Most of our thistles are tall (often 2 m), unpleasantly prickly plants, generally biennial or perennial. Cirsium hillii is normally low and squat, with very large deep-colored heads besides, and hence one of the most attractive species. The tallest C. pitcheri seen was 1.6 m high, but most individuals are shorter. The spines are included in measurements given for involucre widths, but it is hard to determine precise figures, especially for heads that have been flattened in pressing. All of our species, except C. pitcheri, normally have pink to purple flowers, but white-flowered forms may occur, apparently quite rarely. The phyllaries vary within the head, the outermost and usually the middle ones in most species spine-tipped, but the innermost ones spineless. Measurements of spines on leaves and phyllaries are of the pale or brown, non-green portion.
1. Internodes (especially upper ones) with continuous or intermittent spiny wings their full length; aggressive, introduced weeds.
2. Heads small (involucre ca. 1–1.5 cm broad), mostly in dense clusters; phyllaries at most with a spine tip ca. 0.5 (–1) mm long; upper surface of leaves with a few septate hairs.
2. Heads large (involucre ca. 3.5–6.5 cm broad), mostly solitary on short branches; phyllaries with a strong spine tip ca. 2–6 mm long; upper surface of leaves with ± appressed and dense spines mostly 0.5–1.5 mm long.
1. Internodes at most with leaf base slightly decurrent (less than halfway to the next lower node).
3. Leaves green beneath (if loosely tomentose, the green or pale green surface not hidden).
4. Involucre ca. 3.5–6 (–7) cm broad; heads 1–3 (–7) per stem; spines on outer phyllaries mostly 1.5–3.5 mm long.
4. Involucre ca. 1–3 (–3.5) cm broad; heads often 5 or more per stem; spines on outer phyllaries none or at most ca. 1–1.5 mm long.
5. Spines on outer phyllaries ca. 0.5–1 (–1.5) mm long; involucre at most cobwebby on phyllary margins; largest spines on leaves ca. 3.5–7 mm long (as short as 1.5–2 mm in forms with at least the upper leaves unlobed); perennial and colonial from horizontal roots, but not producing rosettes, introduced and mostly in dry ground.
C. arvense (in part)
5. Spines on outer phyllaries none or less than 0.5 mm long; involucre ± cobwebby pubescent; largest spines on leaves less than 2.5 (–3.5) mm long; plant producing rosettes (biennial or perennial) but not horizontal roots, native in moist habitats.
3. Leaves densely tomentose beneath, the green surface fully hidden.
6. Upper internodes (if not all) densely white-tomentose.
7. Cauline leaves deeply pinnate with linear lobes (entire or nearly so) about as wide as the rachis (less than 4 (–8) mm); flowers white or cream (or somewhat pinkish from the style and anthers); native, on sand dunes along the Great Lakes.
7. Cauline leaves with ± triangular lobes, the rachis over 5 mm broad; flowers purple (except in albino forms); a sporadic waif in disturbed places.
8. Lateral lobes of leaves not over 7 mm broad at base; involucre ca. 2–2.7 cm long; achenes ca. 3–4 (–5) mm long.
8. Lateral lobes of leaves mostly over 7 mm broad at base; involucre ca. (2.7–) 3–3.5 cm long; achenes ca. 5–7 mm long.
6. Upper internodes not tomentose or only very lightly so.
9. Involucre ca. 1–1.8 (–2.3) cm long or even shorter, the phyllaries with spines none or up to 1 (–1.5) mm long.
C. arvense (in part)
9. Involucre ca. 2.1–3.3 cm long, the phyllaries with spines 3.5–6.5 mm long.
10. Middle and upper cauline leaves unlobed to shallowly lobed; scarious tips on at least some innermost phyllaries usually expanded at the base (broader than the firm body of the phyllary tip and contracted before tapering to the apex).
10. Middle and upper cauline leaves deeply pinnately lobed; scarious tips of innermost phyllaries all narrowly tapered to the apex, not expanded at base.