Description:
Description and Variation: Diffuse knapweed is an 8 to 40 inch tall, biennial or short-lived perennial species, with a long tap root. The single, upright stem produces several spreading branches. The basal leaves are short-stalked and divided into lobes on both sides of the center vein. The stem leaves are stalkless, becoming smaller and less divided near the top of the stem. The flowers, which are generally white (sometimes pink or lavender), occur in urn-shaped heads that grow in clusters at the ends of the branches. The bracts of the flower heads are leathery, with obvious veins. The lower and middle bracts are yellowish-green with a buff or brown margin; they are edged with a fringe of spines plus a longer, spreading spine at the tip.

• Mechanical: Cultivation will eliminate diffuse knapweed. Grazing or mowing delays flowering and may increase the number of stems, thereby increasing seed production.
• Biological: Five biocontrol agents have been established on diffuse knapweed in Washington. Two seed head weevils, Bangasternus fausti and Larinus minutus, do not occur in collectable numbers at present. Urophora affinis (seed head fly), Urophora quadrifasciata (seed head fly), and Sphenoptera jugoslavica (root boring/gall beetle) are available for mass collections.
• Herbicide: 2,4-D and Roundup provide seasonal control; Banvel and Tordon - longer control, perhaps for two or more seasons varying with soil type and precipitation patterns. Please refer to the annually updated Pacific Northwest Weed Control Handbook for site specific control recommendations. Always follow label instructions Refer to the State Noxious Weed Control Board site

For More Information:
Detailed information about Diffuse Knapweed is available at the Washington State Noxious Weed Control Board Web Site.