Centaurea diffusa - Asteraceae Family - Biennial or short lived perennial
- Flowers: Flowers are predominantly white, occasionally pink-purple. Bracts end in sharp, rigid spines with the terminal spine being distinctly longer. Sometimes can have black spots, but will always have a terminal spine.
- Seeds: Preventing seed production is imperative to control of all knapweeds! Typically, seeds can remain viable up to eight years. Seeds below depths of one and a half inches will not germinate until the soil is disturbed.
- Leaves: Leaves are blue-green, deeply lobed, and sparsely arranged along the stem.
- Flowering Time: June to October.
- Life cycle/other: Can germinate in the spring and fall. Biennial or short-lived perennial. Occasionally an annual.
- Knapweeds are highly competitive plants that can exclude more desirable plants and form large, dense infestations.
- During the winter, knapweed plants will break off at the base of the stem and form tumbleweeds. These tumbleweeds are blown around by the wind therefore spreading seed to un-infested areas.
- Knapweed invasions cause losses averaging up to 63 percent of available grazing forage.
Most effective control methods
- Knapweeds are readily controlled with herbicides. However, the weeds will reinvade unless cultural techniques are used!
- For small sites with limited distribution, pull or dig up plants and remove as much root as possible so the plant will not re-sprout. Roots tend to break off four to six inches beneath the ground.
- Carefully monitor sites throughout the growing season to remove missed plants. Expect the level of control work to be intensive for the first several years due to seed banks and the soil disturbance that occurs when pulling or digging.
- Larger infestations can be treated with an appropriate herbicide for the site. Monitor the site throughout the growing season to catch any missed plants.
- Irrigation (where possible) may help stimulate grass competition. Diffuse knapweed will not tolerate flooding and shade.
- Livestock (sheep, goats, cattle) will eat diffuse knapweed. Recent research completed by Colorado State University shows that cattle grazing diffuse knapweed twice in spring decreased seed set by 50 percent.
- Mowing alone is not recommended for control. Since the plant has the ability to flower below the mower height, mowing alone will not prevent seed production.
Control Methods and timing
Prevention (Prev) Monitor and destroy new plants before seed production.
Mechanical (Mech) Hand pulling, digging, cutting, mowing and tilling.
Cultural (Cult) Biological control agents, livestock grazing, and revegetation practices.
Chemical (Chem) Selective herbicides based on the plant and the specific location. Check our weed fact sheets for specific control information.
Diffuse knapweed: flowers
Diffuse knapweed: foliage
Diffuse knapweed: seedling
Diffuse knapweed: plant
Diffuse knapweed: infestation
Diffuse knapweed: plant
Diffuse knapweed: bolting growth stage
Diffuse knapweed: flowers range from white to pink in color
Beck, K.G. (2013, November). Diffuse and spotted knapweed – 3.110. Retrieved from http://extension.colostate.edu/topic-areas/natural-resources/diffuse-and-spotted-knapweed-3-110/
DiTomaso, J.M., G.B. Kyser et al. (2013). Weed Control in Natural Areas in the Western United States [PDF file]. Weed Research and Information Center, University of California. Retrieved from https://wric.ucdavis.edu/information/natural-areas/wr_C/Centaurea_stoebe.pdf View PDF
Invasive Species Compendium. (2018, July 15). Centaurea diffusa (diffuse knapweed). Retrieved from https://www.cabi.org/isc/datasheet/12039
Lym, R. (2018, April). Know your knapweeds. Retrieved from https://www.ag.ndsu.edu/publications/crops/know-your-knapweeds
Oregon State University. (2008, September). Knapweeds: Centaurea spp., Acroptilon repens [PDF file]. Retrieved from https://catalog.extension.oregonstate.edu/sites/catalog/files/project/pdf/ec1596.pdf View PDF
Wilson, L., & Randall, C. (2005, April). Biology and biological control of knapweed [PDF file]. Retrieved from https://bugwoodcloud.org/resource/files/6223.pdf View PDF