Flower:

Pink to purple, occasionally white, thistle-like flower heads 1 inch across at the end of branching stems. Each head is made up of many tubular flowers with 5 narrow lobes that fan out.

The bracts under the flower head also resemble those of a thistle and are green or brown tipped with brownish black, with coarse hairs all around the edge of the tip. A mature plant has 25 to 100 flower heads.

Leaves and stem:

Grayish blue-green, somewhat hairy leaves are deeply lobed into narrow segments. Near the base of the plant leaves may be up to 8 inches long and 2 inches wide, becoming progressively smaller as they ascend the stem. Leaves near the flowers are typically small, narrow and unlobed. The main stem is ridged and roughly hairy. One plant has up to 15 stems, each heavily branched.

Notes:

I hate this plant—it is taking over everywhere at my favorite local park. The flowers of Spotted Knapweed have been mistaken for Wild Bergamot (Monarda fistulosa), which it vaguely resembles, but the leaves and overall plant structure are very different between the two species. The stems and bracts are stiff and scratchy, making walking through a patch of this an uncomfortable experience. Spotted Knapweed is on the prohibited/control weed list for Minnesota and should be eradicated. It contains chemicals that poison the soil and inhibit native plants. The hundreds of seeds produced by one plant mostly stay within a few feet of the parent plant, thus it tends to form large, monoculture colonies. The first year it sprouts there is just a rosette of leaves, with flowers appearing the second year. Spotted Knapweed is one of 3 targeted plant species in Minnesota with an active bio-control program. The other 2 species are leafy spurge and purple loosestrife. Spotted Knapweed also frequently goes by Latin names Centaurea biebersteinii and Centaurea maculosa.