Scotch thistle: Onopordum acanthium (Asterales: Asteraceae): Invasive Plant Atlas of the United States

Scotch thistle	USDA PLANTS Symbol: ONAC U.S. Nativity: Exotic Habit: Forbs/Herbs
Onopordum acanthium L.

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Taxonomic Rank: Magnoliopsida: Asterales: Asteraceae
Synonym(s): Scottish thistle, cotton thistle, heraldic thistle, Scotch cottonthistle
Native Range: Europe to cent. Asia ()
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.

Identification, Biology, Control and Management Resources

Weed of the Week - USDA Forest Service
Weed Field Guide - USDA Forest Service

Selected Images from Invasive.org

View All Images at Invasive.org

Plant(s); Steve Dewey, Utah State University, Bugwood.org Additional Resolutions & Image Usage	Plant(s); Steve Dewey, Utah State University, Bugwood.org Additional Resolutions & Image Usage	Flower(s); Eric Coombs, Oregon Department of Agriculture, Bugwood.org Additional Resolutions & Image Usage	Flower(s); Steve Dewey, Utah State University, Bugwood.org Additional Resolutions & Image Usage
Plant(s); in flower Steve Dewey, Utah State University, Bugwood.org Additional Resolutions & Image Usage	Feature(s); Flower and foliage Steve Dewey, Utah State University, Bugwood.org Additional Resolutions & Image Usage	Feature(s); Spines on Leaves Leslie J. Mehrhoff, University of Connecticut, Bugwood.org Additional Resolutions & Image Usage	Plant(s); Steve Dewey, Utah State University, Bugwood.org Additional Resolutions & Image Usage
Plant(s); Rosette Steve Dewey, Utah State University, Bugwood.org Additional Resolutions & Image Usage	Flower(s); Lesley Ingram, , Bugwood.org Additional Resolutions & Image Usage	Foliage; Bonnie Million, Bureau of Land Management, Bugwood.org Additional Resolutions & Image Usage	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 Additional Resolutions & Image Usage
Fruit(s); Steve Hurst, USDA NRCS PLANTS Database, Bugwood.org Additional Resolutions & Image Usage	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 Additional Resolutions & Image Usage	Plant(s); Steve Dewey, Utah State University, Bugwood.org Additional Resolutions & Image Usage	Infestation; Steve Dewey, Utah State University, Bugwood.org Additional Resolutions & Image Usage
Infestation; Steve Dewey, Utah State University, Bugwood.org Additional Resolutions & Image Usage	Infestation; on river bank, seeds travel down water way Vince Belleci, , Bugwood.org Additional Resolutions & Image Usage

EDDMapS Distribution:
This map is incomplete and is based only on current site and county level reports made by experts and records obtained from USDA Plants Database. For more information, visit www.eddmaps.org

State List
This map identifies those states that list this species on their invasive species list or law.

U.S. National Parks where reported invasive:

Grand Canyon National Park (Arizona)

Invasive Listing Sources:

California Invasive Plant Council

Connecticut Invasive Plant Working Group

Jil M. Swearingen, Survey of invasive plants occurring on National Park Service lands, 2000-2007

John Randall, The Nature Conservancy, Survey of TNC Preserves, 1995.

Pacific Northwest Exotic Pest Plant Council, 1998

WeedUS - Database of Plants Invading Natural Areas in the United States