Flower:

Flower heads are ¾ inch across, lavender to pale pink or sometimes white. The bracts are flat, except for the tip which peels back away from the flower head. The bract tips are pointed but not spined, and typically a purplish color, sometimes the entire bract is tinged purple. One plant typically has numerous flower heads, single or in small clusters at the ends of branching stems.

Leaves and stem:

Leaves are stalkless, up to 6 inches long with yellow spines scattered along the wavy edges. The lower leaves are up to 2½ inches across and deeply lobed, with a spine at the tip of each lobe. The upper leaves are more narrow, and generally more toothed than lobed. Leaves may be somewhat hairy on the underside. The stem is light green, slightly ridged and may have scattered hairs but is not usually spiny.

Fruit:

Fruit is a brown seed with a tuft of long white hairs to carry it off in the wind, spreading itself far and wide.

Notes:

Canada Thistle is highly aggressive, can form large, dense colonies from its creeping roots, and crowds out native plants pretty much everywhere it grows. It is fairly easy to identify from the numerous small lavender to pinkish flower heads, and bracts that have no spines. While it prefers moist conditions it can sprout up and persist just about anywhere. It is currently designated as a Prohibited-Control noxious weed in Minnesota, but the MN Weed Advisory group is recommending a change to Restricted noxious weed, due to "lack of a viable statewide control method". Unfortunately, the lack of a statewide program applies to pretty much all invasive species in Minnesota at this time and they are in essence giving up the pretense they can do anything to stop its spread. We can only hope that will change someday.