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. Flowering spring-autumn (May-Oct). Mixedgrass prairie, shortgrass prairie, Palouse prairie, sagebrush deserts, pinyon-juniper woodlands, openings in montane coniferous forests, often in disturbed areas; 100-2800 m; Alta., B.C., Man., Sask.; Ariz., Calif., Colo., Ga., Idaho, Ill., Ind., Iowa, Kans. Mich., Minn., Mo., Mont., Nebr., N.Mex., N.Dak., Okla., Oreg., Pa., S.Dak., Tex., Utah, Wash., Wis., Wyo.; Mexico (Chihuahua, Coahuila, Durango, Sonora). 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.

Plant: Perennial 2.5-10 dm, from creeping (sometimes tuber-like) roots; stem generally simple below, few-branched above, white-tomentose Leaves: gray-tomentose above, white- below; lower 15-30 cm, tapered to spiny petioles, elliptic to oblanceolate, ± shallowly lobed, lobes simple or with 2-4 narrow to wide secondary lobes or coarse teeth, main spines 2-4 mm, yellow; middle and upper gradually reduced, decurrent as spiny-margined wings, often spinier than lower, spines 2-10 mm INFLORESCENCE: primary inflorescence a head, each resembling a flower; heads discoid, 1-few in cymes; peduncles 0-15 cm, ± leafy; involucres 2-5 cm, 2-5 cm diam, ± ovoid to bell-shaped, ± loosely tomentose on phyllary margins, becoming glabrous; phyllaries strongly graduated (outer ovate, inner lance-oblong), entire or minutely roughened, tightly appressed, midribs of outer and middle glandular area, spines 2-5 mm, spreading to reflexed, inner with tips flat, erect or recurved, ± twisted Flowers: corollas 25-50 mm, ± bilateral, white to pale lavender or pink, tube 11-25 mm, slender; throat 7-11 mm; lobes 7-15 mm; anther bases sharply sagittate, tips oblong; style tip with slightly swollen node, appendage (above node) long, cylindric, branches very short Fruit: 4-7 mm, light brown, ± compressed, ovoid, glabrous; scar slightly angled; pappus 25-40 mm, bristles many, plumose Misc: Disturbed places; < 1600 m.