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Diffuse Knapweed
(tumble knapweed, bushy knapweed) Asteraceae
(=Compositae), the aster family

Background

Diffuse knapweed is a native of Eurasia, coming to 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.

Description

Diffuse knapweed is an annual, biennial, or short-lived perennial that can grow to a height of 3 feet, with a single, much-branched stem that gives the plant a bushy appearance. Basal leaves are pinnately 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. Bracts surrounding the flower are yellowish green with a light brown, comblike margin. The upper part of each bract narrows into a short, stiff spine. Seeds are brown to gray in color and are tipped by plumes that fall off at maturity.

Distribution

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

Control

Biological control agents include 2 seed head gall flies and a root boring beetle, which have been effective. Herbicides are available for control.

Please contact: Ag Publishing, University of Idaho, Moscow, Idaho 83844-2240; 208 882-7982 visit the Resources for Idaho website at http://info.ag.uidaho.edu, for more information about this or other publications.