2. Cirsium arvense (L.) Scop. (Canada thistle, field thistle)

C. arvense var. horridum Wimm. & Grab.

Pl. 254 c, d; Map 1051

Plants perennial, 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 30–150 cm long, unbranched or more commonly several-branched toward the tip, glabrous or with patches of cobwebby hairs (especially when young), without spiny-margined wings. 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 or occasionally merely coarsely toothed, the lobes otherwise 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. Heads usually relatively numerous, usually in loose clusters at the branch tips, mostly appearing short-stalked. Involucre 10–20 mm long, as long as or slightly longer than wide (often appearing broader when pressed), glabrous or more commonly somewhat cobwebby-hairy, 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. Corollas 12–20 mm long (those of staminate florets slightly shorter than those of pistillate florets), pinkish purple to lavender-purple, rarely white, the lobes 3–5 mm long. Pappus 12–25 mm long at flowering (longer in pistillate than in staminate florets), white or grayish-tinged. Fruits 2.5–4.0 mm long. 2n=34. June–October.

Introduced, scattered, sporadic, most widespread in the northwestern quarter of the state (native of Europe, Asia, introduced nearly throughout the U.S. and Canada). Crop fields, fallow fields, railroads, roadsides, and open, disturbed areas.

Canada thistle is a bad weed of crop fields and rangeland farther north, but although the Missouri legislature declared it a noxious weed in 1909, the species has not yet become the problem in the state that was predicted. Steyermark (1963) knew it as a rare introduction in only five counties. Cirsium arvense apparently was an early introduction into eastern Canada as a contaminant in agricultural seed and was already the subject of a Vermont weed ordinance as early as 1795 (Hansen, 1918). Its southward spread was aided by the transport of contaminated hay during the Civil War era, as well as in impure crop seed harvested from infested sites. Because of its deep, spreading rootstock it is a difficult plant to control once established. Thus far, attempts to develop an effective biological control agent for it have failed. Paradoxically, the species is a good nectar plant for bees and butterflies, and seed-eating birds such as goldfinches (Carduelis species) relish the fruits.

Although sometimes described as dioecious, many staminate clones of C. arvense produce heads containing at least a few perfect florets. Some authors have recognized several varieties. Among the two most commonly treated, the var. horridum is said to differ from var. arvense in its more deeply lobed leaves with spinier margins. Both extremes exist in Missouri along with plants with intermediate characters. Morphological variation among North American populations is complex and rendered more confusing by the great amount of vegetative reproduction through root suckers that often results in populations that are relatively uniform morphologically. Thus, it seems inadvisable to attempt the segregation of varieties among the introduced North American populations of this species at the present time.