White Thistle

Cirsium hookerianum occurs from the Canadian Coast Ranges of British Columbia east to the northern Cascade Range and the northern Rocky Mountains. The relationship between C. hookerianum, C. kelseyi, which I have tentatively included in C. hookerianum, and C. longistylum needs further investigation. A case could be made for including all three in an expanded concept of C. hookerianum, but more investigation of the variation patterns is needed before this is done. Certainly C. kelseyi is better treated within or as a close ally of C. hookerianum than in C. scariosum (var. scariosum), where R. J. Moore and C. Frankton (1974) synonymized it. Cirsium hookerianum is known to hybridize with C. undulatum.

Biennials or monocarpic (sometimes polycarpic?) perennials, 20–150 cm; taprooted. Stems usually 1 and erect, less commonly several and ascending, simple to sparingly short-branched in distal ¹/2, variably villous with jointed trichomes, and/or finely arachnoid, or ± glabrate; branches on distal stems 0–many, short, ascending. Leaves: blades linear-oblong to elliptic, 5–25 × 1–8 cm, subentire to coarsely dentate or deeply pinnatifid, lobes lance-oblong to broadly triangular, spinulose to spiny-dentate or shallowly lobed, main spines 2–10 mm, abaxial faces usually ± densely gray- or white-tomentose with felted arachnoid trichomes, ± villous to tomentose along major veins with septate trichomes, sometimes glabrous or glabrate, adaxial ± green, glabrous to thinly arachnoid, often ± villous or tomentose with septate trichomes; basal often present at flowering, spiny winged-petiolate or sessile; principal cauline well distributed, proximally winged-petiolate, distally sessile, gradually reduced, bases sometimes short-decurrent; distal ± reduced, often narrower than proximal, sometimes with non-pigmented bases, sometimes pectinately spiny. Heads 1–many, borne singly or crowded in spiciform, racemiform, subcapitate, or sometimes more openly branched corymbiform arrays. Peduncles 0–8+ cm. Involucres (green or often purplish), broadly ovoid, 2–3.3 × 1.5–4 cm, loosely to densely villous with septate trichomes to tomentose and/or arachnoid. Phyllaries in 4–8 series, imbricate to subequal, bases short-appressed, entire, abaxial faces with or without narrow glutinous ridge, apices stiffly spreading to ascending, linear, long, plane, spines straight, slender, 3–5 mm; apices of inner flexuous, sometimes expanded and erose. Corollas white, ochroleucous, or occasionally pink, 20–28 mm, tubes 10–13 mm, throats 6.5–9 mm, lobes 5–7 mm; style tips 3–5.5 mm. Cypselae dark brown, 5–6.5 mm, apical collars not differentiated; pappi 18–22 mm. 2n = 34.

Cirsium kelseyi (Rydberg) Petrak

Cirsium hookerianum, common name Hooker's thistle or white thistle, is a North American species of thistle native to western Canada and the northwestern United States. It is found in the northern Rocky Mountains as well as in some of the northern Cascades and Coast Ranges, in Alberta, British Columbia, Washington, Idaho, Montana, and Wyoming.

Cirsium hookerianum is biennial or perennial herb up to 150 cm (60 inches) tall, with a large taproot. Leaves are green on top, usually white and woolly underneath, with spines along the edges. There are usually several flower heads with white or pink disc florets but no ray florets. The species grows in grasslands, meadows, and the edges of forests in mountainous areas.