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Brown knapweed

(Centaurea jacea)

Photo of Brown knapweed
Photo credit: Cindy Roche, University of Georgia

Perennial knapweed with upright stems, that branch near the top. Flowers are lavender-colored with fringed bracts that have brown tips.

Overview

Other names for this plant include:
  • Common names: Brownray knapweed
Ecological threat:
  • Invades open grasslands, prairies, open forests, orchards, cultivated fields, field edges and travel corridors.
  • Aggressively outcompetes desired vegetation, reducing overall biodiversity and forage quality.
  • Plants are easily spread by human activity via seed and the plants' characteristics facilitate increased seed dispersal across long distances.
  • Plants produce up to 800 seeds per plant. Upon maturation, seeds eject and can be dispersed as far as 20 feet from the parent plant.
  • Increases soil erosion and consumes soil nutrients.
Overview map of brown 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 brown knapweed was based upon this literature review [PDF] developed by the department.

Identification

Leaves & stems: A single upright stem branches near the top. Leaves are lanceolate and undivided. Leaf size progressively decreases from bottom to top. Stems are ridged, sometimes with purple-colored striations.

Flowers: Flowers are rose to lavender to purple in color. Bracts (long fringes beneath the flowerhead) tips are brown in color and wider than at the base. They have papery thin margins with a dark brown center.

Fruits & seeds: Seeds are tan in color with fine hairs. There is no pappus. Plants produce on average a few thousand seeds per plant.

Roots: Plants have woody taproots and a woody root crown.

Similar species: Brown knapweed closely resembles other knapweeds, the majority of which are also invasive in Wisconsin.

Control

Mechanical:
  • Handpull the plants, being sure to remove the entire taproot.
  • Mow or cut plants before flowering to prevent seed-set. Note this will not kill the plants.
Chemical:
  • Herbicide treatments are most effective when combined with other control techniques and followed by re-seeding. The timing of herbicide treatments is essential to effectiveness.
  • A 3% solution of triclopyr herbicide mixed with water can be applied to the leaves in early spring or fall.
  • Glyphosate, picloram, 2,4-D and clopyralid have all been used on other knapweeds
Biocontrol:
  • Gall flies (Urophora affinis and Urophora quadrifasciata) are used to feed on the developing seed heads.

Photos

View brown knapweed pictures in our photo gallery!

Resources

Sources for content:
  • Washington State Noxious Weed Control Board
  • USDA Forest Service
Links for more information: