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Diffuse Knapweed

Diffuse Knapweed Management

Cultural Management

Diffuse KnapweedSeeding and maintaining aggressive grasses will help in competing with diffuse knapweed and slow its spread. However seeding alone in infected areas will not provide adequate control. Proper fertilization, grazing, and supplemental irrigation of grasses are always effective cultural control methods.

Supplemental control methods, such as judicious use of herbicides, may be needed to give grasses a chance to compete. Additional herbicide treatments after grass establishment may be needed to keep diffuse knapweed populations reduced to an acceptable level.

It produces early spring growth that is hard to compete against.

More complete information on grasses can be found on the Grass Seeding on the Eastern Front-Range of Colorado page or by contacting the Natural Resources Conservation Service.


Mechanical Management

Mowing (or other mechanical efforts) diffuse knapweed when it is in the bud stage, and again when it flowers, will significantly reduce seed production. Preventing seed set for a number of years will eventually eliminate an infestation, once the seed source in the soil is depleted. This assumes no seed enters the area from an outside source. This weed has been known to flower at a plant height below mower level.

Diffuse knapweed can be easy to pull by hand. Get at least 2/3 of the root. Start in May as plants bolt. Leave plants on the ground if they haven't flowered; otherwise bag them and dispose. Wear gloves and long-sleeved shirt. Handpulling is effective on small patches, but is not recommended for large infestations.

Biological Management

Two species of seedhead flies (Urophora affinis and U. quadrifasciata) reduce the seed production of this weed. When combined with a root-boring beetle (Sphnoptera jugoslavica), diffuse knapweed control is increased. Sheep will graze young plants from spring to early summer. Cattle will eat diffuse knapweed young growth. Goats will also eat it.

Herbicide Management

Few Herbicides are labeled for use on diffuse knapweed. Those that have been effective when used independently or in combination with each other are: picloram (Tordon), dicamba (Banvel), clopyralid (Stinger, Transline), clopyralid + 2,4-D (Curtail) and 2,4-D. The latter used alone works poorly. Refer to the table below for more information.

Labeled site*
Rate (per acre)
Application time

Tordon 22K



Apply in spring prior to stem elongation and/or to rosettes in the fall.

Banvel +

2,4-D Amine


1pt. + 1pt.

Apply in the spring prior to stem elongation and/or to rosettes in the fall.




Apply in the spring after the majority of Basal leaves emerged to pre-bud stage



2/3 to 1.0 pt.

In spring, up to bud stage.

2,4-D Amine






Contact weed department, online information will be posted soon.

* R & P = Range and Pasture; NC = noncrop; Crop = cropland; F = fallow; All = all of these sites.

Read the label to insure the herbicide is labeled for your application site.

Additional Information

Diffuse Knapweed Identification

Information Sheet - Diffuse Knapweed

CSU Fact Sheet # 3.110 - Diffuse & Spotted Knapweed

Adams County Extension web pages do not endorse any commercial providers or their products, other herbicide applications may be possible. Consult with your local pesticide dealer,county weed supervisor, or extension agent for specific recommendations. Always read and follow label instructions when making any application.


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