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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.
a few brown knapweed plants laid on a blue background

How to identify brown knapweed

  • Perennial plant 1–4 feet tall with multiple upright stems that are reddish, ridged and may have purple stripes.
redish colored stem of brown knapweed plant
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

two brown knapweed buds
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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