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Cirsium

Given the polymorphic nature of Cirsium and its ability to form hybrids that can reproduce sexually (and backcross), not all collections will be referable to a species by the following key. Careful comparison with museum specimens and technical descriptions may be necessary. References: Moore and Frankton (1974), Keil (2006c).

See list of 2 species in 1a
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1a. Upper stem and branches with conspicuous, prickly and undulate-margined wings formed by decurrent leaf bases
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  C. vulgare
  2a. Involucre 25–40 mm tall; involucral bracts (or most of them) with prominent spine-tips 2–6 mm long; leaf blades adaxially with abundant, appressed spines
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  C. palustre
  2b. Involucre 10–15 (–20) mm tall; involucral bracts lacking spine tips or with short, vestigial spines up to 1 mm long; leaf blades adaxially with sparse, septate hairs
See list of 7 species in 1b
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1b. Neither the stem nor the branches with decurrent wings
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  C. arvense
  3a. Involucre 10–20 mm tall; capitula with, or with mostly, unisexual flowers by abortion; plants perennial, colonial from deep-seated, creeping roots, the reproductive stems not arising from a rosette of leaves
- See list of 6 species in 3b
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  3b. Involucre 20–50 mm tall; capitula with bisexual flowers; plants biennial or short-lived perennial, not colonial, the reproductive stems arising from a rosette of leaves
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    C. horridulum
    4a. Capitula closely invested by series of narrow, spiny-margined leaves that form a false involucre sometimes overtopping the true involucre [Fig. 390]; corollas yellow (rarely purple); scarious apex of innermost (i.e., longer) involucral bracts slender and  not expanded
  - See list of 5 species in 4b
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    4b. Capitula either without subtending leaves or with 1 or 2 approximate, reduced leaves; corollas purple to pink (rarely white); scarious apex of innermost involucral bracts expanded or not
    - See list of 3 species in 5a
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      5a. Leaf blades persistently white-tomentose on the abaxial surface
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        C. flodmanii
        6a. Adaxial surface of leaf blades thinly tomentose (rarely glabrate); stem thinly, uniformly, and persistently tomentose; pappus bristles 20–30 mm long; plants from taproots and also with horizontal roots that produce new shoots
      - See list of 2 species in 6b
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        6b. Adaxial surface of leaf blades glabrate or sparsely villous with septate hairs; stem glabrate, infrequently with thin tomentum apically; pappus bristles 12–25 mm long; plants from taproots and sometimes also a cluster of coarse, fibrous roots, but without prominent horizontal roots
        
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        C. altissimum
        7a. Stem leaf blades with flat margins, unlobed or with short lobes separated by shallow sinuses (rarely with deep sinuses and then with broad-triangular lobes); tips of innermost involucral bracts usually dilated and erose to serrulatae
        
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        C. discolor
        7b. Stem leaf blades with ± revolute margins, evidently pinnately lobed  with narrow-lanceolate lobes; tips of innermost involucral bracts slender, not dilated, entire
    - See list of 2 species in 5b
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      5b. Leaf blades abaxially thinly tomentose or arachnoid-pubescent when young, becoming glabrate in age
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        C. pumilum
        8a. Outer involucral bracts tipped with a coarse spine 3–6 mm long; cypsela body (3–) 3.5–4 mm long
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        C. muticum
        8b. Outer involucral bracts lacking a spine tip or tipped with a vestigial spinule up to 0.5 (–1) mm long; cypsela body 4.5–5.5 mm long

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Show photos of: Each photo represents one species in this genus.