Wild Flowers of Pittsburgh

Canada Thistle (Cirsium arvense), White Form

Part of our continuing mission is to bring you common flowers blooming in the wrong colors. (See also our white Burdockpurple Queen Anne’s Lacewhite Deptford Pink, white Bluebellslavender Bugles, and white New England Aster.) Few weeds are more common than Canada Thistle, but its flowers are supposed to be pinkish-purple. Every once in a while, however, a plant appears that grows white flowers, as this one did. It grew by the side of a street in Banksville, where it was blooming in early August.

Gray describes the genus and the species:

CIRSIUM [Tourn.] Hill. COMMON or PLUMED THISTLE. Heads many-flowered; flowers all tubular, perfect and similar, rarely imperfectly dioecious. Bracts of the ovoid or spherical involucre imbricated in many rows, tipped with a point or prickle. Receptacle thickly clothed with soft bristles or hairs. Achenes oblong, flattish, not ribbed; pappus of numerous bristles united into a ring at the base, plumose to the middle, deciduous. Herbs, mostly biennial; the sessile alternate leaves often pinnatifid, prickly. Heads usually large, terminal. Flowers reddish-purple, rarely white or yellowish; in summer. (Name from kirsos, a swelled vein, for which the Thistle was a reputed remedy.) CNICUS of many auth., not L. By some recent Am. auth. included in CARDUUS.

Outer bracts of the appressed involucre barely prickly-pointed; heads mostly small and numerous. None of the leaves strongly decurrent.

C. arvénse (L.) Scop. (CANADA THISTLE.) Perennial, slender, 3-9 dm. high, the rootstock extensively creeping; leaves oblong or lanceolate, smooth, or slightly woolly beneath, finally green both sides, strongly sinuate-pinnatifid, very prickly-margined, the upper sessile hut scarcely decurrent; heads imperfectly dioecious; flowers rose-purple or whitish. (Carduus Robson; Cnicus Hoffm.) Cultivated fields, pastures, and roadsides, common; a most troublesome weed, extremely difficult to eradicate. (Nat. from Eu.) Var. VEST!TUM Wimm. & Grab. Leaves permanently white-lanate beneath. Locally established. (Nat. from Eu.) Var. INTEGRIFOLIUM Wimm. & Grab. Leaves  chiefly plane and uncut, or the lowest slightly pinnatifid. — Local, Que., N. E., and N. Y. (Nat. from Eu. )

In Wild Flowers East of the Rockies, Chester Albert Reed gives us this copious description:

CANADA THISTLE (Cirsium arvense) (EUROPEAN) is a small flowered, perennial species that has strayed across the ocean and became a pernicious weed. Individual plants are not themselves any more of a pest than are our native thistles but they have a dangerous, latent or potential power, in that they are far more prolific than our native species, due perhaps more to the number of the flowering heads than to any physical qualities of the plant.

The stem is rather slender, branching and grows from 1 to 3 feet in height. It grows from a perennial, creeping rootstalk that is, as farmers have discovered, very difficult to eradicate from the soil. It grows in extensive colonies and, unless strenuous efforts are made to destroy them, they very soon take possession of a field to the exclusion of almost everything else.

The leaves, that grow alternately and closely together on the stem, are long, lance-shaped, deeply cut into sharply-prickled lobes. Numerous flower heads, about one inch across, terminate the branches. When in full bloom, the florets vary in color from rose-purple to white; the involucre is almost globular and covered with over-lapping bracts, each with a tiny, sharp, out-turned point.

All the thistles yield an abundance of nectar and are frequented by bees and butterflies, by one of the latter so persistently that it has been named the Thistle Butterfly or Painted Lady (Pyrameis cardui); in fact this butterfly usually begins its career, as a caterpillar, on the thistle and lives chiefly upon its nectar and pollen through life.

In Wild  Flowers Every Child Should Know (1914), Frederic William Stack gives us a description that seems a bit technical for the juvenile market; but perhaps it is a mistake to underestimate the interest of children in botanical details.

CANADA THISTLE. CREEPING, CURSED, WAY, CORN, OR HARD THISTLE

Cirsium arvense. Thistle Family.

The Canada Thistle has been severely condemned by farmers in this country because of its rapid spread and the extreme difficulty with which its creeping roots are eradicated from the soil. It grows in extensive colonies, and quickly monopolizes our fertile meadows and pasture lands. The slender, leafy stalk is grooved and branching at the top, and grows from one to three feet high, from a perennial creeping rootstalk. The long, lance-shaped leaf is deeply cut into very prickly lobed or coarsely toothed segments, which bristle with many prickers, as they become curled or ruffled. The colour is grayish green, and the midrib is whitish. They slightly clasp the stalk, and the lower ones are stemmed. The numerous small, purple or whitish flower heads are loosely clustered on the tips of the branches. Many tubular florets with prominent purple stamens and white pistils compose the head. The latter is set in an egg-shaped, grayish green cup, which is covered with short, weak prickers. The flowers are fragrant and pleasing, but after they mature they become anything but sightly. This species is very common in cultivated fields and pastures and along roadsides from Newfoundland to Virginia, Minnesota and Nebraska, from July to September.

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