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Black knapweed

Black knapweed
(Centaurea nigra)

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



Overview

Regulated areas of black knapweed
This species is listed as Restricted (Orange counties)

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.
  • Increase soil erosion and consume soil nutrients.

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 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 at 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 bristle 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.

Distribution

Centaurea_nigra
Counties in WI where black knapweed has been reported (as of April 2013). Both vouchered and unvouchered reports included.

Do you have black knapweed in your county but it isn't shaded on the map? Send us a report.

Control

Mechanical:

  • Handpull the plants, being sure to remove the entire tap root.
  • 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. Timing of herbicide treatments is essential to the 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!

Last revised: Monday December 11 2017