Plant of the Week
Pitcher’s Thistle (Cirsium pitcheri)
By Morty Harwood
Pitcher’s thistle, or dune thistle, occurs along the shores of Lake Michigan, Huron, and Superior. Originally discovered around 1820 by Dr. Zina Pitcher in what is now Pictured Rocks National Lakeshore, this small thistle grows to a maximum height of about three to four feet when flowering, considerably less that the size reached by its larger cousin Canada thistle (C. arvense). A native of the Great Lakes dune ecosystem, it thrives in sandy soil with the help of an impressive taproot (up to six feet long!) that helps the plant obtain water. For much of its life the Pitcher’s thistle exists as a small, inconspicuous cluster of leaves called a rosette, the massive taproot encompassing a majority of the plants mass. Flowering occurs after the Pitcher’s thistle survives in this rosette stage for up to eight years. After flowering and producing seed, the plant dies.
Pitcher’s thistle can be identified by its deeply toothed, wooly leaves and by the presence of spines at the tips of each point on the leaf. The wooly coating gives the leaves and stems their characteristic silvery appearance, as well as helping the plant to conserve water. The plant flowers from mid-June through September and shows a flower head that ranges from creamy white to light pink. This trait can be helpful in distinguishing it from other, more common thistles, which typically possess pink or purple flowers. Other varieties of thistle also lack the extremely wooly leaves and silvery appearance of the Pitcher’s Thistle.
From an ecological perspective, all thistles are important food sources for certain birds, especially American Goldfinches, which may feed on thistle seed almost exclusively during later summer and early fall. Small mammals such as ground squirrels may also consume the seeds after they have fallen from the plant.
Although it was likely never extremely widespread, Pitcher’s thistle was once fairly common in sand dune ecosystems of Michigan, Wisconsin, Indiana, Illinois, and the Canadian province of Ontario. Numbers of Pitcher’s thistle, however, have declined in recent decades due to habitat destruction associated with development and recreational use. The sensitivity of the Pitcher’s thistle to these types of disturbances led to its designation as a federally threatened species in 1996. A threatened species is a species that is at risk of becoming endangered, while an endangered species is one that is at risk of becoming extinct. Pitcher’s Thistle is also protected at the state level in each state where it occurs, and at both provincial and national levels in Canada. Due to continued development and overuse of the sensitive Great Lakes sand dune ecosystem, there is still a risk to the Pitcher’s thistle and other native plants sharing this habitat. Invasive plants species – particularly spotted knapweed – also pose a threat to the habitat of the Pitcher’s thistle. Recently, concern has arisen that the non-native beetle species Rhinocyllus conicus may be praying on Pitcher’s thistle. This beetle preys on the flower heads of the plant, and was originally introduced to control populations of non-native thistles.