Native Range

Europe and western Asia

Appearance

Scotch thistle is an upright forb that typically grows as a biennial but can also behave as short-lived perennial or even an annual. On average, plants grow 7–8 ft (2.1–2.4 m) tall from a fleshy taproot. Leaves are large with wavy, toothed margins armed with sharp yellow spines. Stems are robust, have numerous branches, and have spiny wings extending along their entire length. Leaves and stems are covered with a thick mat of gray-green woolly hairs. The large flower heads have rows of needle-like bracts and up to 150 purplish florets. Seeds are grayish-brown and 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

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

Selected Images

Maps

Invasive Listing Sources

• Arizona Noxious Weeds
• California Invasive Plant Council
• California Noxious Weeds
• Colorado Noxious Weeds
• Connecticut Invasive Plant List
• Connecticut Invasive Plant Working Group
• Idaho 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.
• Missouri Noxious Weeds
• Nevada Noxious Weeds
• New Mexico Noxious Weeds
• Nonnative Invasive Species in Southern Forest and Grassland Ecosystems
• Oklahoma Noxious Weeds
• Oregon Noxious Weeds
• Pacific Northwest Exotic Pest Plant Council, 1998
• Utah Noxious Weeds - Contain
• Washington Noxious Weeds
• WeedUS - Database of Plants Invading Natural Areas in the United States
• Wyoming Noxious Weeds

References