Native Range

Western and southern Europe, the Mediterranean, and northward to Scandinavia

Appearance

Slenderflower thistle is an upright forb that typically grows as a winter annual but occasionally grows as a biennial. Plants grow 3½ ft (1 m) tall on average from shallow taproots. Leaves are alternate, lance-shaped and gray-green, with matted hairs on their undersides and spines on lobe tips. Stems are stout and have spiny triangular wings along their entire length. Flower heads appear in tight clusters of 5–20 at branch tips. They have pinkish-purple florets and triangular bracts tipped with spines. There are two types of seeds: cream-colored with tufts of fine hair and brown-colored without.

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

Selected Images

Maps

Invasive Listing Sources

• California Invasive Plant Council
• California Noxious Weeds
• New Invaders of the Northwest
• 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