New weeds to watch for in Douglas County: Scotch Thistle

By Aaron Rosenblum
Foster Creek Conservation District

 

This is the fifth in a series of articles that introduces readers to new weed invaders in Douglas County. The weed species featured are known from only a few locations in the county, but have the potential to become wide-spread and problematic. The goal of these articles is to inform and educate Douglas County residents, so that you can help us with the early detection and rapid treatment of small weed populations before they become major issues throughout the county.

 

Globe-shaped flower heads form on mature Scotch thistle plants in mid-summer. (Provided photo/Aaron Rosenblum)

This week’s weed to watch for is Scotch thistle (Onopordum acanthium). Scotch thistle is a tall and robust plant that is native to the Mediterranean, Europe, and Central Asia and was introduced into the U.S. in the late 19th century, likely as an ornamental. It is now widespread across the western U.S. and

can be found in all Eastern Washington counties.

Scotch thistle is commonly found on rangeland, pastures, shrub steppe, wet meadows and draws, CRP lands and any other disturbed area. Wherever found, it reduces habitat and forage values through the displacement of native and beneficial species. Livestock and wildlife use of the land can be essentially reduced to nothing if dense stands are allowed to develop. This is because animals refuse to pass through the large spiny plants.

Branching stems with spiny, leaf-like “wings” form in the Scotch thistle’s second year. (Provided photo/Aaron Rosenblum)

In fact, legend has it that Scotch thistle played a role in helping Scotland to defend itself from a Viking invasion. Vikings reportedly landed on Scotland planning a stealthy attack. However, as they crossed through an infestation of Scotch thistle they cried out in pain. This alerted the Scots and allowed them to fend off the attack.

A mature Scotch thistle plant growing in favorable conditions can reach 8-12 feet tall. (Provided photo/WSU Extension-Jennifer Andreas)

Scotch thistle usually has a biennial life cycle, meaning it matures, produces seed and dies during its second growing season. However, it can be an annual (completes lifecycle in a single year) or perennial (persists for multiple years) under certain conditions. After seeds germinate in the fall and in their first year of growth, Scotch thistle plants produce a rosette, which is a circle of leaves close to the ground that lack a stem. The leaves are coarsely lobed with each lobe ending in a very sharp spine. Erect, branching stems with spiny, leaf-like “wings” form in the plant’s second year. All surfaces of the plant are covered with a thick mat of wooly hairs, giving the plant a gray-green appearance. A mature Scotch thistle plant growing in favorable conditions can reach 8-12 feet tall and 6 feet wide!

Numerous globe-shaped flower heads form on mature plants in mid-summer. The flowers are generally 1-2 inches in diameter and range in color from dark pink to lavender to purple. The lower portion of the flower heads are composed of structures known as bracts. Scotch thistle’s bracts are long, stiff, needle-like structures that are sure to keep any potential herbivore at bay. Reproduction occurs by seed only, with each plant producing 8,400 to 40,000 seeds that can remain viable in the soil for up to 20 years.

Foster Creek Conservation District is leading the development of the Douglas County Cooperative Weed Management Area, and has funding to assist landowners with the control of Scotch thistle as well as the other weeds featured in this series of articles. If you believe you have Scotch thistle on your land please contact me at 423-5990 or at arosenblum@fostercreekcd.org; or Dale Whaley with the WSU Extension at 745-8531, ext. 6352.

 

Aaron Rosenblum is the natural resource specialist for the Foster Creek Conservation District.