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slough thistle

Canada or creeping or field thistle, Canada thistle, Canadian thistle, chardon du Canada ou des champs, cirse des champs, creepiing thistle, creeping thistle, lettuce from hell thistle

Habit Annuals or biennials, (60–)100–300 cm; taprooted. Perennials, dioecious or nearly so, 30–120(–200) cm; colonial from deep-seated creeping roots producing adventitious buds.
Stems

usually 1, erect, stout, (hollow, 2–10 cm diam. at base), openly branched distally, thinly arachnoid, villous with jointed trichomes, at least proximally.

1–many, erect, glabrous to appressed gray-tomentose;

branches 0–many, ascending.

Leaves

blades elliptic to broadly oblanceolate, 15–70 × 30–150+ cm, flat, pinnatifid 1/2–2/3 distance to midvein, larger usually with broad sinuses, lobes broad, few lobed or dentate, main spines 3–8 mm, abaxial faces gray-tomentose, adaxial thinly arachnoid-tomentose, sometimes midveins with jointed trichomes;

basal present or withered at flowering, winged-petiolate;

principal cauline sessile, progressively reduced distally, bases clasping or short-decurrent 1–2 cm;

distal cauline reduced, becoming bractlike, sometimes spinier than proximal.

blades oblong to elliptic, 3–30 × 1–6 cm, margins plane to revolute, entire and spinulose, dentate, or shallowly to deeply pinnatifid, lobes well separated, lance-oblong to triangular-ovate, spinulose to few-toothed or few-lobed near base, main spines 1–7 mm, abaxial faces glabrous to densely gray-tomentose, adaxial green, glabrous to thinly tomentose;

basal absent at flowering, petioles narrowly winged, bases tapered;

principal larger cauline proximally winged-petiolate, distally sessile, well distributed, gradually reduced, not decurrent;

distal cauline becoming bractlike, entire, toothed, or lobed, spinulose or not.

Peduncles

0–15 cm.

0.2–7 cm.

Involucres

ovoid to campanulate, 1.5–3 × 1.5–3 cm, ± glabrous.

ovoid in flower, ± campanulate in fruit, 1–2 × 1–2 cm, arachnoid tomentose, ± glabrate.

Corollas

pale rose-purple (white), 19–26 mm, tubes 9–12 mm, throats 4–6 mm, lobes 5–9 mm;

style tips 3.5–4.5 mm.

purple (white or pink);

staminate 12–18 mm, (remaining longer than pappus when head is fully mature), tubes 8–11 mm, throats 1–1.5 mm, lobes 3–5 mm;

pistillate 14–20 mm, (overtopped by pappi in fruit), tubes 10–15 mm, throats ca. 1 mm, lobes 2–3 mm;

style tips 1–2 mm.

Phyllaries

in 5–7 series, weakly unequal, dark green to brownish, lanceolate (outer) to linear (inner), abaxial faces without (or with very obscure) glutinous ridge;

outer and middle appressed or apices spreading, at least outer irregularly spiny-fringed, finely serrulate, spines slender, 3–5 mm;

apices of inner erect, abaxial faces gray-tomentose, ± twisted.

in 6–8 series, strongly imbricate, (usually purple-tinged), ovate (outer) to linear (inner), abaxial faces with narrow glutinous ridge, outer and middle appressed, entire, apices ascending to spreading, spines 0–1 mm (fine);

apices of inner phyllaries flat, ± flexuous, margins entire to minutely erose or ciliolate.

Heads

1–several at branch tips, closely subtended by clustered leafy bracts or not, collectively forming open, corymbiform or paniculiform arrays.

1–many, borne singly or in corymbiform or paniculiform arrays at tips of main stem and branches.

Cypselae

dark brown, 5–5.5 mm, collars narrow, ± stramineous;

pappi 15–20 mm.

brown, 2–4 mm, apical collar not differentiated;

pappi 13–32 mm, exceeding corollas.

2n

= 32.

= 34.

Cirsium crassicaule

Cirsium arvense

Phenology Flowering spring (Apr–Jun). Flowering summer (Jun–Oct).
Habitat Freshwater marshes, canal banks Disturbed sites, fields, pastures, roadsides, forest openings
Elevation 5–100 m (0–300 ft) 0–2600 m (0–8500 ft)
Distribution
from FNA
CA
[BONAP county map]
from FNA
AK; AL; AR; AZ; CA; CO; CT; DC; DE; IA; ID; IL; IN; KS; KY; MA; MD; ME; MI; MN; MO; MT; NC; ND; NE; NH; NJ; NM; NV; NY; OH; OR; PA; RI; SD; TN; TX; UT; VA; VT; WA; WI; WV; WY; AB; BC; MB; NB; NF; NS; NT; ON; PE; QC; SK; YT; SPM; Greenland; native; Eurasia [Introduced in North America]
[WildflowerSearch map]
[BONAP county map]
Discussion

Cirsium crassicaule is known only from a few sites in the San Joaquin Valley. Some populations are threatened by habitat modification and development pressures.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Cirsium arvense is one of the most economically important agricultural weeds in the world. It was introduced to North America in the 1600s and soon was recognized as a problem weed. Weed control legislation against the species was passed by the Vermont legislature in 1795 (R. J. Moore 1975). Canada thistle is now listed as a noxious weed in most areas where it occurs. It has very high seed production, and the runner roots readily survive the fragmentation that accompanies cultivation.

Numerous variants of Cirsium arvense have been named based upon such features as pubescence, extent of leaf division, and spininess. Although extreme variants can be strikingly different, they are connected by such a web of intermediates that there seems to be little value in according any of them formal taxonomic recognition.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Source FNA vol. 19, p. 132. FNA vol. 19, p. 109.
Parent taxa Asteraceae > tribe Cardueae > Cirsium Asteraceae > tribe Cardueae > Cirsium
Sibling taxa
C. altissimum, C. andersonii, C. andrewsii, C. arizonicum, C. arvense, C. barnebyi, C. brevifolium, C. brevistylum, C. canescens, C. carolinianum, C. ciliolatum, C. clavatum, C. cymosum, C. discolor, C. douglasii, C. drummondii, C. eatonii, C. edule, C. engelmannii, C. flodmanii, C. foliosum, C. fontinale, C. grahamii, C. helenioides, C. hookerianum, C. horridulum, C. hydrophilum, C. inamoenum, C. joannae, C. kamtschaticum, C. lecontei, C. longistylum, C. mohavense, C. muticum, C. neomexicanum, C. nuttallii, C. occidentale, C. ochrocentrum, C. ownbeyi, C. palustre, C. parryi, C. perplexans, C. pitcheri, C. praeteriens, C. pulcherrimum, C. pumilum, C. quercetorum, C. remotifolium, C. repandum, C. rhothophilum, C. rydbergii, C. scariosum, C. texanum, C. tracyi, C. turneri, C. undulatum, C. vinaceum, C. virginianum, C. vulgare, C. wheeleri, C. wrightii
C. altissimum, C. andersonii, C. andrewsii, C. arizonicum, C. barnebyi, C. brevifolium, C. brevistylum, C. canescens, C. carolinianum, C. ciliolatum, C. clavatum, C. crassicaule, C. cymosum, C. discolor, C. douglasii, C. drummondii, C. eatonii, C. edule, C. engelmannii, C. flodmanii, C. foliosum, C. fontinale, C. grahamii, C. helenioides, C. hookerianum, C. horridulum, C. hydrophilum, C. inamoenum, C. joannae, C. kamtschaticum, C. lecontei, C. longistylum, C. mohavense, C. muticum, C. neomexicanum, C. nuttallii, C. occidentale, C. ochrocentrum, C. ownbeyi, C. palustre, C. parryi, C. perplexans, C. pitcheri, C. praeteriens, C. pulcherrimum, C. pumilum, C. quercetorum, C. remotifolium, C. repandum, C. rhothophilum, C. rydbergii, C. scariosum, C. texanum, C. tracyi, C. turneri, C. undulatum, C. vinaceum, C. virginianum, C. vulgare, C. wheeleri, C. wrightii
Synonyms Carduus crassicaulis Serratula arvensis, Breea arvensis, Carduus arvensis, C. arvense var. argenteum, C. arvense var. horridum, C. arvense var. integrifolium, C. arvense var. mite, C. arvense var. vestitum, C. incanum, C. setosum
Name authority (Greene) Jepson: Fl. W. Calif., 506. (1901) (Linnaeus) Scopoli: Fl. Carniol. ed. 2, 2: 126. (1772)
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