Centaurea stoebe (Spotted Knapweed)
|Also known as:
|biennial, short-lived perennial
|sun; dry fields, roadsides, waste areas
|June - October
|2 to 3 feet
|Wetland Indicator Status:
|MN county distribution (click map to enlarge):
|National distribution (click map to enlarge):
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Thistle-like flower heads 1 inch across, single at the ends of numerous branching stems. Each head consists of set of ray flowers around the outer edge and numerous, shorter disk flowers in the center. 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, usually appressed, green to purplish-brown, with obvious parallel veins and a dark brown to blackish appendage at the tip that is fringed in long, comb-like teeth, the teeth white to blackish-brown. The phyllary body is often wider than the appendage. The entire set of phyllaries (involucre) is 3/8 to ½ inch long and usually longer than wide. A mature plant has 25 to 100 flower heads.
Leaves and stem:
Leaves are basal and alternate, gray-green to blue-green, somewhat hairy, 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, more linear, and unlobed. Stems are stiff, ridged and roughly hairy, multiple from the base, and heavily branched. First-year plants produce only a basal rosette of leaves, with flowering stems emerging the second year.
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 was formerly (or incorrectly) known as Centaurea biebersteinii and Centaurea maculosa. There are at least 2 recognized subspecies of C. stoebe, though they are poorly documented; subsp. micranthos is the species in North America.
The flower heads are very similar to other non-native, pink to purple flowered Centaurea species known to be in (or coming soon to) Minnesota, but the phyllaries of each species should be distinctive. Perhaps the most similar is Alpine Knapweed (Centaurea nigrescens); its phyllary body is not usually noticeably wider than the appendage, is narrowed just below the appendage so the appendage is more distinct from the body, and the teeth are more slender and always dark brown to black, never pale. Spotted Knapweed has the added distinction of leaves deeply lobed into narrow segments, where the other Knapweeds are either unlobed or less finely lobed.
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- Spotted Knapweed plant
- Spotted Knapweed plants
- Spotted Knapweed plants
- an infestation of Spotted Knapweed
- emerging in spring
- first year rosette
- more leaves
- more flowers
- a white Spotted Knapweed flower
- comparison of Centaurea stoebe and C. nigrescens phyllaries
Photos by K. Chayka taken in Ramsey County. Photos by Peter M. Dziuk taken in Anoka, Kanabec and Pine counties.
Have you seen this plant in Minnesota, or have any other comments about it?