Diffuse/Spotted Knapweed

Photo: Diffuse/Spotted KnapweedSpotted Knapweed

Biennial or short lived perennial. Multi stemmed plant with several stems arising from the crown. Flowers purple or rarely cream colored. The tips of the flower head bracts are black thus the name "spotted."


Both plants will form dense stands on any open ground, excluding more desirable forage species. Once established, the necessary extensive control measures are often more expensive than the income potential of the land.

Chemical Control

Residual chemicals are preffered due to higher percentages of control demonstrated in test plots by OSU Extension Service. Other selective herbicides are available. Contact your local weed control supervisor, or chemical consultant for additional information.

Biological Control

Agents are becoming well established, check with local weed control personnel on availability.

Diffuse Knapweed

Biennial, single stemmed plant with numerous lateral branches. Flowers white to rose, sometimes purpleish. Flowerheads slender with pointed fringed bracts.

This tumbling hitchhiker has made his presence know across the county in the last 20 years. Earliest record of diffuse knapweed in western North America is from an alfalfa field in Bingen, Washington, in 1907. The 1930s appear to be the decade of rapid movement of diffuse knapweed along roadsides and railroads in the region. The spread of diffuse knapweed has been like a wildfire: sending out fingers along roads, hot spots flaring up at disturbed sites and eventually the jointing of smaller sites into large scale infestations. Diffuse knapweed is generally a biennial. Seed germinate in the fall or early spring forming low-lying rosettes. Some plants flower and produce seed the first year, with the majority producing seed in the second year of growth. Seed production in northeastern Washington averaged 11,200 to 48,100 seeds per square meter according to work completed by Roland Schirman, WSU Extension agent in Dayton, Washington.

Image: knapweed drawingManual and Mechanical

Pulling and digging is feasible for controlling scattered diffuse knapweed plants. Rosettes should be cut 2-4 inches below the crown to reduce the amount of re-sprouting. Hand pulling is recommended three times per year, early spring, late June and just before seed dispersal. Mowing is not an effective control. A single cultivation may stimulate weed populations


Mixed results are reported following burning. Diffuse knapweed resprouts following even intense fire. Fire can be used to remove plant debris and improve herbicide efficacy.


Several herbicides are registered for control of diffuse knapweed on range and pasture land. An herbicide with residual activity is usually desirable in range sites because of the long germination period for diffuse knapweed.


Twelve different insects are established biological control agents on diffuse knapweed. Urophora flies are widespread but have failed to show any significant impacts on populations. Larinus minutus, a seed head weevil, is showing excellent control and is widely distributed in Oregon. Contact the County Weed Control program for collection and release options.


Grazing can be utilized if timing is closely followed. It is most likely to be grazed by sheep when it is green and succulent (rosette through bud stage), especially when the associated desirable plants are dry, or when it is the only plant available.

Integrated Management

A program designed to use all or several of the above tools should be designed specific to local needs and resources. The establishment of a competitive grass cover is imperative for long-term control. In trials conducted by Roche in 1988, the combination of herbicides and fertilizer maximized weed control and forage production. The plots also showed the total reestablishment of diffuse knapweed eight years after treatment, in the absence of grazing or clipping. Roche notes the best case is to establish competitive forage species, and with help from effective biological control agents and prescribed grazing practices one could maintain diffuse knapweed at low levels.

Source: Biology and Management of Noxious Rangeland Weeds., Oregon State University Press. 1999

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