Spotted Knapweed (Centaurea stoebe)
Spotted knapweed originated in Europe and Asia. It was first brought to North America in the 1890s as a contaminant in agricultural seed and in soil discarded from ship ballast. Since then, this species has seriously impacted pastures and most rangelands of the United States.
This noxious weed rapidly colonizes disturbed areas. Spotted knapweed produces a substance that is toxic to other plants causing it to spread quickly and establish. The highly invasive nature of this plant decreases the biodiversity of the environment.
Spotted knapweed can grow up to 2-4 feet tall and has a long tap root that anchors it in the ground. The plants’ stem has various branching. It has grayish-green leaves that are somewhat like hair, deeply lobed into narrow segments and a pink/purple oval flower. The flower has a bract that is stiff and is marked with upside down “V”s, producing a spotted like appearance. These flowers bloom from June to the end of August.
Controlling efforts depend on the size of infestation.
Continuous hand pulling can be effective when controlling small patches of spotted knapweed. It’s crucial to remove as much of the tap root as you can to prevent regrowth. To prevent skin irritation, wear long sleeves and gloves.
Mowing before the plants flower can prevent them from producing seeds. Although, they flower throughout most of the spring/summer so it’s necessary to mow multiple times to reduce the possibility of resprouting and seed production.
Herbicide use has been successful for larger patches. Applications should be applied during early spring and late fall in order to target rosettes. Typical herbicides used include clopyralid + 2,4-D (Curtail) and clopyralid + triclopyr. All herbicides should be applied according to label instructions.
Multiple insects and fungi have been introduced to control spotted knapweed. More information about spotted knapweed biocontrol can be found in the U.S. Forest Service’s Biology and Biological Control of Knapweed Guide.
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Centaurea stoebe (spotted knapweed). Minnesota Wildflowers. (n.d.). https://www.minnesotawildflowers.info/flower/spotted-knapweed
Spotted knapweed. Minnesota Department of Agriculture. (n.d.). https://www.mda.state.mn.us/plants/pestmanagement/weedcontrol/noxiouslist/spottedknapweed
Spotted knapweed. WNY PRISM. (2023, March 3). https://www.wnyprism.org/invasive_species/spotted-knapweed/#:~:text=Spotted%20knapweed%20is%20an%20herbaceous,of%20its%20many%20branching%20stems.
Spotted knapweed control. King Washington State Noxious Weed Control Board. (n.d.). https://www.nwcb.wa.gov/images/weeds/Spotted-Knapweed-control_King.pdf