Scotch thistle

Onopordum acanthium L.

Description from Invasive.org

Appearance
Onopordum acanthium is an herbaceous biennial plant that can grow up to about 6.5 ft. (2 m) in height. The plant is coarse, many-spined and is highly branched. The stems of O. acanthium are winged. The whole plant is densely tomentose, giving it a bluish-white appearance.
Foliage
The leaves are oblong and prickly, being toothed or slightly lobed along the margins. The apex of the leaf is acute. The leaves are mostly sessile, with some of the lower leaves having petioles. The blades of the lower leaves can measure up to 1 ft. (30 cm) long. Since this plant is a biennial, only the basal rosette of leaves is present in the first year of its growth.
Flowers
The flower heads are purple and measure 1-2 in. (2.5-5 cm) in diameter. All of the bracts of the involucre are tipped with flat, pale, orange-colored spines. Flowering occurs from July to October.
Fruit
The seeds of this plant are 0.2 in. (4-5 mm) long. They are gray in color, and attached to a brown-colored pappus that can be two times as long as the seed.
Ecological Threat
O. acanthium is a major agricultural weed in western United States. With enough moisture, it can resprout from roots cut up during cultivation. This plant spreads easily because each plant can produce over 20,000 wind dispersed seeds. The seeds can also be dispersed by water or by being caught in the fur of animals.

Resources


Selected Images from Invasive.org

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Plant(s)
Steve Dewey, Utah State University, Bugwood.org
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Plant(s)
Steve Dewey, Utah State University, Bugwood.org
Additional Resolutions & Image Usage
Flower(s)
Eric Coombs, Oregon Department of Agriculture, Bugwood.org
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Flower(s)
Steve Dewey, Utah State University, Bugwood.org
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Plant(s) in flower
Steve Dewey, Utah State University, Bugwood.org
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Feature(s) Flower and foliage
Steve Dewey, Utah State University, Bugwood.org
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Feature(s) Spines on Leaves
Leslie J. Mehrhoff, University of Connecticut, Bugwood.org
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Plant(s)
Steve Dewey, Utah State University, Bugwood.org
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Plant(s) Rosette
Steve Dewey, Utah State University, Bugwood.org
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Flower(s)
Lesley Ingram, , Bugwood.org
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Foliage
Bonnie Million, Bureau of Land Management, Bugwood.org
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Plant(s) Britton, N.L., and A. Brown. 1913. Illustrated flora of the northern states and Canada. Vol. 3: 556.
USDA NRCS PLANTS Database , USDA NRCS PLANTS Database, Bugwood.org
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Fruit(s)
Steve Hurst, USDA NRCS PLANTS Database, Bugwood.org
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Fruit(s) Young fruit with pappus and corolla remnant still attached. Note the reddish color of the bristles.
D. Walters and C. Southwick, USDA, Bugwood.org
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Plant(s)
Steve Dewey, Utah State University, Bugwood.org
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Infestation
Steve Dewey, Utah State University, Bugwood.org
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Infestation
Steve Dewey, Utah State University, Bugwood.org
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Infestation on river bank, seeds travel down water way
Vince Belleci, , Bugwood.org
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Distribution