Centaure maculosa • Class B

Spotted knapweed grows from 2 to 4 feet tall and has a thick tap root. The plant is hairy and rough and each plant has from 1 to 15 stems. Its alternate, pale bluish green leaves are egg-shaped to oblong, and are once or twice divided. Numerous pink to purple flowers (infrequently they are white) dot the tops of this bushy plant. It is easily identified by involucral bracts at the base of the flower; these leaf-like structures are arranged almost like shingles on a roof. The bracts are black-tipped which gives them a spotted look, thus the name spotted knapweed. 

Spotted Knapweed contains Sesquiterpene lactones (SQL) which are a class of chemicals found in many plants, and can cause allergic reactions and toxicity if consumed in large quantities, particularly in grazing livestock. These lactones also inhibit germination and root growth of nearby native grasses, trees, and weeds, under laboratory conditions. 

Spotted knapweed is found along roads & railroads, gravel pits, vacant lots, pastures, and forest clearings. 

Spotted Knapweed is a very aggressive species and is one of the most dominant weed species in the western United States. It can infest large areas very quickly. The species has little value as forage for cattle and increases production costs for ranchers. It impairs the quality of wildlife habitat, decreases plant diversity, increases soil erosion rates on valuable watershed areas, and poses a wildfire hazard. 

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

• The large, stout, taproots of the Spotted knapweed plant make it very difficult to pull manually. Small, isolated infestations may be dug out   of damp or sandy soil. Care must be taken to collect and bag up all flowering parts of the plant to prevent seed spread. Mowing is not very effective because knapweeds are persistent and the plant will produce flowers below the mowed height. 

• Ten biocontrol agents have been released on diffuse knapweed in Washington State. 

• Spot spraying with an herbicide containing the active ingredient glyphosate (Roundup Pro, Glyfos, etc.) may be used effectively while the plant is actively growing, repeat as needed. For most effective treatments, apply before plants bloom and produce seed. Be aware, glyphosate is non-selective and will injure any plants that it comes in contact with, including grass. 

• 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 applied anytime the plant is actively growing. Applications of aminopyralid are also effective in the fall before a killing frost. Aminopyralid products will not harm grass and can be used around livestock (provided all label precautions are followed).  

• 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.) 

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