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Spotted Knapweed
Asteraceae
(= Compositae), the aster family
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Background
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 Spotted
knapweed, native to Europe, is thoroughly established in North America. In
Idaho the weed is widely dispersed. Each plant produces up to 25,000 seeds
that wind, animals, and people disperse. Seeds may remain viable for 8 years |
Description
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Spotted knapweed is a biennial or short-lived
perennial.
Mature plants may be 3 feet tall and are much branched. The weed forms a
basal rosette the first year with leaves up to 6 inches long; both basal and
stem leaves are
pinnately divided. Flower heads are abundant, 1/2 inch wide,
and solitary on branch tips. Flowers are pink to purple, or occasionally
white, and appear from midsummer to fall. Each stiff flower head
bract has a
dark comblike fringe resembling a black spot at the tip. Seeds are dark
brown to tan and are tipped by plumes that falloff at maturity.
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Distribution
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 Spotted
knapweed is found in the northeast and north central U.S. as well as along
the Pacific Coast, most of Idaho, and east through Montana.
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Control
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Several biological control agents are available. A seed head moth and 2 seed
head gall flies have been effective. Herbicides are available for control of
spotted knapweed.
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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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