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Canada Thistle
(creeping
thistle) Asteraceae (=Compositae), the aster family.
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Background
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 Canada
thistle is an invader from Eurasia. It was introduced to Canada probably as
a crop seed contaminate before 1800. It is an aggressive weed that spreads
both by seed and extensive root systems. The deep roots grow horizontally
and sent up shoots along their length, forming dense colonies. |
Description
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 Canada
thistle is a perennial plant up to 5 feet tall. Leaves are wavy
margined to
lobed, up to 6 inches long, and armed with yellowish spines.
Stems are single, branched near the top, ridged, and hollow. Flower heads
bloom in midsummer, 1/2 inch in diameter, and are not particularly spiny.
Flowers are purple to lavender, occasionally white, with male and female
flowers borne on separate plants. Seeds are slender, tan, 1/8 inch long, and
bear fine plumes.
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Distribution
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 Canada
thistle is found in the northern U.S. and southern Canada. It is one of
Idaho's most widespread and damaging noxious weeds.
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Control
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Biological control agents are available, but control in only poor to fair.
Herbicides are available that can control the weed if consistently used.
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© 1999 University of Idaho:
Text and photographs for these pages from Idaho's Noxious Weeds, by
Robert H. Callihan and Timothy W. Miller (revised by Don W. Morishita
and Larry W. Lass).
Please contact: Ag Publishing, University of Idaho, Moscow,
Idaho 83844-2240; 208 882-7982 visit the Resources for Idaho website at
http://info.ag.uidaho.edu, for more information about this or other
publications.
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