Plants mostly 25-60 cm, branches 0-4, short. Roots often tuberous-thickened. Leaves usually shallowly lobed, main spines ± fine, 3-6 mm. Heads 5-7 cm. Phyllaries: outer 3-6 mm wide at base, spines slender, 1.5-3 mm. Corollas usually 45-55 mm. Cypselae usually 4.5-5 mm. 2n = 30.

Flowering summer (Jun-Aug). Sandy or gravelly soils, prairies, limestone barrens, pastures, pine barrens, open woods, and oak savannas; 200-400 m; Ont.; Ill., Ind., Iowa, Mich., Minn., Wis.

Variety hillii occurs mainly in prairie areas from Minnesota and Iowa east to southern Ontario, Michigan, and northern Indiana. It has often been treated as a species distinct from Cirsium pumilum. As R. J. Moore and C. Frankton (1966) pointed out, the differences separating these taxa are, for the most part, metric characters that show considerable overlap. Some specimens, especially those from Ohio and western Pennsylvania, are difficult to place, and scattered individuals within the area of var. pumilum would readily be assigned to var. hillii were they growing in the area of that taxon. Moore and Frankton chose to recognize the infraspecific taxa within C. pumilum as subspecies, a rank that is seldom employed in North American Cirsium taxonomy.

Hill´s thistle is a taxon that has attracted the attention of conservationists because of declining populations. Although the U.S. Fish and Wildlife Service determined in 1999 that this plant does not warrant elevation to Candidate Species status, it is recognized as a species of concern in several states. Throughout much of its range the populations have declined due to habitat destruction and fragmentation that have accompanied agricultural development and urbanization.

This thistle prefers dry, sandy or gravelly soil and all of our records are from within the area shown on the map. My specimens from Benton, Fulton, and White Counties are from the right-of-way of railroads. The others are from open dunes, open woodland, and the high bank of a stream.

Perennial herb 25 - 60 cm tall

Stem: relatively delicate, unbranched, and lacking spiny wings.

Fruit: of numerous, small achenes, one from each flower.

Basal leaves: in a rosette, persistent, large (12 - 25 cm long, 2 - 7 cm wide), wavy-lobed, with reddish midveins, and numerous, short spines along the edges, with scattered larger spines intermixed.

Stem leaves: alternate, green beneath, similar in shape to basal leaves, but a bit smaller.

Roots: stout, elongate, thick, and hollow.

Flower heads: few, large, with the involucre about 3.5 - 7 cm wide and more than 3.5 cm tall. Spines on the outer series of bracts (phyllaries) 1.5 - 3.5 mm long, and erect.

Individual flowers: rich rose-purple.

Similar species: Cirsium hillii is probably most similar to C. muticum and C. arvense, but both of those species normally have more than five flowering heads, which are less than 3 cm broad, less than 3.5 cm high, and the spines on the phyllaries are less than 1 mm long. However, when flowers are not present these three species can often be confused. Our other pinkish flowered thistles do not have as large or as few flowering heads, the stems have spiny wings, and the underside of the leaves often are not green.

Flowering: June to August

Habitat and ecology: Dry prairies and other open places, often in sandy soil, but rare in the Chicago Region.

Occurence in the Chicago region: native

Notes: The original collection of C. hillii was from a Chicago Region locale, Pine Station (now part of Gary) in Lake County, Indiana. Reverend Ellsworth J. Hill, a common Chicago area collector near the end of the nineteenth century and beginning of the twentieth century, collected this plant in 1890, and it was named for him. This specimen, called a type specimen, is deposited in the herbarium at the New York Botanical Garden and is available for viewing online. Many current authorities believe this taxon is not a separate species from the eastern C. pumilum, under which it has been placed as either a subspecies or variety, still retaining the epithet hillii. Today, the continued existence of C. hillii is under threat from habitat loss. Fire suppression is also thought to have severely diminished populations of this species since it requires open bare soil for seedlings to develop.

Etymology: Cirsium comes from both the Greek words kirsion, meaning a kind of thistle, and cirsos, meaning "swollen vein", for which thistles were once thought a remedy. Hillii is named after Rev. E. J. Hill (1833 - 1917), a respected botanist who first collected this plant in 1890.

Author: The Field Museum

Much like no. 4 [Cirsium pumilum (Nutt.) Spreng], but perennial with long, thickened roots, these traversed longitudinally by several cavities; glutinous dorsal ridge of the invol bracts avg larger and better developed; achenes mostly 4-5 mm; 2n=30. Prairies and other open places, often in sandy soil; s. Ont. to Pa., w. to Minn. and S.D. June-Aug.

Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.

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