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Class B Noxious Weed

Diffuse Knapweed
Centaurea diffusa
 

Diffuse knapweed closeup

Other common names: White knapweed
 Plant family: Compositae (Asteraceae)
Year listed: 1995
Native to: Europe and northcentral Ukraine.

Why is it a noxious weed?

Diffuse knapweed is a very aggressive species. The species has little value as forage for cattle and limited seasonal value for big game. Knapweed infestations increase production costs for ranchers, decrease plant diversity and wildlife habitat, increase soil erosion rates, and poses wildfire hazards.

How would I identify it?
General Description: Diffuse knapweed grows from 8 to 40 inch tall, it is a biennial or short-lived perennial, with a long tap root.  The lower and middle bracts are yellowish-green with a buff or brown margin; they are edged with a fringe of spines plus a longer, spreading spine at the tip.

Leaves:

  • basal leaves are short stalked, divided into lobes on both sides of the center vein
  • stem leaves are stalkless
  • smaller and less divided at top

Flowers:

  • usually white
  • can be pink or lavender
  • urn shaped heads
  • occur in clusters
  • bracts are leathery, have obvious veins

Stems:

  • single
  • upright
  • several spreading branches protrude from stem

How does it reproduce?

While plants may regenerate from the crown, diffuse knapweed reproduces primarily by seed:

  • A single flower stalk can produce 1200 seeds.

  •  When the plant is broken, the base behaves as a tumbleweed and seeds are dispersed.

Where does it grow?
Diffuse knapweed grows in a variety of habitats in Washington including river shores, rangeland and pastures. Diffuse knapweed thrives in gravel pits, roadsides, railroad tracks, vacant lots, airports, trails and heavily grazed pasture.

What is the distribution in Washington State?

Click on map to enlarge.

 

  • Distribution map is based on 2003 data

How do I control it?

General control methods:

Cultural: Cultivation will eliminate infestations.

Herbicide: Please refer to the PNW Weed Management Handbook, or contact your county noxious weed coordinator.

Mechanical: Mowing may delay flower production, therefore potentially increasing seed production.

Biological: Five biocontrol agents have been established on diffuse knapweed in Washington:

  • Two seed head weevils, Bangasternus fausti and Larinus minutus, do not occur in collectable numbers at present.
  • Urophora affinis (seed head fly), Urophora quadrifasciata (seed head fly) , and Sphenoptera jugoslavica (root boring/gall beetle) are available for mass collections.

Are there plants that may be confused with Diffuse knapweed?

Many species of Centaurea look very similar to each other, making identification difficult. If you need help with plant identification, please contact  your county noxious weed coordinator.

 

For more information
For more information, download our Written Findings.

 

More Photos-Click photo to enlarge
Feel free to download, or contact us to receive high-resolution JPEG or TIFF formats
Diffuse knapweed infestation diffuse knapweed flowers diffuse knapweed white
Diffuse knapweed infestation. Another view of Diffuse knapweed flowers. Diffuse knapweed flowers.

 Last updated September 10, 2008