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Spotted knapweed

Quick facts

Spotted knapweed is an invasive species. 

  • Spotted knapweed is found in artificial corridors such as gravel pits, railroad beds and field margins, and can spread to adjacent intact woodlands and prairies.
  • Seeds are dispersed by rodents, livestock and commercial hay.
  • Allelopathic (able to stop the germination or growth of other plants), and can hybridize, making identification difficult.

CAUTION: Can cause skin irritations for some. Wear gloves, long sleeves, and pants when handling.

Spotted knapweed should be reported. The Minnesota Department of Natural Resources provides detailed recommendations for reporting invasive species.

close up of purple spotted knapweed flower

How to identify spotted knapweed

  • Short-lived (two to five years) perennial herbaceous plant, two to four feet high.
  • Can persist as a rosette for one to four years.

Stem

  • Basal leaves form a rosette that grows one to eight inches wiry, hoary, branched stems during final year.
  • Often has many branches leading off a main stem.

Leaves

  • Rosette leaves are bluish green, covered in rough hairs, deeply lobed and can be up to six inches long.
  • Leaves of bolted plant are alternate, grayish, hoary; divided into lance-shaped lobes.
  • Lower stem leaves resemble rosette and decrease in size and frequency as they ascend the plant.

Flowers

  • Small (0.3–0.6 inch diameter), oval, pink to purple flowers radiate from egg-shaped flower buds at the tips of terminal and axillary stems.
  • Flower bud bracts have dark, upside-down “V” markings, giving them a spotted appearance.
  • Blooms June to November.

Seeds

  • Small, brownish, oblong seeds have small tuft of bristles; appear from June to February.
  • One plant can produce 1,000 seeds that can survive in the soil for seven years.

Roots

  • Stout perennial taproot.
  • Lateral shoots form new rosettes near the parent plant.
Spotted knapweed plant without blossoms growing in the grass
Spotted knapweed leaves

Angela Gupta, Extension educator; Amy Rager, Extension educator; Megan M. Weber, Extension educator

Reviewed in 2019

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