Spotted Knapweed

spotted knapweed

Centaurea stoebe • 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: stoebe  (sto-bee)
 
Description:

Spotted knapweed grows up to 5 feet tall and has a thick tap root.

Plants form basal rosettes during winter and early spring and develop highly branched flowering stems in late spring and summer.

Its alternate, pale grayish-green leaves are egg-shaped to oblong, and are once or twice divided and covered with hairs.

Numerous pink to purple flowers (sometimes white) dot the tops of this bushy plant from May to September.

It is easily identified by the egg-shaped bracts at the base of each flower. Bracts look spotted due to their dark colored, triangle shaped tips, with a comb-like fringe.

Its brown/black oval seeds have short bristles (pappus) on one end.

 Why Is it a Noxious Weed?

This is a very aggressive species. 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.


Where Does it Grow?

It is found along roads, railroads, gravel pits, vacant lots, pastures, dry meadows and fields, and forest clearings.  


Facts:

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


Control Options:
  • 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. 

  • A number of biocontrol agents have been released 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 an herbicide containing the active ingredient glyphosate 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 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.) 
  • 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.

 

More Information:

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

 


More Pictures:  

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