Scotch Thistle

Onopordum acanthium

Scotch Thistle

Family: Asteraceae

Other Common Names: cotton thistle, woolly thistle
Weed class: B
Year Listed: 1988
Native to: Europe and Asia
Is this Weed Toxic?:

not known to be

Legal listings:

WA Quarantine list, WAC 16-752

Why Is It a Noxious Weed?

Scotch thistle is a problem in rangeland. Infestations of Scotch thistle reduce forage production and virtually prohibit land utilization for livestock. Dense stands of the large, spiny plants exclude animals from grazing and access to water.

How would I identify it?

General Description

Scotch thistle is a branched, biennial or annual with a broadly winged stem that can grow up to 8 feet or more in height and 6 feet in width.

Flower Description

Plants flower in mid-summer. The globe-shaped flower heads are borne in groups of 2 or 3 on branch tips. Flower heads are up to 2 inches in diameter, with long, stiff, needle-like bracts at the base. Flowers range from dark pink to lavender.

Leaf description

Leaves are up to 2 feet long and 1 foot wide, are covered with sharp yellow spines and have a gray-green appearance from being covered with a thick mat of cotton-like or woolly hairs.

Stem description

Stems have vertical rows of prominent, spiny, ribbon-like leaf material or wings that extend to the base of the flower heads.

Fruit Seed Description

Seeds are smooth, slender, and plumed.

Where does it grow?

Scotch thistle will grow in wet meadows and pastures as well as dry pastures and rangelands. It may also be found alongside streams and rivers. Please click here to see a county level distribution map of Scotch thistle in Washington.

How Does it Reproduce?

Scotch thistle reproduces by seed. Each plant can produce 8,400 to 40,000 seeds.

How Do I Control It?

General Control Strategy

Establishing a dense well-maintained pasture is effective in preventing a Scotch thistle infestation.

Mechanical Control

Small areas can be dug out. Mowing has limited effectiveness for controlling Scotch thistle, usually only prevents seed production.

Biological Control

Goats will graze Scotch thistle, preventing seed production.

Herbicide Control

Please refer to the PNW Weed Management Handbook, or contact your county noxious weed coordinator.

For More Information

See our Written Findings for more information about Scotch thistle (Onopordum acanthium).

Thurston County NWCB Fact Sheet on Scotch thistle

Franklin County NWCB Fact Sheet on Scotch thistle

Stevens County NWCB Fact Sheet on Scotch thistle

Whatcom County NWCB Fact Sheet on Scotch thistle

Pierce County NWCB Fact Sheet on Scotch thistle

Lincoln County NWCB Brochure on Scotch thistle

Additional Photos