Scientific name: Onopordum acanthium
What Is It?
Scotch thistle was introduced to the United States as an ornamental in the 1800s. It can grow up to 8 feet tall and 5 feet wide, with small pink-purple flowers. It is native to Europe and Asia.
Is It Here Yet?
Yes, Scotch thistle has been documented in many parts of Washington, particularly eastern Washington.
Why Should I Care?
Scotch thistle invades disturbed areas, such as roadsides, ditch banks, pastures (especially when overgrazed), campgrounds, and burned areas. Its dense stands compete with native plants for resources and can form a physical barrier to water and grazing for animals.
How Can We Stop It?
Do not purchase, plant, trade, sell, or release this species. Scotch thistle is on Washington’s Terrestrial Noxious Weed Seed and Plant Quarantine list, meaning it is prohibited to transport, buy, sell, offer for sale, or distribute Scotch thistle plants, plant parts, or seeds. Scotch thistle also is listed as a Class B noxious weed in Washington, meaning it is designated for control in certain state regions. Promoting native and desired species also should help slow the spread of Scotch thistle.
What Are Its Characteristics?
- Grows 6-8 feet tall and wide.
- Stems are coarse with many spines, and are gray-green and highly branched.
- Leaves are oblong and prickly with toothed margins.
- The plant forms dark pink to purple, globe-shaped flower heads, 1-2 inches in diameter, which stand alone on branch tips and bloom July-October.
- Whorl of bracts beneath the flower is tipped with flat, pale, orange spines.
- Seeds are less than 1/4 inch in length, slender, smooth, and plumed.
How Do I Distinguish It From Native Species?
Scotch thistle may be distinguished from all other thistles by the dense, white, woolly covering on its stems and leaves.