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Canada
Thistle
Common
Name(s):
Canada Thistle
Scientific
Name:
Cirsium
arvense (L.) Scop.
Scientific
Name Synonyms:
Carduus arvensis (L.)
Robson
Symbol:
CIAR4
Description:
Life
Span: Perennial
Origin:
Introduced
Season: Cool
Growth Characteristics:
A
colony-forming weed, growing from deep and extensive horizontal
roots, and forming dense clones. Stems are 1 to 6 ½ feet
tall, rigid, and branching above. Flowering occurs during July and
August. It reproduces mainly from rhizomes, but can also reproduce
by seed.
Flowers/Inflorescence:
Flowers
are unisexual (found on separate plants). Flowers are purple, occasionally
white, in head ½ to ¾ inch in diameter, clustered
at the top of the stems. Bracts below flower are spineless, and
not painful to touch.
Fruits/Seeds:Seeds
are somewhat flattened, brownish, with a tuft of hairs at the top.
They grow to 1/8 long.
Leaves: Alternate
placement on stem, lacking petioles, oblong or lance-shaped, divided
into spiny-tipped irregular lobes.
Ecological
Adaptations:
Canada
thistle establishes and develops best on open, moist, disturbed
areas, including ditch banks, overgrazed pastures, meadows, tilled
fields or open waste places, fence rows, roadsides, and campgrounds;
and after logging, road building, fire and landslides in natural
areas. It differs from other species of the true thistles in that
there are male and female flower heads, and these are on separate
plants.
Soils: Adapted
to a broad range of soils.
Associated Species:
Widespread
Uses and
Management:
Canada
thistle has little or no forage value for livestock. It can be a
minor component of the winter and spring diet of mule deer, and
is used somewhat by bears.
Canada thistle is a listed noxious weed in Utah. It is very aggressive
and difficult to control. Breaking up the roots by plowing only
increases the number of plants.
When reproducing asexually, it is possible that a colony of male
plants could produce no fruit, but still maintain itself.
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