Native Range

Southern Europe, the Mediterranean, and North Africa

Appearance

Milk thistle is an upright forb that typically grows as a winter annual but can sometimes grow as a biennial. Plants grow 5 ft (1½ m) tall on average from a long taproot. Stems are stout, rigid, and not spiny. Leaves are alternate, lobed, and have yellowish margins tipped with long woody spines. They are dark green with white marbling along veins. Flower heads appear solitary at the ends of stems and have magenta to purple florets. The bracts are broad, leathery, and tipped with very stiff spines. Seeds are shiny dark brown with a rippled surface and a tuft of minutely barbed hairs that falls off at maturity.

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

• Element Stewardship Abstract - The Nature Conservancy

Selected Images

Maps

Invasive Listing Sources

• Alabama Invasive Plant Council
• California Invasive Plant Council
• John Randall, The Nature Conservancy, Survey of TNC Preserves, 1995.
• Maine Invasive Plants
• New Invaders of the Northeast and Northcentral
• Oregon Noxious Weeds
• Pacific Northwest Exotic Pest Plant Council, 1998
• Washington Noxious Weeds
• WeedUS - Database of Plants Invading Natural Areas in the United States

References