Invasive Species: Diffuse Knapweed

Centaurea diffusa

Diffuse knapweed is most common in the north part of central and eastern Oregon. It does have some limited presence in Yamhill County.

Why is it important to control?

Diffuse knapweed is a biennial that out-competes grasses and other pasture species and is difficult to control.

How can you identify it?

Image of Diffuse KnapweedFlower
Habitat: Sunny locations including pastures, forest openings, and roadsides.
Life Cycle: Biennial
Height: Up to 3 ft
Leaf Description: Rosette leaves are deeply lobed. Mature leaves are pale green and hairy.
Fruit & Flower Description: White to purplish flowers have spiny, yellow brown bracts. Flowers are solitary.
Bloom Time: June to September

How do you control it?

  • Small patches can be dug up or hand pulled.

  • Dispose of plant materials in a tied plastic bag. Put the bag in the trash.

  • Herbicides effectively control meadow knapweed when used according to the label directions.

  • The Oregon Department of Agriculture has released three biocontrol agents for knapweeds.

How to report it?

There are various ways you can report this noxious weed.

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