Surf Thistle

Cirsium rhothophilum is endemic to the dunes of southern San Luis Obispo and northern Santa Barbara counties. It rarely forms hybrids with C. occidentale var. occidentale and C. scariosum var. citrinum.

Biennials or short-lived, usually monocarpic perennials, 10–100 cm; taprooted with simple or branched caudices. Stems 1–several, spreading to erect, bushlike or forming low rounded mounds, gray-tomentose with appressed feltlike trichomes; branches 0–several, inclined to ascending, stiff. Leaves: blades elliptic to ovate, 10–25 cm, strongly undulate, usually broadly pinnatifid, lobes entire or coarsely few-toothed or -lobed, main spines abrupt, 1–4 mm, faces gray-white-tomentose with appressed feltlike, non-septate trichomes; basal present or withered at flowering, winged-petiolate; principal cauline well distributed, winged-petiolate to sessile, gradually reduced, bases clasping with expanded auricles; distal reduced, spines to 8 mm. Heads 1–many, erect, terminal on branches in subcapitate to congested, corymbiform arrays, closely subtended by clustered, ± leafy bracts. Peduncles 0–7 cm. Involucres hemispheric or campanulate, 3–4 × 4–6 cm, densely arachnoid. Phyllaries in 8–10 series, imbricate, linear, abaxial faces without glutinous ridge; outer and mid bases short- appressed, margins spiny-ciliate, apices long, spreading to erect, spines straight, 2–5 mm; apices of inner flattened or spine-tipped, serrate to scabrid, sometimes pectinately fringed. Corollas white to pale yellow, 20–34 mm, tubes 11–15 mm, throats 5–8 mm, lobes 5–8 mm; style tips 3–4 mm. Cypselae light–brown to black, 5–7 mm, apical collars whitish, 0.2–0.3 mm; pappi 15–25 mm. 2n = 34.

Carduus maritima Elmer, Bot. Gaz. 39: 45. 1905, not Cirsium maritimum Makino 1910

Cirsium rhothophilum is a rare North American species of thistle known by the common name surf thistle. It is endemic to California, where it is known only from the coastline around the border between San Luis Obispo and Santa Barbara Counties. It grows in sand dunes and coastal scrub near the beach.

Cirsium rhothophilum grows up to 100 cm (39 in) tall with fleshy, woolly herbage usually forming a mound. The thick leaves are wavy and covered in feltlike hairs. They may have smooth, lobed, or toothed edges and small spines. The largest leaves at the base of the plant may reach 25 centimetres (9.8 inches) in length. The inflorescence bears several clustered flower heads, each head up to 4 centimeters long and 6 cm wide. The heads are lined with woolly phyllaries and filled with off-white to pale yellowish disc florets but no ray florets. The fruit is an achene a few millimeters long with a pappus up to 2 centimeters in length.