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  • Noxious Weed Identification List

    Canada Thistle


    (Cirsium arvense)

    Info Sheet Adobe PDF Icon

     

    Colorado List B - Control required in Jefferson County

    General

    • Family: Sunflower (Asteraceae)
    • Elevation: 4,000 to 9,500 feet
    • Herbaceous
    • Perennial
    • Colony-forming unisexual plants

     

    Habitat

    • Infestations are found in cultivated fields, riparian areas, pastures, forests, lawns and gardens, roadsides and in waste areas; due to its seeding habits, vigorous growth and extensive underground root system, control or eradication is difficult; Canada thistle is one of the most widespread and economically damaging noxious weeds in Colorado

     

    PLANT

    Vegetation

    • Mature plants are 2 to 5 feet tall
    • Leaves are 4 to 8 inches long, usually dark green with spiny, serrated edges

     

    Roots

    • Horizontal growth is quick, roots can regenerate from small pieces
    • Horizontal roots may extend 15 feet or more, vertical roots may grow 6 to 15 feet deep.

     

    Flower

    • Color: purple, pink or white
    • Season: June to August
    • Buds are 3/4 inch or less in diameter

     

    Seed

    • Seed can be viable 8 to 10 days after flowering
    • Seeds germinate from late May through early fall
    • Can remain dormant in soil up to 20 years

     

    Seedling

    • Seeds germinate in spring or fall

     

    Reproduction

    • Seed and horizontal roots
    • Root propagation is main source for new plants

     

    CONTROL

    Biological

    • Not effective in our area

     

    Chemical

    Please refer to our Info Sheet Adobe PDF Icon

    Cultural

    • Prevention – maintain health of site

     

    Mechanical

    • Burning  - N/A 
    • Grazing  - N/A 
    • Mowing - Mowing is best before seed set; mow every 3 to 4 weeks June through September to weaken plants; works best when combined with fall herbicide treatments. 

     

    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

    weed-pest-b-canada-thistle-flower

    weed-pest-b-canada-thistle-plant

    weed-pest-b-canada-thistle-rosette


    Last Updated: 8-7-2013