Cirsium undulatum (Nutt.) Spreng.

Cirsium undulatum is widely distributed in the wstern half of North America from the dry plains and plateaus of the Pacific Northwest eastward across the Great Plains to Manitoba and the Dakotas and south to Texas, New Mexico, and northwestern Mexico. It occurs in scattered localities in the Rocky Mountains and northeastern Great Basin region. At least some of the few widely scattered records from the eastern United States are probably introductions. Cirsium undulatum is both widespread and variable. Plants of the Great Plains region tend to be low-growing with a few large heads and elongate corollas. Plants of the Pacific Northwest are usually taller and produce smaller, more numerous heads with shorter corollas. A detailed study of this species might reveal races worthy of recognition as infraspecific taxa.

Wavyleaf thistle is listed by California as a noxious weed. However, most reports of Cirsium undulatum in California are based upon misidentifications of C. canescens. Cirsium undulatum is known to hybridize with C. flodmanii, C. hookerianum, and C. scariosum var. coloradense. J. T. Howell (1960b) reported that C. undulatum was suspected to hybridize with C. brevifolium in the Pacific Northwest.

Perennials, 20–230 cm; deeply seated runner roots that produce adventitious buds. Stems 1–several, erect or ascending, densely gray-tomentose; branches 0–few, usually above middle, ascending. Leaves: blades elliptic to oblong or ovate, 10–40 × 1–10 cm, margins strongly undulate, coarsely dentate or shallowly to deeply lobed, lobes ascending to spreading, ± triangular, well separated to closely spaced, spinulose and coarsely dentate or usually cleft into 2–3 lanceolate to triangular, often entire-margined, spine-tipped divisions, main spines (yellowish), 2–12+ mm, abaxial densely gray-tomentose, adaxial faces thinly tomentose; basal sometimes present at flowering, winged-petiolate; principal cauline becoming sessile and progressively reduced distally, widest at base, bases ± auriculate-clasping to short-decurrent; distal reduced, spinier. Heads 1–10+, terminal on branches, in leafy, ± corymbiform arrays. Peduncles 0–25+ cm. Involucres ovoid to hemispheric or broadly campanulate, 2.5–4.5 × 1.5–4.5 cm, loosely arachnoid on phyllary margins or glabrate. Phyllaries in 8–12 series, imbricate, ovate to lanceolate (outer) to linear-lanceolate (inner), abaxial faces with prominent glutinous ridge; outer and middle appressed, spines spreading, 1.5–5 mm; apices of inner narrow, often flexuous, flat, ± entire, spineless or weakly spiny. Corollas lavender to pink, purple, or white, 24–50 mm, tubes 12–28 mm, throats 6–14 mm, lobes 6.5–13 mm; style tips 5–7.5 mm. Cypselae light to dark brown, 6–7 mm, bodies and apical collars concolorous, narrow; pappi 20–38 mm (usually scabridulous). 2n = 26.

Carduus undulatus Nuttall, Gen. N. Amer. Pl. 2: 130. 1818; Cirsium megacephalum (A. Gray) Cockerell ex Daniels; C. undulatum var. megacephalum (A. Gray) Fernald

Cirsium undulatum is a species of thistle known by the common names wavyleaf thistle and gray thistle. It is native to much of central and western North America from British Columbia east to Manitoba and south as far as the State of Durango in Mexico. It has also been found outside of its native range as an introduced species.

Cirsium undulatum is widespread and found in many habitat types. It is a perennial herb exceeding 200 cm (79 in) in height. The stem branches a few times toward the top of the plant if at all. The leaves are very wavy along the edges and usually cut into shallow toothed lobes. The longest near the base of the plant are up to 30 centimetres (12 inches) long. The inflorescence holds one or more flower heads each up to 5 centimetres (2.0 inches) long and wide. The head is lined with spiny phyllaries of different shapes. The flowers in the head are white to lavender to pink and up to 5 centimetres (2.0 inches) long. The fruit is an achene a few millimeters long with a pappus which may be up to 4 centimeters in length.

Cirsium undulatum has been shown to have its seed production reduced by an exotic weevil Larinus planus which was released to control Canada thistle.

Cirsium undulatum là một loài thực vật có hoa trong họ Cúc. Loài này được (Nutt.) Spreng. mô tả khoa học đầu tiên năm 1826.