primarily from freshwater wetlands. It has been found in only a few locations in Kern, Kings, and San Joaquin Counties.

Cirsium crassicaule is an annual or biennial herb known to reach 300 cm (10 feet) in height. The thick stem is hollow and may be nearly 10 centimeters (4 inches) wide at the base. It is coated in hairs and cobwebby fibers. The woolly, webby, spiny leaves are deeply cut into many lobes, the lobes often lined with teeth. The longest leaves near the base of the plant may be 70 centimeters (28 inches) long.

The inflorescence is a cluster of several flower heads each up to 3 centimeters long by 3 wide. The head is lined with spiny phyllaries and filled with pale pink or occasionally white flowers. The fruit is an achene with a flat, dark brown body about 5 millimeters long and topped with a pappus which may be 2 centimeters in length.