Cirsium arvense (L.) Scop.

Canada Thistle

© SRTurner

Family - Asteraceae/Cardueae

Habit - Perennial forb, imperfectly dioecious (some plants with only pistillate florets, others with a combination of staminate and perfect florets), suckering and forming potentially large, clonal colonies from widely creeping, deep-set, black roots.

Stems - Erect, to 1.5 m, usually several-branched toward the tip, glabrous or with patches of cobwebby hairs (especially when young), hollow, striate, lacking spiny-margined wings.

	Stem.
© DETenaglia

Leaves - Basal and alternate. Basal leaves usually absent at flowering, 8-30 cm long, 2-6 cm wide, narrowly oblong-lanceolate to elliptic or ovate, tapered at the base, sharply pointed at the tip, shallowly to deeply pinnately lobed, the lobes spiny and sometimes irregularly toothed, the upper surface appearing green, glabrous to thinly pubescent with woolly or cobwebby hairs, the undersurface appearing green or more commonly gray, glabrous to densely pubescent with woolly or felty hairs. Stem leaves well developed nearly to the inflorescence, mostly 2-20 cm long, slightly expanded and sometimes minutely decurrent at the base, often somewhat narrower than but otherwise like the basal leaves.

	Stem and leaf.
© SRTurner

	Leaf.
© DETenaglia

Inflorescence - Loose clusters or solitary heads at branch tips. Branches of inflorescence (peduncles) pubescent with cobwebby hairs.

	Inflorescence.
© SRTurner

Heads - Involucre 10-20 mm long, as long as or slightly longer than wide, urn-shaped, glabrous or more commonly somewhat cobwebby-hairy. Involucral bracts imbricate, appressed, the lower and median bracts tapered to an appressed or ascending, nonspiny tip, this often with a minute, sharp point, green or purplish-tinged, usually also somewhat sticky along the midrib. Bract tips usually slightly spreading. Outer bracts with sparse to moderate arachnoid pubescence. Receptacle flat.

	Head.
© DETenaglia

	Involucre.
© SRTurner

Ray flowers - Absent.

Disk flowers - Corollas 12-20 mm long (those of staminate florets slightly shorter than those of pistillate florets), pale pinkish to lavender-purple, rarely white, the 5 lobes 3-5 mm long, thickened at apex with small protuberance. Pappus of dense plumose bristles, 12-25 mm long at flowering (longer in pistillate than in staminate florets), white or grayish-tinged.

	Florets.
© SRTurner

Fruits - Achenes to 2.5-4.0 mm long, glabrous, terete to weakly 4-angled.

	Fruiting head.
© DETenaglia

Flowering - June - October.

Habitat - Crop fields, disturbed areas, roadsides, railroads.

Origin - Native to Eurasia.

Lookalikes - Other thistles, e.g. Cirsium discolor, C. vulgare, etc.

Other info. - This is a nasty, noxious weed throughout much of the U.S., particularly in states to our north and west. Fortunately, it is still relatively uncommon in Missouri, having been collected in only nine counties thus far. In addition to its general thistle-like appearance, the plant is recognized by usually having numerous, smallish flowering heads. The stems are unwinged, the leaf undersides are greenish rather than whitened, and the involucral bracts are pointed but not spiny. The plant is quite aggressive, and due to its deep and extensive rhizomes is very difficult to eradicate.

The plant is not without redeeming qualities. Like most thistles it is enjoyed by birds as a source of seeds, and by insects as a source of nectar. Nevertheless, the plant should never be deliberately cultivated or spread, as there are numerous species, including other thistles, which are far less noxious and better behaved within the natural communities they inhabit.

	Albino form.
© DETenaglia

Photographs taken in Spencer, WI., 8-6-04 (DETenaglia), also along a crop field near Fremont, Newaygo County, MI, 7-2-2014 (SRTurner).