Diffuse Knapweed

diffuse knapweed on pierce county, wa

Centaurea diffusa • Class B

Family Name: Asteraceae family (ass-ter-AY-see-ee)
Common: Aster, daisy, or sunflower family
Genus: Centaurea (sen-TAR-ee-uh) 
Meaning: Latin, referring to the Centaur Chiron who discovered the medicinal use of the Centaury plant
Species: diffusa  (dy-FEW-sa)
Meaning: Loosely spreading

Diffuse knapweed grows from 8 to 40 inches tall, with branching stems covered in short, dense hairs and a very long tap root. It's basal leaves grow 7 to 8 inches long, with smaller stem leaves reducing in size up the stems. Flowers are white to lavender and grow out of urn-shaped bracts at the tips of the many branches. The bracts are leathery and yellowish green with obvious veins and are edged with a fringe of spines, plus a longer spreading spine/thorn at each tip. 

 Why Is it a Noxious Plant?

It decreases plant diversity and wildlife habitat due to its competive nature, it also increases soil erosion and creates wildfire hazards.   This is a very aggressive species. 

Where Does it Grow?

Diffuse knapweed is invades along roadsides, on disturbed land and in waste places, often on gravelly, or light soil. It has shown wide adaptability, but does not tolerate flooding or shade. 


It decreases the value of hay and can decrease the value of the land. Other losses include soil erosion, wildlife population reduction, soil and water resource depletion, native species reduction, biodiversity reduction, and the disflavoring of milk. 

A single flower stalk can produce 1200 seeds. When the plant is broken, the base behaves as tumbleweed and seeds are dispersed. 

Control Options:

The most effective control of Diffuse Knapweed is prevention. Above all, prevent plants from going to seed. 

  • Small, isolated infestations can be dug out if the soil is damp or sandy. Be  careful to collect and dispose of all the pieces of roots and crown to prevent them from re-establishing, and double bag flowering parts to prevent seed spread. 

  • Anyone working with diffuse knapweed or other knapweed species should wear protective clothing and avoid getting knapweed sap into cuts or open abrasions as it may cause irritation.

  • Five biocontrol agents have been established on diffuse knapweed in Washington State.  For information about the biological control of this or any other noxious weed, see the WSU Extension Integrated Weed Control Project . .    

  • Spot spraying with glyphosate (example: Roundup Pro®, Glyfos®, etc.) is effective in controlling Diffuse Knapweed. Spot application means the herbicide is applied only to the plants and not on the surrounding plants or soil. Spray each plant thoroughly on the stems and leaves enough to be wet but not dripping. Be aware that Glyphosate is non-selective, and will injure any plants that it comes in contact with, including grass. Apply either glyphosate anytime plants are actively growing. 
  • For selective control of knapweed in agricultural settings (pastures, hayfields, etc.): an herbicide containing the active ingredient aminopyralid (example: Milestone™, Milestone VM™, etc.) may be a preferred choice. Aminopyralid products will not harm grass and can be used around livestock (provided all label precautions are followed). As with glyphosate, apply aminopyralid anytime plants are actively growing. For the most effective treatments, apply before plants bloom and produce seed. Applications of aminopyralid are also effective in the fall before a killing frost.
  • When using herbicides, read and follow all label instructions and obey all label precautions. (Note: pesticide product registration is renewed annually and product names and formulations may vary from year to year.)   

More Information:

 Download our Flyer or visit Washington State Noxious Weed Control Board Here. Photo by Leo Michels


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