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Brown knapweed

Quick facts

Brown knapweed is an invasive species on the Prohibited Eradicate List. Plant must be destroyed and transportation, propagation, or sale of these plants is prohibited.

  • Brown knapweed grows in disturbed sunny areas such as road ditches and woodland clearings.

  • It outcompetes native vegetation by forming large monocultures.

  • It can hybridize with other knapweeds, allowing for aggressive seed dispersal.

  • This plant is toxic to horses.

  • The Minnesota Department of Agriculture monitors this invasive species. Please report any black swallow-wort you spot at Arrest the Pest.

How to identify brown knapweed

  • Brown knapweed (Centaurea jacea) is a perennial plant 1–4 feet tall with multiple upright stems that are reddish, ridged and may have purple stripes.
Stems have ribbed rough texture

Stem

  • Erect to ascending, can be both single or multiple from the base.
  • Branched more in the upper part of the plant.
  • Ribbed rough texture with vertical ridges on the stem and a cobweb-like appearance.

Leaves

  • Alternate, green, lance-shaped leaves with waxy margins and a hairy texture.
  • Leaves at the base of the plant may be stalked, lobed or toothed; they are 10" long and 1" wide.
  • Leaves become smaller moving up the plant and become lance shaped.

Flowers

Flower clusters form one composite flower head
  • Multiple small flowers clusters are 1 to 1-1/2 inch across.
  • Together flower clusters form one composite flower head.
  • Flowers are pink/purple in color and occasionally have a white center structure.
  • Bracts are more rounded and wide at the tip, with fine fringes and a dark brown base.
  • Flower heads are wide at the top.
  • Blooms July to August.

Seeds

  • White to light brown in color with short plumes.

Roots

  • Deep, vigorous taproot.

 

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

Reviewed in 2019

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