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Spotted Knapweed Centaurea biebersteinii (Centaurea maculosa)


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Spotted Knapweed
credit: Matt Lavin from Bozeman, Montana, USA/CCSA

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Family: Asteraceae, Aster view all from this family

Description A noxious, invasive weed with many thistle-like lavender flowers that persist after drying.
Habit: introduced biennial or short-lived perennial herb; 1-15 or more branched, persistent, hairy stems; clumping or mat-forming: deep taproot.
Height: 8 inches to 4 feet (0.2 - 1.2 m)
Leaf: in basal rosette, gray-green, rough, deeply pinnately divided, narrowly lobed, to 8 in (20 cm) long and 2 in (5 cm) wide; on stem, alternate, becoming smaller and linear.
Flower: thistle-like, terminal, solitary or in clusters of 2 or 3, .25-1 in (6-25 mm) wide, 0.5 in (12 mm) long; bracts tipped with black fringe; up to 60 flowering heads per plant.
Fruit: dry oval seed, to .25 in (6 mm) long; tipped with short bristles.

Warning If this noxious weed is extensively handled, gloves are advised; there is some evidence that it causes tumors on the hands.

Flower June to October.

Habitat Varied: open dry sites with poor, gravel, or sandy soil; fields, forest openings, grasslands, prairie, streambanks, disturbed areas, roadsides, railroads, rangelands, dunes, barrens, overgrazed pastures.

Range Primarily a weed of the northwestern states and southwestern Canada, but reported in every U.S. state except Alaska, Texas, Oklahoma, and Mississippi.

Discussion Sometimes classified as Centaurea stoebe ssp. micranthos. Spotted knapweed has been declared a noxious or restricted weed in at least 15 states in the U.S. and 4 Canadian province. Native to eastern Europe, it was introduced to North America in the late 1800s, probably as a contaminant in alfalfa seed or ship's ballast. It naturalized first in the San Juan Islands of Washington, and has spread rapidly across North America. It spreads aggressively and, once established, chokes out native vegetation.