Canada thistle Cirsium arvense (L.) Scop., [CIRAR, chardon des champs, Canadian thistle, Creeping thistle, Field thistle, chardon du Canada] Perennial, reproducing by seed and by horizontal roots which produce new shoots, often forming dense patches. Stems erect, 30-150 cm (1 - 5 ft.) high, usually branched, slender, smooth or occasionally with a few, narrow, spiny-margined leaf-like wings on the lower part; leaves more or less lobed and spiny, alternate (1 per node), elliptic to oblong in outline, stakless and often clasping the stem; the wide variations in lobing, spininess, hairiness, texture and colour of leaves divide the species into 4 botanical varieties and are described in detail below; flower heads numerous comparatively small, 5 - 15 cm (1/5 - 3/5 in.) wide and about twice  as long, the involucral bracts weakly spiny or almost smooth, ray florets absent but disk florets prominent with purplish or sometimes white corollas; plants unisexual, although stamens and stigmas are sometimes present in the same flower, the flowers are functionally unisexual, all the flowers in 1 head and all the heads on a plant being either male or female; heads with male (pollen-producing) flowers somewhat shorter and narrower than heads with female (seed-producing) flowers; seeds light brown or straw-coloured, smooth, 2.5 - 4mm (1/10 - 1/6 in.) long. Flowers from June to late autumn.

Four botanical varieties of Canada thistle occur in Ontario. They are distinguished by differences in leaf characteristics.

Common names have not gained general acceptance for them but descriptive terms area applicable as follows:

Spiny Canada thistle, Cirsium arvense (L.) Scop. Var. horridium Wimm & Grab., [CIRAH, chardon des champs à feuilles très épineuses, typical C. arvense var. arvense in some other publications] leaves deeply and uniformly lobed along each side; the lobes pointed, twisted and wavy with many long, stiff, stout yellowish spines sticking out in all directions from their margins; the whole blade firm, stiff, mostly hairless and bright green to slightly yellowish-green. This is by far the most common variety, occurring throughout Ontario in grainfields, pastures, meadows, woodlands, waste places, roadsides, gardens and lawns in all soil textures and in saline (alkaline) soils as well as rich fertile soils.

Entire-leaved Canada thistle, Cirsium arvense L. var. integrifolium Wimm. & Grab., [CIRAI, chardon des champs à feuilles entières, C. setosum (Willd.) MB. ], all leaves entire (without teeth or labes), thin and flat, or the upper ones entire and only the lower ones shallowly and uniformly lobed or wavy-margined, with only a few short (3mm, 1/8 in.) slender marginal spines, hairless and green to rather dark green. Occurs in only a few localities in Ontario but able to grow in most habitats listed for the first variety.

Gentle Canada thistle, Cirsium arvense L. var. arvense [CIRAA, chardon des champs à feuilles peu èpineuses, C. arvense L. var. mite Wimm. & Grad. in some other publications], lower leaves shallowly to deeply lobed, the lobes often of unequal length andwidely spaced; upper leaves entire (without teeth or lobes) or nearly so, all leaves thin, flat, hairless, green, and with only a few short (3mm, 1/8 in.), slender marginal species. Occurs in only a few localities in Ontario but able to grow in most habitats listed for the first variety.

Woolly Canada thistle, Cirsium arvense L. var, vestitum Wimm. & Grab., [CIRAV, chardon des champs inerme, C. incanum S. G. Gmel. Fisch. Ex MB.], leaves gray-woolly on the undersurface, green above, entire (without teeth or lobes) or shallowly lobed, flat and with a few weak marginal spines; and stalk below each flower head also white woolly. Occurs only in a few localities in Ontario but able to grow in most habitats listed for the first variety.

All four varieties of Canada thistle are distinguished from the biennial thistles in non-flowering stages by their perennial habit reproducing from spreading underground roots, by the absence of a distinct circular rosette of basal leaves, and by their mostly non-winged stems with generally slender stature, and from Bull thistle by the absence of prickles from the surface of the leaf blades (apart from spines along the margins); flowering stages are distinguished by their generally smaller flower heads, mostly less than 25 mm ( 1 in.) long and 15 mm (3/5 in.) wide, these unisexual, and the involucral bracts surrounding each head either without spiny tips or with very short weak ones. They are distinguished from the Sow-thistles by the absence of white milky juice from stems and leaves.

(Source: Ontario Ministry of Agriculture and Food, Publication 505, Ontario Weeds)