WSU Extension

Hortsense

Weeds : Knapweeds : Centaurea spp.
(revision date: 4/7/2021)

Family: Asteraceae (Compositae)
Cycle: Biennial/perennial
Plant Type: Broadleaf
Use Integrated Pest Management (IPM) for successful weed management.

Biology
Knapweeds are short-lived perennials with stout taproots. They can have one or more stems, reaching one to three feet in height. Depending on species, leaves may be narrow and lance-shaped, lobed, or somewhat toothed to deeply divided. Spotted knapweed (Centaurea stoebe, C. biebersteinii) flower heads are solitary at the ends of branches, with stiff bracts tipped with a dark comb-like fringe. Ray flowers are pinkish-purple or rarely cream-colored. Fruits are about an 1/8 inch long, tipped with a tuft of persistent bristles. Diffuse knapweed (C. diffusa) flower heads are numerous and narrow. Ray flowers are white to rose or purplish. Bracts under the flower have yellow spines with teeth appearing as a comb along the spine margins. The leaves are pinnately divided. Meadow knapweed (C. jacea x nigra, C. debeauxii) flowers are borne in large pink to purplish-red heads at the ends of branches, with deeply lobed bracts below the flower. The lower leaves have long stalks and may be toothed. Upper leaves are smaller, lack stalks, and are smooth-margined. SPECIAL INFORMATION: In WASHINGTON and OREGON, several knapweed species are designated as Class 'A' or 'B' noxious weeds. Management or eradication may be REQUIRED by law in your county. In addition, several species are on the Washington and Oregon noxious weed quarantine lists, which prohibit sale, purchase, and transport of plants, seeds, and plant parts. Consult your local Noxious Weed Control Board for more information.
Habitat
Several species of knapweeds establish themselves on roadsides, waste areas, fields, and pastures both east and west of the Cascades. Their early spring growth makes them competitive for soil moisture and nutrients. Some evidence suggests that knapweeds release chemicals which inhibit surrounding vegetation.

Management Options

Non-Chemical Management
  • Cultivation (rototilling or hoeing) will effectively eliminate plants.
Select non-chemical management options as your first choice!

Chemical Management
IMPORTANT: Visit Home and Garden Fact Sheets for more information on using pesticides

Apply according to label instructions. Use glyphosate products as spot treatments only! NOTE: Some ingredients listed here are only available in combination. Read the label carefully on combination products to make sure the product is suitable for your specific situation.

Landscape areas
  • glyphosate
  • products containing 2,4-D
Turf areas
  • products containing 2,4-D
Bare ground areas
  • glyphosate
  • products containing 2,4-D
Images
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Caption: Spotted knapweed flowerheads
Photo by: B.F. Roche
Caption: Spotted knapweed flower
Photo by: T. W. Miller
Caption: Diffuse knapweed flowers
Photo by: D.G. Swan
Caption: Meadow knapweed flower
Photo by: Washington Noxious Weed Board slide collection
Caption: Diffuse knapweed
Photo by: D.G. Swan
Caption: Spotted knapweed
Photo by: D.G. Swan