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Cirsium

See list of 9 species in this genus

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).

  • 1a. Upper stem and branches with conspicuous, prickly and undulate-margined wings formed by decurrent leaf bases
    • 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
    • 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
  • 1b. Neither the stem nor the branches with decurrent wings
    • 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
    • 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
      • 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
      • 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
        • 5a. Leaf blades persistently white-tomentose on the abaxial surface
          • 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
          • 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
            • 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
            • 7b. Stem leaf blades with ± revolute margins, evidently pinnately lobed 
with narrow-lanceolate lobes; tips of innermost involucral bracts slender, not dilated, entire
        • 5b. Leaf blades abaxially thinly tomentose or arachnoid-pubescent when young, becoming glabrate in age
          • 8a. Outer involucral bracts tipped with a coarse spine 3–6 mm long; cypsela body (3–) 3.5–4 mm long
          • 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.