CANADA THISTLE — Cirsium arvense (L.) Scop.

Asteraceae — (Sunflower family)

Seedlings of Canada thistle are several spiny upward growing leaves that become almost flat to the ground in a wheel and spoke arrangement.  The leaves are dark green, an inch or so wide and up to about 8 inches long, having ragged ruffled spiny edges.  The leaves get smaller and farther apart upward on the stem.  

The branching central stems grow 2-4 ft tall or more.  Each stem supports several vase shaped buds about ½ inch deep. 
Pink-purple (rarely white) ¾ flowers protrude.  The seeds are 1/8 inch long with a white plume which assists in wind dispersal.  

The roots are extensively creeping and produce numerous new plants each spring.  Broken roots can grow new plants so plowing or roto-tilling will increase the population.  Although the thistles may appear to be separate they are not; the underground root system allows them to nurture each other if they are threatened.  As an example; plants on the edge of a field are sprayed and seem almost to die but return vigorously because adjacent plants on the ditch bank or adjacent field are healthy.  Another example: the thistle in a lawn is mowed weekly or sprayed but does not die because the thistles in the flower bed or in the neighbor’s yard are not addressed. 

Seeds fed to birds are a prime source for lawn weeds including Canada Thistle. 

The following is courtesy of Weeds of the West:

Canada thistle is a native of Southeastern Eurasia. It was introduced to Canada as a contaminant of crop seed in the late 18th century. An aggressive weed that spreads both by seed and extensive root systems, Canada Thistle first establishes itself in disturbed soils. A colony-forming perennial whose roots grow horizontally sending up shoots along their length, Canada Thistle forms dense stands with erect stems which are one to four feet tall, ridged, and branching above. Leaves are alternate, lacking petioles, oblong or lance-shaped, divided into spiny-tipped irregular lobes.

The base of the flower head lacks spines. Canada thistle differs from other species of the true thistle in that there are male and female flower heads, and these are on separate plants. Asexual reproduction makes it possible for a colony of male plants to maintain itself. Flowers are purple (occasionally white) in heads ½ to ¾ inch in diameter; involucral bracts spineless. Fruits are about 1/8 inch long, somewhat flattened, brownish, with a tuft of hairs at the top. Flowering occurs during June and August.

Growth Habit: Perennial, erect, up to 4` tall.

Leaves: Varies from light to dark green, oblong or lance shaped, deeply cut, spiny toothed margins (some may be smooth); slightly hairy below. Tremendous leaf variability.

Stems: Smooth to slightly hairy, branched at top.

Flower: Small bristly clusters, 3/8 to 5/8 " in diameter, light lavender to deep rose purple. Plants are male or female.

Roots: Extensive, fleshy, creeping rootstocks.

Seeds: Smooth, light to dark brown, tipped by a cupped conical point, approximately 1/8" long.

Other: Reproduces by seed and creeping rootstocks.