Photographer: Rob Routledge Affiliation: Sault College Source: www.bugwood.org Copyright: CC BY 3.0
Knapweed (Centaurea stoebe) is a perennial star thistle found in the same family as sunflowers. Knapweed grows to a height of 2 to 4 feet with pale green leaves that are 1 to 3 inches long. The surface of the leaves is rough, have a linear shape, and slender stems. Knapweed typically blooms from July to August. Flowers are pink to light purple and can be surrounded by a stiff black bract, giving the plant a spotted appearance.
View entire knapweed species list provided by Integrated Taxonomic Information System
As an aggressive and hearty plant, knapweed is able to become established rapidly in new environments, preventing native plants and crops from growing. Knapweed is also known for releasing a toxin into the soil as it grows, which can effect growth of native plants even after the knapweed has been removed.
Knapweed produces only by seed, but is capable of producing 1000 or more seeds from a single plant. Once seeds have been placed in soil, they are able to remain viable for up to five years if environmental conditions are not favorable. This also allows for re-infestation of knapweed after all visible plants have been removed.
Knapweed was introduced to the United States in the late 1800's through contaminated seeds or ballast.
Habitat: In several states knapweed has been found in a variety of habitats consisting of road sides, agriculture fields, waterways, railroad tracks, and pipelines.
U.S. Present: IA, IN, MI, MN, MT, ND, NY, OH, PA, WI
The best management method is prevention. Knapweed is most often distributed by humans through bales of hay or under vehicles. As a producer, it is important to be aware of what knapweed looks like to prevent further spreading of the plant by distributing contaminated hay. Purchasers should also be weary of buying hay from states that have a known knapweed infestation and carefully look over purchased hay. Chemical management can be achieved by applying Picloram (Tordon) at 0.25 to 0.5 pounds per acre. Other chemicals have been found to be effective, but Picloram (Tordon) has the longest lasting effectiveness, prevents knapweed growth for up to three years.
Sheley, Roger L., James S. Jacobs and Michael F. Carpinelli. 1998. Distribution, Biology, and Management of Diffuse Knapweed (Centaurea diffusa) and Spotted Knapweed (Centaurea maculosa). Weed Technology 12(2): 353-362.
Know Your Knapweed Species provided by North Dakota State University