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Spotted knapweed

(Centaurea biebersteinii)

Photo of spotted knapweed
Photo credit: Kenneth Sytsma

Herbaceous, short-lived perennial, 2-4’ tall. Persists as a rosette 1-4 years before bolting. Flowering plants usually have 1-6 stems, but many have up to 20.

Overview

Other names for this plant include:
  • Scientific names: C. stoebe; C. stoebe ssp. micranthos; C. maculosa
Ecological threat:
  • Invades dry areas, including prairie, oak and pine barrens, dunes and sandy ridges. It also invades roadsides and disturbed areas.
  • Roots exude allelopathic chemicals (compounds that inhibit the growth of other vegetation).
  • It is not palatable as a forage plant and is avoided by both livestock and native grazers.
  • Infestations cause increased runoff, sedimentation, and decreased water-holding capacity in soil.
Overview map of spotted knapweed classification in WI
Restricted (orange) counties

Classification in Wisconsin: Restricted

Species Assessment Groups (SAG) were assembled to recommend a legal classification for each species considered for NR 40. The recommendation for spotted knapweed was based upon this literature review [PDF] developed by the department.

Identification

CAUTION: Wear long sleeves and gloves when handling. Spotted knapweed exposure can irritate the skin.

Leaves: Gray-green, covered in rough hairs, and deeply divided. Rosette leaves grow up to 6” long. Stem leaves alternate, with lower stem leaves resembling rosette leaves, becoming small (1-3” long), entire and linear higher up the stem.

Flowers: Thistle-like, pink to purple flower heads, rarely white. Flower heads have stiff bracts tipped with black, fringed hairs. Bloom mid-summer to early fall.

Fruits & seeds: Wind-dispersed for short distances but carried long distances by humans, livestock, or rodents. Viable in the soil for up to 7 years.

Roots: Strong taproot. Some plants produce a shallow mat of fibrous roots extending from the plant for several feet. Some sprouting from lateral roots occurs.

Similar species: There are several non-native Centaurea species that look similar. Spotted knapweed can be distinguished by the bracts on the flower head that are tipped with black.

Control

Mechanical: Small infestations can be repeatedly hand-pulled making sure to remove the entire root. Continual mowing as close to the ground as possible, prior to seed-set, may suppress populations.

Chemical: Foliar spray of glyphosate, clopyralid or aminopyralid and a non-ionic surfactant during bolting or flower bud stage.

Biological: There are currently 13 biological control agents. Successful treatment requires one seed head and one root mining agent. Contact the Wisconsin Department of Agriculture, Trade and Consumer Protection.

For more information on control techniques, visit the Spotted knapweed factsheet [exit DNR] by University of Wisconsin-Extension.

Photos

View spotted knapweed pictures in our photo gallery!

Resources

Sources for content:

  • Czarapata, Elizabeth; Invasive Plants of the Upper Midwest: an illustrated guide to their identification and control. University of Wisconsin Press. 2005. Pg. 52-55
  • University of Wisconsin-Extension Weed Science

Links for more information: