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Black knapweed

(Centaurea nigra)

Photo of Black knapweed
Photo credit: Joseph M. DiTomaso, University of California-Davis, Bugwood.org

Perennial knapweed, with purple flowers and black bracts, growing up to 5 feet tall.

Overview

Other names for this plant include:
  • Common names: Lesser knapweed, hardheads, Spanish buttons, common knapweed
  • Scientific names: Centaurea nigra var. radiata
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.
  • It increases soil erosion and consumes soil nutrients.
Overview map of Black 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 black knapweed was based upon this literature review [PDF] developed by the department.

Identification

Leaves & stems: Stems are erect and covered in fine hairs, branching near the middle. Basal leaves are borne on leaf stalks and are narrow. The widest section of the leaf is in the middle. Leaves progressively get smaller up the stalk. Leaf margins (edges) are mostly smooth to lightly serrated.

Flowers: Flowering occurs at branch tips. Flower heads are composed of 40 to over 100 individual flowers, purple to white in color with black bracts.

Fruits & seeds: Seeds may have short bristles on one end. Reproduces prolifically by seed.

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

Similar species: Black 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 but suppress them.
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.
  • 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 black knapweed pictures in our photo gallery!