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

Diffuse knapweed (Centaurea diffusa) is a native of Eurasia, introduced into the U.S. in the early 1900s. It spreads by seed, aided by the tumbling of windblown mature plants, and it grows under a wide range of conditions.

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DESCRIPTION

Diffuse knapweed is an annualPlant that germinates, flowers, seeds, and dies during one growing season, 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 that can grow to a height of 3 feet, with a single, much-branched stem that gives the plant a bushy appearance. BasalAt the base of plant or plant part leaves are pinnatelyWith 2 rows of leaflets, like a feather divided and up to 6 inches long; stem leaves are entire and smaller. Tips of each branch have a 1/3-inch wide white or sometimes pinkish flower head that appears from midsummer to fall. BractsLeaflike structure at the base of flowers or leaves surrounding the flower are yellowish-green with a light brown comblike marginThe edge of the leaf. The upper part of each bract narrows into a short, stiff spine. Seeds are brown to gray in color and are tipped by plumesA hairlike or featherlike structure, often on a seed that fall off at maturity..

DISTRIBUTION

Diffuse knapweed is widespread in the Northwest and many other states.

CONTROL

Several biological control agents, including a root boring beetle and moth, 2 seed head gall flies, and a seed head weevil are available but are not proven effective. Herbicides are available for control.