Canada thistle
(Cirsium arvense (L.) Scop.)

Overview

Appearance

Cirsium arvense, a rhizomatous perennial grows from 1-5 ft. (0.3-1.5 m) tall. Roots can grow deep into the ground. Stems do not have conspicuous spines.

Foliage

Leaves are dark green and lanceolate to oblong-lanceolate. They are glabrous above, but their undersides have short, white hairs. They may be pinnatifid and very prickly. Basal leaves are 5-8 in. (12-20 cm) long. Leaves are usually sessile to slightly clasping. Leaf characteristics are variable across different varieties and subspecies.

Flowers

It has male and female plants. Female flowers are flask-shaped, 0.4-0.6 in. (1-1.5 cm) in diameter, and 0.4-0.75 in. (1-2 cm) tall. Female flowers are fragrant, the male flowers are not. Male flowers are smaller and more globose than the female flowers. Flowers range from purple to pink or white. It blooms from June to August.

Fruit

Fruits are tiny, 0.1 in. (2-3 mm) long, about 0.04 in. (1 mm) in diameter, with a white to light brown pappus.

Ecological Threat

Cirsium arvense is most commonly found in agricultural and disturbed sites, or sites that are undergoing restoration. It is shade intolerant and therefore is rarely found within wooded sites, except in clearings. It is found in some dry, sandy sites, but more commonly on the edges of wet habitats such as stream banks and lake shores. In the western and northern U.S. it presents a significant problem in prairie and riparian habitats.

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 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.
• Kansas Noxious Weeds
• 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 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 Carolina Exotic Pest Plant Council
• 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