Spotted knapweed leaves and flowers

Spotted knapweed (Centaurea biebersteinii)

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

Regulated areas of spotted knapweed
Spotted knapweed is Restricted (Orange counties)

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.

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 developed by the department.

Identification

CAUTION: Wear long sleeves and gloves when handling. Spotted knapweed exposure can irritate 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 min-summer to early fall.

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

Roots: Strong taproot. Some plants produce a shallow mat of fibrous roots extending from 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.

Distribution

Known county distribution of spotted knapweed
Counties in WI where spotted knapweed has been reported (as of July 2011). Both vouchered and unvouchered reports included.

Do you have spotted knapweed in your county but it isn't shaded on the map? Send us a report.

Control

Mechanical: Small infestations can be repeatedly hand pulled making sure to remove 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

Last revised: Monday October 07 2013