Centaurea diffusa • Class B

Diffuse knapweed is a bushy, taprooted bienniel or short-lived perennial. 

It grows from 8 to 40 inches tall, with branching stems covered in short, dense hairs and a very long tap root. 

Its basal leaves (leaves at the stem base) have short stalks and grow 7 to 8 inches long. It has smaller stem leaves that are stalkless and reduce in size growing 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 spine/thorn at each tip. 

It decreases plant diversity and wildlife habitat due to its competive nature, it also increases soil erosion and creates wildfire hazards. 

Diffuse knapweed releases a chemical which suppresses the growth of other species enabling it to create a monoculture.  

Diffuse knapweed 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.

Losses due to Diffuse knapweed 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. 

• 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  is effective in controlling Diffuse Knapweed.  Be aware that Glyphosate is non-selective, and will injure any plants that it comes in contact with, including grass. Apply glyphosate anytime plants are actively growing. 

• For selective control of knapweed in agricultural settings (pastures, hayfields, etc.): an herbicide containing the active ingredient aminopyralid may be a preferred choice. Aminopyralid products will not harm grass.  Apply aminopyralid anytime plants are actively growing.  Applications of aminopyralid are also effective in the fall before a killing frost. 

• To minimize any harmful impact on bees and other pollinators, timing is important.  Ideally, treat plants before blooming.  If treatment after blooming is necessary, do control work early in the morning, or in the evening when bees are less active. 

For more information on this noxious weed Download our Flyer or visit Washington State Noxious Weed Control Board Here. Photo by Leo Michels