Flower:

A thick oval to cylindric spike 2 to 4 inches long at the top of the plant and the tips of branching stems in the upper plant. A spike consists of hundreds of tiny flowers that typically bloom in a series of rings around the spike. Flowers are tubular, white with 4 lobes that are commonly lavender but range from pink to purple. Projecting from the tube are purple-tipped stamens.

At the base of each flower is a flexible, spine-like bract that is straight to slightly curved. At the base of the spike are several stiff, narrow bracts that are spreading to upward curving, variable in length, often with some longer than the spike, and most with scattered prickles along the edges.

Leaves and stems:

Leaves of flowering plants are opposite, mostly lance-oblong, pointed at the tip, the lowest leaves largest, up to 12 inches long, coarsely toothed or scalloped around the edges, wavy-edged or flat. The bases of leaf pairs are fused together around the stem (perfoliate), forming a cup where water may collect.

Leaves become smaller as they ascend the stem, the uppermost leaves often toothless and barely connected at the stem. All leaves are hairless, have a prominent white midvein, and scattered prickles along the midrib on the underside. Some leaves also have scattered prickles on the upper surface, usually in lines parallel to the midvein.

Stems are single from the base, typically branched in the upper plant and ribbed, with scattered white prickles along the ribs, more densely prickled near the flower heads.

Leaves of non-flowering plants are in a basal rosette, smaller than flowering plants, coarsely toothed, crinkly-wavy around the edges and puckered on the surface. The plants remain in this state one or more years until they store up enough energy to produce a flowering shoot. This is known as a monocarpic perennial.

Fruit:

A spike creates a seed head with hundreds of seeds that ripen to a dark brown. Stems and seed heads can persist to the next season, or even longer.

Notes:

Like its cousin Cut-leaf Teasel (Dipsacus laciniatus), Common Teasel was brought into the US in colonial times, using the spiny heads on spindles to raise the nap of fabric, though I read one account of that story that claims this honor actually belongs to another cousin, Cultivated Teasel (Dipsacus sativus). More on that in a moment... Common Teasel is just beginning to spread in Minnesota—it was first recorded in Winona County in 2006 and quickly made the Eradicate list by the MN Department of Agriculture. Both Cut-leaved and Common teasels are highly invasive and can form dense monocultures. Both are typically found in open disturbed roadsides, fields and waste places but can move into higher grade habitat and native woodlot margins.

As one might guess from the common name, Cut-leaf Teasel differs most significantly by its deeply lobed stem leaves, where Common Teasel has unlobed leaves. Distinguishing Common (D. fullonum) from Cultivated (D. sativus) teasel is less easy, particularly since there is so little documentation on Cultivated Teasel. California has the largest populations of it, and the Jepson Herbarium one of the few resources describing it. Per their descriptions:

• D. fullonum floral bracts are flexible and relatively straight, D. sativus are stiff and curve down at the tip (these characteristics are what made it more useful for raising fabric naps).
• The bracts at the base of D. fullonum spikes are generally erect or upcurved, D. sativus are more spreading or reflexed.
• D. fullonum leaf pairs are narrowly fused around the stem, D. sativus are narrowly to widely fused.

D. fullonum also typically has lavender flowers where D. sativus are white. In 2017 a population of D. fullonum was recorded in Hennepin County and of course I ran right over to investigate. What I found was not what I expected: flowers were white, not lavender. Other aspects seemed reasonable for D. fullonum, but at that time I had no knowledge of D. sativus. The flower color kept nagging at me. After extended research I learned about D. sativus and have since concluded that may well be what's at the Hennepin County site, especially after seeing textbook D. fullonum in Dakota County. Besides the white flowers, it also had floral bracts curved at the tip, larger leaves were all widely fused, and the basal rosettes did not really resemble those of D. fullonum. It's not confirmed, but is a distinct possibility—more research is needed. And a more complete reference.