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