Flower:

Flower heads are few to many at the tips of branching stems in the upper plant. Flowers are purple to pink, rarely white, 1 to 2 inches wide.

Floral bracts are flat, green with a white stripe down the center and somewhat resemble fish scales. Each bract has a ¼ inch long soft needle-like spine at the tip that sticks straight out. At the base of these floral bracts surrounding the stalk are several narrow leafy bracts.

Leaves and stem:

Leaves are alternate, up to 18 inches long and 8 inches wide. Basal and lower leaves may be deeply lobed or have no lobing, tapering to a winged stalk. Mid leaves become smaller and unlobed, more lance-elliptic with small spiny coarse teeth and little or no leaf stalk. Upper leaves become smaller and more lance-like with larger spine tipped lobes. 

Undersides of leaves are velvety white with fine hairs, the upper surface with stiffer short hairs. Stems are ridged and covered with crisp, bristly hairs but are not spiny; branching is in the upper portions of mature plants.

Notes:

While Tall Thistle as been described as attaining nearly ten feet in height, found in a variety of open spaces, including disturbed sites, and open places we have encountered it only in fairly deeply shaded woods of southeastern Minnesota and never much over 6 feet tall. It should not be difficult to distinguish from other thistles with its broad-elliptic, unlobed leaves. Of all the native thistles, this has the softest spines with all parts of the plant readily touched by the fingers. The floral bracts and white undersides of leaves are much the same as Field Thistle (Cirsium discolor), which has deeply lobed leaves all the way up the stem.