Centaurea solstitialis (Yellow star-thistle)

Thistles are a symbol of independence in Scotland where they hold distinction as the national flower. “No one provokes me with impunity” is the motto of the Scottish Order of the Thistle, a chivalrous title that carries the highest honor. Yellow star-thistle, an annual and biennial in the Asteraceae family, is native to southern Europe and north Africa, and grows extensively in the US West. Introduced in the mid 1800’s from the Mediterranean in contaminated seed, yellow star thistle grows up to 6 feet tall and produces more than 150,000 seeds per plant that remain viable for 10 years. The leaves are green, grey and covered in wooly hairs, and the sharp spines on the bracts grow underneath the flowerhead in a star shape. A drought tolerant species that loves full sun, they grow well in poor soils, and are thus a pioneer in disturbed areas such as overgrazed lands. The stem can grow 6 feet tall. A deep taproot brings up micronutrients into surface soils that benefit other plants. In Europe, this thistle is kept in check by co-evolution with grazing animals such as goats and sheep. When removed from Yellowstone National Park where it grows in monocultures, 180 native plants emerged.

Five native thistle species in the US are on the endangered species list, while yellow star-thistle proliferates. The sharp spines can be harmful to livestock and foraging for horses causes fatal chewing disease. Captain Lewis on the Lewis and Clark expedition ate a cooked thistle root, from Circium edule. The flowers of yellow star-thistle are used in Turkish folk medicine for cancer, and studies are underway for cancer drug development. Other invasive species, like Japanese knotweed, has robust antioxidant and anti-cancer effects in the body. Thistles can indicate soil deficiencies in iron and copper.

Elizabeth Oriel 2022