Canada Thistle

(Cirsium arvense L.)

Asteraceae (Sunflower Family)

▲ seedling

▲ small flowering Canada thistle

young shoots from creeping roots ▲▼

▲ short, thin, upright, slightly branched stems of pre-flowering plants

▲ flowering plants with small inflorescences (usually 1" diameter or smaller)

large 30+ foot diameter clonal colony of Canada thistle in Minnesota ▲▼

▲ fruit of Canada thistle

▲ creeping root system of Canada thistle

Canada thistle:  (pp. 132-133, Weeds of the Northeast; pp. 108-109, Weeds of the Great Plains)

• a creeping perennial noxious weed in mostly northern U.S. (northern Missouri)

• not tall, anywhere from 6" to 3' tall, averaging around 2' tall; often of varying heights in colonies

• usually does not have leafy wings on stems

• lower leaf surface green to whitish hairy (variable among populations & locations)

• flower bracts not cobwebby, outer bracts taper to long tip

• solid stem, creeping perennial root

• has a lot of genetic variability in response to herbicides

• has creeping roots that can spread horizontally 10-20 ft. diameter/yr

• roots can go >10 ft. deep; 60% of roots >2' deep

• tends to be dioecious, with separate male and female plants

• produces many seeds which can remain viable >20 yrs.

• seedlings grow slowly at first, produce creeping roots at 7-9 wks

• creeping root shoots emerge in March-May

• blooms May-August

• inactive after bloom until September, then put on new growth up into November

• can distinguish from tall thistle, bull thistle, musk thistle, scotch thistle and field thistle by the shorter stature, presence of creeping roots (creating large, dense colonies) and smaller flower heads on canada thistle.

• biological control agents are being evaluated for its control—a gall fly, a stem-mining weevil and a seedhead weevil