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 Appearance:
Perennial forb, 2-5' tall with slender grooved stems that branch
only at the top. It has male and female plants. Leaves:
Alternate, smooth, oblong, tapering, and directly attached to the
stem, deeply divided with prickly margins. Flower:
Numerous small purple flowers appear on top of the upper branched
stems between June and September. Seed:
Small light brown seeds are tufted for wind dispersal. Seeds remain
viable in the soil for over 20 years. Root:
Each plant has a fibrous taproot with wide spreading horizontal roots.
Each small section of root can form a new plant, enabling the species
to spread vegetatively. |
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Canada thistle invades
natural areas such as prairies, savannas, glades and dunes if some
degree of disturbance already exists. It also invades wet areas with
fluctuating water levels such as streambanks, sedge meadows and wet
prairies. Once it has established itself it spreads quickly, replacing
native plants and diminishing diversity. It grows in circular patches
spreading vegetatively through horizontal roots which can spread 10-12'
in one season. Canada thistle occurs in Canada and throughout the northern
U.S. from northern California to Maine and southward to Virginia.
Legal Status:
Canada thistle has been declared a noxious weed in 43 states as one
of the most tenacious and economically important agricultural weeds.
This plant is a prohibited noxious weed in Minnesota and must be
controlled or eradicated as required by Minnesota Statutes, section
18.78.
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 Wisconsin
Manual of Control Recommendations for Ecologically Invasive Plants,
1997 |
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