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  • Noxious Weed Identification List

    Diffuse Knapweed


    Centaurea diffusa

    Info Sheet  Adobe PDF Icon

     

    Colorado List B

    Eradication required in parts of Jefferson County

    Control required in other areas of Jefferson County

    Map of Diffuse Knapweed Eradication Zone Adobe PDF Icon

    General

    • Family: Sunflower (Asteraceae)
    • Herbaceous
    • Biennial (short-lived perennial)
    • Rosettes formed the first year and bolt early in May to June of the next year
    • Dead plants break off near ground and tumble

     

    Habitat

    • Found in disturbed areas, such as overgrazed pastures and rangelands, roadsides, right-of-ways and gravel pits
    • Prefers dry, light, porous soils
    • Intolerant of shade and flooding

     

    PLANT

    Vegetation

    • Grayish green
    • Contains chemicals that may inhibit other plants and irritate skin
    • Mature plants are 1 to 2 feet tall with numerous single flower heads at the tips of shoots

     

    Roots

    • Tap root

     

    Flower

    • White, sometimes light purple
    • Second season: June to September
    • Size: small - 0.13 inch wide and 0.5 to 0.67 inch long
    • Bracts are comb-like and tipped but can be distinguished from Spotted knapweed by the long, terminal spine

     

    Seed

    • 15,000-plus seeds per plant
    • Seed may remain dormant in soil for more than 6 years
    • Seed spread by plants as they break off at the base and tumble

     

    Seedling

    • Seedlings spend their first growing season as rosettes

     

    Reproduction

    • Seed

     

    CONTROL

    Biological

    • Agapeta zoegana, root-boring moth
    • Cyphocleonus achates, root weevil
    • Larinus minutus, seedhead weevil
    • Sphenoptera jugoslavica, root borer

     

    Chemical

    Please refer to our Info Sheet  Adobe PDF Icon

    Cultural

    • Prevention – maintain health of site
    • Hand pulling works when populations are small and isolated
    • Revegetation of highly disturbed sites
    • Grass competition – promote grass growth with fertilization, irrigation and reseeding
    • Cultivation (when plants are 3 to 6 inches tall)

     

    Mechanical

    • Burning - Not effective alone.  Conditions don’t support an effective fire.  May promote flush of weeds, which can then be treated with herbicide 
    • Grazing - Sheep, goats and cattle will feed on the plant 
    • Mowing - Not effective alone.  Plants will regrow from crown and produce as many seeds as plants that don’t get mowed 

     

    More Information

     

    Use all chemicals according to the manufacturer's label. Listing the above methods or products does not imply a specific recommendation or endorsement.



    Alicia Doran
    Weed & Pest Management Specialist
    303-271-5989
    adoran@jeffco.us

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    weed-pest-b-diffuse-knapweed-habit

    weed-pest-b-diffuse-knapweed-rosette


    Last Updated: 8-1-2013