Canada Thistle Information

(Cirsium arvense)

Erect, rhizomatous perennial
Kansas has many ecotypes, all with varying degrees of herbicide susceptibility

Distinguished from all other thistles by:
            *Creeping, horizontal roots.
            *Small, dioecious flower heads
            *High variability in leaf size and shape

Habitat:

            *Invades nearly every type of upland community
            *Prefers fertile, moist soils, can tolerate salt content of 2%
            *Prairies and grasslands
            *Roadsides and wasteland
            *Range and pasture
            *Cropland

Flowers:
            *Dioecious
            *Female flowers lack pollen and have a "vanilla" scent
            *Flowering occurs 12-14 weeks after emergence

Seeds:
            *Seeds are viable 8-10 days after onset of anthesis
            *Dissemination occurs 2-3 weeks after pollination
            *Majority of seeds germinate during the first year, remainder during the following spring

Roots:
            *Plants produce horizontal and vertical roots
            *Vertical roots can grow to 20 feet, however most are in top 24".
            *Roots live an average of 2 years
            *Carbohydrate reserves are lowest just prior to flowering
            *Roots begin to increase carbohydrate reserves in early fall as shoot growth declines

Shoots:
            *Primary shoots grow as rosettes for 2-3 weeks
            *Bolting occurs 24 weeks after emergence
            *Several growth stages can occur on the same plant, during the summer

Growth and Reproduction:
            *18 week old plant can produce 36 feet of roots, 26 above ground stems, 154 underground shoots
            *Average lateral root growth is 14-16 feet per year
            *Reproduction mainly from rootstocks, some seed

Management:
            *Frequent mowing during summer can enhance fall herbicide application
            *Herbicides: clopyralid (2,4-D, dicamba, metsulfuron not effective)

Threats:
            *Moderately allopathic, can change ecosystem structure/composition
            *Aggressively competes for light, nutrients and water, decreases yields
            *Displaces native, desirable plants and reduces species diversity.