Idaho OnePlan recommends the website of the Idaho Weed Awareness Campaign as the best resource for up-to-date information about Idaho's noxious weeds, and their control.
Source for this page: Idaho's Noxious Weeds by Robert H. Callihan & Timothy W. Miller

BACKGROUND

Spotted knapweed (Centaurea maculosa), native to Europe, has thoroughly established itself in North America. In Idaho the weed is widely dispersed. Each plant produces up to 25,000 seeds that are dispersed by wind, animals, and people. Seeds may remain viable for 8 years.

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DESCRIPTION

Spotted knapweed is a biennialPlant that germinates in one growing season, then flowers, seeds, and dies during a second or short-lived perennialPlant that lives for more than 2 growing seasons. Mature plants may be 3 feet in height and are much branched. The weed forms a basalAt the base of plant or plant part rosetteA circular, normally basal, clump of leaves the first year and stem leaves are pinnatelyWith 2 rows of leaflets, like a feather divided. Flower heads are abundant, inch wide, and generally solitary on branch tips. Flowers are pink to purple, or occasionally white, and appear from midsummer to fall. Each stiff flower head bractLeaflike structure at the base of flowers or leaves has a dark comblike fringe resembling a black spot at the tip. Seeds are dark brown to tan and are tipped by plumesA hairlike or featherlike structure, often on a seed that fall off at maturity.

DISTRIBUTION

Spotted knapweed is found in the northeast and north central U.S. as well as along the Pacific Coast, most of Idaho and east through Montana.

CONTROL

Several biological control agents are available. A seed head moth and 2 seed head gall flies have been effective. Effective herbicides are also available for control of spotted knapweed.