The Jepson Herbarium
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Taprooted annual, biennial, or short-lived perennial herb that flowers once, or multi- flower per with taprooted rosettes arising from runner roots or from simple to branched caudex; glabrous to cobwebby or ± densely tomentose with long, fine, slender hairs, sometimes with thicker multicellular, jointed hairs that often appear crinkled, shining, iridescent when dry.
Stem: generally erect.
Leaf: basal and proximal cauline generally tapered or ± wing-petioled, generally wavy-margined, dentate to generally pinnately lobed and ± dentate, lobes and teeth spine-tipped, generally spiny- ciliate, faces glabrous to tomentose, especially abaxially; distal generally sessile, ± reduced.
Inflorescence: heads discoid, 1–many, center head of cluster generally larger, generally erect; involucre ± cylindric to ovoid, spheric, or bell-shaped, persistent when dry; phyllaries many, graduated in 5–20 series, generally entire (spiny- ciliate or with irregularly toothed or cut scarious margin or distal appendage), outer and middle generally spine-tipped, in some species midrib with sticky-resinous ridge (milky when fresh, dark when dry, occasionally very narrow); inner phyllaries generally narrow, flat, tips straight or twisted; receptacle flat, long-bristly, epaleate.
Flower: ± many, generally bisexual ( unisexual in Cirsium arvense); corolla ± radial, white to red or purple, tube long, narrowly cylindric, throat cylindric, lobes linear; anther tube colored same as corolla or not, anther base sharply sagittate, tip linear or oblong; style generally exserted, tip cylindric, branches very short.
Fruit: ovoid, thick or ± compressed, straw-colored or tan to dark brown, glabrous; attachment scar slightly angled; pappus bristles many, ± flattened proximally, plumose, weakly fused at base, often deciduous in ring, white to brown.
± 200 species: North America, Eurasia. (Greek: thistle) [Keil 2006 FNANM 19:95–164] Taxa difficult, variable, incompletely differentiated, hybridize. Exceptional white-flowered plants occur in most taxa with pigmented corollas; these generally not treated in key.
Unabridged references: [Kelch & Baldwin 2003 Molec Ecol 12:141–151]
Unabridged note: Native thistles are part of an apparently actively evolving group of species with many geog and ecological races and growth forms. Morphologically divergent species often are able to hybridize; unrecognized hybridization or intergradation often complicates identification. Stature, growth form, and proportions are subject to environmental influence.

Key to Cirsium

1. Adaxial leaf face with slender appressed bristle-like prickles ..... C. vulgare

1' Adaxial leaf face without prickles

2. Plants dioecious or nearly so ..... C. arvense

2' Plants with bisexual flowers

3. Corolla lobes 1–2+ × tube

4. Corolla pink-purple to red-purple, red-pink, or red; heads generally erect; leaf adaxially tomentose (occasionally ± glabrous in age) ..... C. arizonicum

5. Corolla bright red or ± red-pink; corolla throat 2–5 mm, lobes 13–18 mm; SNH, W&I ..... var. arizonicum

5' Corolla pink- to ± red-purple; corolla throat 5–8 mm, lobes 10–14 mm; e DMtns ..... var. tenuisectum

4' Corolla white to pink or lavender; heads generally nodding; leaf adaxially densely velvety, with short, erect, jointed hairs, also cobwebby — serpentine wetlands; SnFrB, SCoRO ..... C. fontinale

6. Fr 4.1–5 mm; phyllary spines 1–2 mm ..... var. fontinale

6' Fr 3.4–4.1 mm; phyllary spines generally 2–6 mm

7. Longer spines of cauline leaves 10–18 mm; most phyllaries without marginal spines ..... var. campylon

7' Longer spines of cauline leaves 4–7 mm; many outer phyllaries with marginal spines ..... var. obispoense

3' Corolla lobes < tube

8. Abaxial face of outer and middle phyllaries with long, sticky-resinous ridge (milky when fresh, dark when dry, occasionally very narrow), occasionally poorly developed or 0 in Cirsium mohavense

9. Margins of phyllaries minutely spiny-fringed ..... C. hydrophilum

10. Heads generally 2.5–3 cm; fruit ± 5 mm, oblong, tapered near base; tidal marshes; deltaic GV (Suisun Marsh, Solano Co.) ..... var. hydrophilum

10' Heads generally 3–3.5 cm; fruit 4–5 mm, oblong or elliptic; serpentine springs; n SnFrB (Mount Tamalpais, Marin Co.) ..... var. vaseyi

9' Margins of phyllaries entire

11. St evidently coarse-hairy with multicellular hairs, also cobwebby to densely tomentose with long, fine hairs; cauline leaves generally ± clasping, with expanded basal lobes; leaf veins generally prominently raised abaxially ..... C. cymosum

12. Larger heads 15–25 mm diam; outer phyllaries generally << inner, sticky-resinous ridge prominent, well developed, appearing dark brown on dry specimens ..... var. canovirens

12' Larger heads 20–35 mm diam; outer phyllaries long, often nearly = inner, sticky-resinous ridge weakly developed ..... var. cymosum (2)

11' St with few or 0 coarse, multicellular hairs, thinly to densely cobwebby or tomentose with long fine hairs; cauline leaves generally not clasping with expanded basal lobes; leaf veins generally not prominently raised abaxially

13. Involucre 2.5–4.5 cm

14. Mid and distal leaves not or scarcely decurrent ..... C. undulatum

14' Mid and distal leaves evidently decurrent as spiny wings to 5 cm

15. Phyllary spines generally 2–4 mm; corolla ± white ..... C. canescens

15' Phyllary spines generally 5–12 mm; corolla white to pale lavender or pink ..... C. ochrocentrum var. ochrocentrum

13' Involucre 1.5–2.5(3) cm

16. Rosette arising from runner roots; leaf adaxially green, thinly gray-tomentose — n KR ..... C. ciliolatum

16' Rosette arising from taproot or woody caudex; leaf adaxially gray-tomentose or becoming ± green

17. Phyllaries ± loosely appressed, generally ascending to spreading above appressed base

18. Phyllary tip-spine generally 1–3 mm; SCoR and c SNF to mtns of SW ..... C. occidentale var. californicum (4)

18' Phyllary tip-spine generally 3–10 mm

19. Involucre, including phyllary tips generally ovoid to ± spheric; tip-spines of phyllaries generally 3–7 mm; outer phyllaries generally spreading or ascending — SNH, GB ..... C. inamoenum var. inamoenum (2)

19' Involucre, incl phyllary tips often wider than long; tip-spines of phyllaries generally 5–10 mm; outer phyllaries often reflexed ..... C. neomexicanum (2)

17' Phyllaries tightly appressed, only tip-spines spreading

20. Corolla ± white to pink or pale lavender; involucre pale green or straw-colored

21. Distal leaves generally conspicuously decurrent as spiny wings; wet soils, streams, springs; SNE, DMoj ..... C. mohavense

21' Distal leaves not or only weakly decurrent; dry sites; SCoR and c SNF to mtns of SW ..... C. occidentale var. californicum (4)

20' Corolla generally ± dark rose-purple (white); involucre generally dark green or ± purple — n CA-FP, MP ..... C. douglasii

22. Larger spines of distal cauline leaves generally <= 7 mm ..... var. breweri

22' Larger spines of distal cauline leaves generally 7–20 mm ..... var. douglasii

8' Abaxial face of outer and middle phyllaries without sticky-resinous ridge

23. Margins of outer phyllaries irregularly toothed to spiny-ciliate

24. Lvs densely gray- or white-felty-tomentose on faces — coastal dunes, headlands; s CCo ..... C. rhothophilum

24' Lvs adaxially green and glabrous to ± gray-hairy with multicellular hairs and/or finely cobwebby, abaxially sometimes densely gray- or white-tomentose

25. Corolla 30–45 mm, ± bright rose-purple ..... C. andersonii (2)

25' Corolla 17–28 mm, white or cream to purple

26. Heads generally not closely subtended by well-developed distal leaves or clustered leafy bracts; phyllary tips flattened, generally ascending, connected side-to-side by cobwebby hairs or not

27. Plant stout, generally 10–30 dm; stem 2–10 cm diam near base, evidently hollow; middle and inner phyllaries entire — SnJV ..... C. crassicaule

27' Plant slender, generally 1–11 dm; stems generally < 2 cm diam near base; middle and inner phyllaries irregularly toothed or spiny-ciliate ..... C. remotifolium

28. Phyllary margins irregularly toothed or cut ..... var. odontolepis

28' Phyllary margins ciliate with minute spreading to recurved spines ..... var. rivulare

26' Heads generally closely subtended by ± well-developed distal leaves or clustered bract-like leaves; phyllary tips needle-like, ascending to radially spreading, connected side-to-side by long, cobwebby, or stouter, multicellular hairs

29. Corolla lobes linear, tips not knob-like; style conspicuously exserted; stem cobwebby, ± glabrous in age, coarser, multicellular hairs 0 ..... C. andrewsii

29' Corolla lobes thread-like, tips knob-like; style included or exserted 1–2 mm; stem evidently hairy with ± coarse multicellular hairs, often cobwebby to densely tomentose with long, fine hairs ..... C. brevistylum (2)

23' Margins of outer phyllaries ± entire

30. Outer and middle phyllaries lance-linear to ovate, appressed; tip-spines 1–12 mm, generally ascending to erect

31. Plant erect; stem openly branched distally — biennial; dry sites; SCoR and c SNF to mtns of SW ..... C. occidentale var. californicum (4)

31' Plant generally ± compact, stless or low-growing and branched ± throughout (if erect, generally distally unbranched)

32. Perennial from runner roots; generally from dry sites; NCo, NCoRO, CW ..... C. quercetorum

32' Biennial or perennial herb that flowers once, then dies, taprooted; generally from moist sites; widespread ..... C. scariosum

33. Corolla pink to purple; SNH, SnBr, SNE ..... var. congdonii

33' Corolla white or faintly pink- or lilac-tinged

34. Abaxial leaf face generally gray-tomentose; CaRH, SNH, Wrn ..... var. americanum

34' Abaxial leaf face generally ± glabrous (occasionally tomentose in Cirsium scariosum var. citrinum)

35. Plant generally stless, or stless and occasionally short-stemmed individuals in population

36. Main leaf spines generally 2–7(10) mm; Teh, WTR, PR ..... var. citrinum (2)

36' Main leaf spines generally (3)7–10(12) mm; s CCo (sw San Luis Obispo, nw Santa Barbara cos.) ..... var. loncholepis (2)

35' Plant generally with evident branched or unbranched stem, or occasionally stless individuals present in population

37. St generally erect, proximally unbranched; heads generally ± tightly clustered at stem tip; corolla 20–28 mm ..... var. scariosum

37' St generally branched proximally or throughout, plant often growing as low, rounded mound; most heads on short to ± long lateral branches; corolla 26–36 mm

38. Inner phyllary tips generally expanded as scarious, irregularly toothed appendage — KR, nw MP ..... var. robustum

38' Inner phyllary tips acuminate, entire or rarely toothed

39. Main leaf spines generally 2–7(10) mm; Teh, WTR, PR ..... var. citrinum (2)

39' Main leaf spines generally (3)7–10(12) mm; s CCo (sw San Luis Obispo, nw Santa Barbara cos.) ..... var. loncholepis (2)

30' Outer and middle phyllaries with short-appressed bases and lanceolate to linear or needle-like, ascending to stiffly spreading tips; tip-spines generally 1–35 mm

40. Outer phyllaries ± equal, narrowly lanceolate, rigidly spreading, tip-spines 5–10 mm; middle and inner phyllaries ± graduated, bodies appressed, tips spreading, spineless — s SnFrB (Palo Alto, Santa Clara Co.) ..... C. praeteriens

40' Outer and middle phyllaries ± equal or graduated, tips ascending to spreading, needle-like to linear or lanceolate; middle phyllaries spine-tipped

41. Phyllary tips conspicuously connected side-to-side by network of cobwebby or multicellular hairs

42. Corolla lobes thread-like, tips knob-like; style included or exserted 1–2 mm; stem ± hairy, hairs coarse, jointed, generally also cobwebby to densely tomentose with long, fine hairs ..... C. brevistylum (2)

42' Corolla lobes linear, tips not knob-like; style conspicuously exserted; stem cobwebby to densely tomentose, hairs long, fine ..... C. occidentale (2)

43. Plant compact, low-growing, stless or proximally branched, rounded, mound-like; heads generally not much elevated above leaves — CCo ..... var. compactum

43' Plant erect, main stem generally 1, proximally unbranched; heads generally elevated above leaves

44. Phyllary tips ± equal, all needle-like, 13–30 mm; corolla light purple to rich red-purple or red ..... var. coulteri

44' Phyllary tips ± graduated, outer generally < middle and inner, ± needle-like to lanceolate, 5–15 mm; corolla lavender to bright purple ..... var. occidentale

41' Phyllary tips glabrous or hairy to densely tomentose but generally not conspicuously connected side-to-side by network of cobwebby or multicellular hairs

45. Corolla red to ± red-pink or bright rose-purple

46. Perennial, rosettes from runner roots; corolla throat ± abruptly narrowed to tube; corolla ± bright rose-purple ..... C. andersonii (2)

46' Biennial; corolla throat gradually narrowed to tube, without a clear external separation between tube and throat ..... C. occidentale (2)

47. Plant densely white-tomentose; phyllaries persistently white-tomentose (except tip-spines); outer phyllaries generally very long, spreading or reflexed ..... var. candidissimum

47' Plant variably tomentose, occasionally ± glabrous in age; phyllaries glabrous to ± loosely tomentose, occasionally some loosely cobwebby hairs extending between adjacent phyllary tips; outer phyllaries short to long, erect to spreading or reflexed

48. Corolla 20–24 mm, red or dark red-purple; s SCoRO (s Santa Lucia Range, San Luis Obispo Co.) ..... var. lucianum

48' Corolla 23–35 mm, generally bright red-pink to red; widespread in CA-FP, W&I, w DMoj ..... var. venustum

45' Corolla white or cream to lavender, purple, or pale rose

49. At least some inner phyllaries ± fringed ..... C. remotifolium var. remotifolium

49' Inner phyllaries entire

50. Base of cauline leaves generally clasping with expanded basal lobes; leaf veins generally prominently raised abaxially ..... C. cymosum var. cymosum (2)

50' Base of cauline leaves generally not clasping with expanded basal lobes; leaf veins not prominently raised abaxially

51. Phyllary tip-spine generally 1–3 mm; SCoR and c SNF to mtns of SW ..... C. occidentale var. californicum (4)

51' Phyllary tip-spine generally 3–10 mm

52. Involucre, incl phyllary tips generally ovoid to ± spheric; tip-spines of phyllaries (2)3–6 mm; outer phyllaries generally spreading or ascending — SNH, GB ..... C. inamoenum var. inamoenum (2)

52' Involucre, incl phyllary tips often wider than long; tip-spines of phyllaries generally 5–10 mm; outer phyllaries often reflexed ..... C. neomexicanum (2)

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Citation for the whole project: Jepson Flora Project (eds.) [year] Jepson eFlora, http://ucjeps.berkeley.edu/IJM.html [accessed on month, day, year]
Citation for an individual treatment: [Author of taxon treatment] [year]. [Taxon name] in Jepson Flora Project (eds.) Jepson eFlora, [URL for treatment]. Accessed on [month, day, year].
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