Flower:

Thistle-like flower heads 1 to 1½ inches across, single at the ends of branching stems. Each head consists of set of ray flowers around the outer edge and numerous, shorter disk flowers in the center, though sometimes the ray flowers are absent. Ray flowers are sterile, widely spreading, narrowly tubular with 5 slender lobes as long as or longer than the tube. Disk flowers are fertile, erect to ascending, with a column of white-tipped stamens and a divided style. Flower color ranges from pink to purple, occasionally white, with the center disk flowers sometimes much paler than the outer flowers.

The bracts (phyllaries) surrounding the base of the flower are in several layers, green to pinkish with obvious parallel veins and a dark brown to blackish appendage at the tip, the phyllary body appressed with the appendage erect to somewhat spreading. The appendage is generally triangular in outline, about as long or longer than wide, and fringed with about 8 long, comb-like teeth on each side, the teeth also blackish-brown. The phyllary body is narrowed just below the appendage and may be a bit narrower than the appendage, including teeth. The entire set of phyllaries (involucre) is ½ to ¾ inch long and usually longer than wide.

Leaves and stems:

Leaves are basal and alternate, green, somewhat hairy. Basal and lower stem leaves may be stalked, up to 10 inches long and 1+ inch wide, often widest above the middle, sometimes with a few lobes or teeth, becoming progressively smaller, more lance-elliptic, unlobed and stalkless as they ascend the stem.

Stems are erect to ascending, single or a few from the base, branched in the upper plant, ribbed, rough textured to sparsely covered in cobwebby hairs.

Fruit:

Fruit is a dry seed, elliptic, brown, finely hairy, 2.5 to 3 mm long. A tuft of hairs at the tip may be absent, or when present the hairs are of unequal lengths and may shed early

Notes:

The Knapweeds are a confusing bunch. The differences between several species boil down to the phyllaries, more specifically, the appendage at the tip. Its size relative to the phyllary body, color and shape are key to a positive ID. Alpine Knapweed phyllaries have a green body, sometimes pink-tinged, and narrows at the tip just below the dark brown to black appendage. The appendage is triangular in outline, has about 8 narrow, comb-like teeth along each side, and is about as wide or a little wider than the phyllary body. The bodies are not completely hidden by the appendages so from a little distance the involucre can look green with blackish polka dots.

Alpine Knapweed has been most easily confused with both Spotted Knapweed (Centaurea stoebe) and Meadow Knapweed (Centaurea ×moncktonii). Meadow Knapweed, a hybrid between Brown Knapweed (C. jacea) and Black Knapweed (C. nigra), has phyllary appendages with up to 15 comb-like teeth, are light to medium brown, and are rather larger, mostly hiding the smaller phyllary bodies. Spotted Knapweed phyllaries have green to purplish-brown bodies that are mostly wider than the appendage, the body is not much narrowed just below the appendage especially on lower phyllaries, and the appendage teeth are frequently whitish especially when young. Spotted Knapweed also has the distinction of mid and lower stem leaves deeply divided into narrow segments where other Knapweeds are not much lobed, if at all.

Note that there is a record of Centaurea nigrescens collected in northern St. Louis County, but after seeing photographs of that collection we believe it was mis-identified so is not included on our Minnesota map. And the location in Clearwater County where we photographed our specimens was originally identified as C. X moncktonii, which we also believe is incorrect. That illustrates the degree of expertise on these new Knapweeds—I wasn't kidding when I said they are a confusing bunch. We may be incorrect ourselves, but stand by our accounts pending further discussion with the experts.