Native Range

Eurasia

Appearance

Canada thistle is an upright perennial forb often found in dense infestations. Plants grow 1–4 ft (0.3–1.2 m) tall with taproots and extensive, creeping lateral roots. Leaves are lobed with very prickly, ruffled margins and a slightly downy lower surface. Leaves grow alternately along the slender, grooved stems which can be finely haired but are typically not spiny. Upper branches are topped by clusters of small, compact flower heads with non-spiny bracts and florets that vary from white to deep lavender. Male and female flowers occur on different plants. Each female floret produces one small brown seed topped by a tuft of fine hairs.

Impact

Introduced thistles currently infest millions of acres across North America, primarily occurring in or along roadways, waterways, agricultural fields, rangelands, pastures, forests, and disturbed areas. They are responsible for millions of dollars of damage annually in reduced agricultural yields and lowered forage value in grazing systems. Introduced invasive thistles can also displace native vegetation, negatively impacting wildlife and threatening the delicate ecological balance within many habitats.

Reference

Randall, C.B., J.E. Andreas, and J. Milan. 2024. Introduced Thistles: History and Ecology in North America. In: R.L. Winston, Ed. Biological Control of Weeds in North America. North American Invasive Species Management Association, Milwaukee, WI. NAISMA-BCW-2024-8- INTRODUCED THISTLES-P. https://bugwoodcloud.org/resource/files/29168.pdf

Resources

• Biological Control of Invasive Plants in the Eastern United States - USDA Forest Service
• Weeds Gone Wild: Alien Plant Invaders of Natural Areas - Plant Conservation Alliance
• Element Stewardship Abstract - The Nature Conservancy
• Fire Effects Information System - USDA Forest Service
• Federal Noxious Weed Disseminules of the U.S. - USDA-APHIS
• Weed of the Week - USDA Forest Service
• Weed Field Guide - USDA Forest Service
• Plant Invaders of Mid-Atlantic Natural Areas - National Park Service and U.S. Fish and Wildlife Service

Selected Images

Maps

Invasive Listing Sources

• Alabama Invasive Plant Council
• Alaska Noxious Weeds
• Alberta Weed Control Act
• Arizona Noxious Weeds
• California Invasive Plant Council
• California Noxious Weeds
• City of Ann Arbor Michigan Parks and Recreation
• Colorado Noxious Weeds
• Commonwealth of Pennsylvania, Department of Conservation and Natural Resources, 1994.
• Connecticut Invasive Plant List
• Connecticut Invasive Plant Working Group
• Connecticut Noxious Weeds
• Delaware Invasive Species Council
• Delaware Noxious Weeds
• Delaware Regulations for Noxious Weed Control
• Hawaii Noxious Weeds
• Hoffman, R. & K. Kearns, Eds. 1997. Wisconsin manual of control recommendations for ecologically invasive plants. Wisconsin Dept. Natural Resources, Bureau of Endangered Resources. Madison, Wisconsin. 102pp.
• Idaho Noxious Weeds
• Illinois Noxious Weeds
• Indiana Invasive Species Council - Invasive Plant List
• Indiana Noxious Weeds
• Invasive Plant Association of Wisconsin
• Invasive Plant Species of West Virginia
• Iowa Forest Invasive Plants
• Iowa Noxious Weeds
• 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.
• Kentucky Exotic Pest Plant Council - Severe Threat
• Kentucky Noxious Weeds
• Maryland Code and Regulations Noxious Weed & Seed Law
• Maryland Cooperative Extension Service.  2003. Invasive Plant Control in Maryland. Home and Garden Information Center, Home and Garden Mimeo HG88. 4 pp.
• Maryland Department of Natural Resources Policy: Restriction on Planting Exotic Invasive Plants
• Maryland Invasive Species Council - Invasive Species of Concern in Maryland
• Maryland Noxious Weeds
• Michigan Noxious Weeds
• Minnesota Noxious Weeds
• Missouri Noxious Weeds
• Montana Noxious Weeds
• National Park Service, Mid-Atlantic Exotic Plant Management Team Invasive Plant List
• Native Plant Society of Oregon, 2008
• Nebraska Noxious Weeds
• Nevada Noxious Weeds
• New Hampshire Invasive Species Committee. 2005. Guide to Invasive Upland Plant Species in New Hampshire. New Hampshire Department of Agriculture,  Markets and Food Plant Industry Division and New Hampshire Invasive Species Committee.
• New Jersey Invasive Species Strike Team 2017 Invasive Species List
• New Mexico Noxious Weeds
• New York Regulated and Prohibited Invasive Species - Prohibited
• Non-Native Invasive Plants of Arlington County, Virginia
• Non-Native Invasive Plants of the City of Alexandria, Virginia
• Nonnative Invasive Species in Southern Forest and Grassland Ecosystems
• North Carolina Noxious Weeds
• North Dakota Noxious Weeds
• Ohio Invasive Species Council
• Ohio Noxious Weeds
• Oklahoma Noxious Weeds
• Ontario’s Invading Species Awareness Program Tracked Species List
• Oregon Noxious Weeds
• Pacific Northwest Exotic Pest Plant Council, 1998
• Pennsylvania Department of Conservation and Natural Resources Invasive Plants
• Pennsylvania Noxious Weeds
• Pennsylvania Regulations for Controlled Plants and Noxious Weeds
• Plant Invaders of Mid-Atlantic Natural Areas
• Rhode Island Natural History Survey
• South Dakota Noxious Weeds
• Utah Noxious Weeds - Contain
• Virginia Invasive Plant Species List
• Virginia Invasive Species Management Plan 2012
• Washington Noxious Weeds
• WeedUS - Database of Plants Invading Natural Areas in the United States
• West Virginia Invasive Species Strategic Plan and Volunteer Guidelines 2014
• West Virginia Native Plant Society, Flora West Virginia Project, and West Virginia Curatorial Database System, September 3, 1999
• Wisconsin Noxious Weeds
• Wisconsin's Invasive species rule – NR 40
• Wyoming Noxious Weeds

References