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Diffuse Knapweed

(Centaurea diffusa)

Diffuse Knapweed

Diffuse knapweed is native to the eastern Mediterranean region and to western Asia. In the beginning of the 1900's, diffuse knapweed found it's way to North America in alfalfa seed shipments. This plant is considered a noxious weed and is included on the Colorado and the Adams County Noxious Weed Lists.

Diffuse Knapweed FlowerDiffuse knapweed is a biennial, which means its first year of growth is a basal rosette of leaves from a deep taproot. Rosettes can appear in either spring or fall. During the second year, the stem bolts 1.5 to 3 feet tall. Stem leaves become smaller higher up the stem. Urn-shaped flower heads are mostly white, sometimes purple, and are located on each branch tip. The bracts surrounding each flower bear 4 to 5 pairs of lateral spines which are painful to the touch.

Diffuse knapweed has been credited with reducing biodiversity, increasing soil erosion, reducing land value, increasing cost of roadside maintenance, threatening Natural Area Preserves, and replacing wildlife and livestock forage on rangeland and pasture. During the winter, these plants will break off at the base of the stem and form tumbleweeds. These tumbleweeds are blown around by the wind therefore spreading seed to uninfested areas.

At least 3 million acres in western United States are infested with diffuse knapweed. Of that 3 million, at least 100,000 acres are infested in Colorado. In Adams County, diffuse knapweed has infested numerous areas along the South Platte River and near the towns of Bennett and Strasburg. If you think you might have this weed on your property, or would like more information regarding noxious weed identification and management, please call the Adams County Weed Department at 303-637-8115.



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