Pitcher's Thistle

The Pitcher’s thistle is a native thistle that grows on the open sand dunes and low open beach ridges along the shorelines of Lakes Michigan, Superior, and Huron. It is now found in Indiana, Michigan, and Wisconsin and in Ontario Canada. Pitcher’s thistle was extirpated from Illinois but has been reintroduced in Lake County. It is most often found in near-shore plant communities but it can grow in all nonforested areas of a dune system.

Pitcher’s thistle is endemic to the beaches and grassland dunes of Lakes Michigan, Superior, and Huron . The majority of known sites of Cirsium pitcheri occur along the shores of Lake Michigan. The species ranges from the north shore of Lake Superior south to Indiana, and formerly occurred in northern Illinois, where it is has been experimentally reintroduced.

Pitcher’s thistle colonizes patches of open, windblown areas of the landscape, and gradually declines locally as the density of vegetation and ground litter increases through plant succession.

Seed dispersal commences in late July at the northern limits of its range, but can occur from June to August. Primary seed dispersal is through individual seeds blowing from the inflorescence head or by the whole plant and heads falling to the ground at the end of the flowering season.

Once conditions are right, the seed dormancy is broken by cold, moist stratification, with seed germination occurring in May and June. Seedlings produce 1 to 6 leaves in the first season, and then juveniles grow or maintain a constant size throughout the growing season, but may diminish in size over the winter. 

Age of reproduction ranges from 5 to 8 years and appears to be correlated with habitat. Pitcher’s thistle blooms from May to September, with the date of peak bloom occurring later with increasing latitude (mid-July at Sleeping Bear Dunes). 

Pitcher’s thistle appears to have a small between-year seedbank, with seeds remaining viable for about three years. This suggests a buried seedbank may not strongly buffer population stability when plants are destroyed. Seed dispersal to nearby suitable habitats may be more important for population stability than the seedbank.

This monocarpic (once-flowering) plant produces a vigorous rosette that may mature for ca. 5-8 years or more before it flowers.

The stout, prickly, dune species, Pitcher’s thistle, grows for five to eight years before it flowers. Its non-flowering form is a rosette or cluster of silvery leaves and its flowering form typically has one stem with many branches. The entire flowering plant may grow 3 feet tall, but can also be as little as 6 inches and flower. Cream or pink flowers grow at the end of branches and from the leaf axils. Leaves are finely and deeply lobed and may be one foot long.

The leaves and entire plant are blue-green in color and densely covered with white-woolly hairs. The mature leaves are deeply divided into narrow, spine-tipped segments. The prickly, spine-tipped flower heads are relatively large and strikingly cream-colored, though they may occasionally have a slightly pinkish tint, yielding seeds with feathery bristles. Pitcher's thistle is unlikely to be easily confused with any other thistle species in Michigan, including both native and nonnative species, all of which can be distinguished by their deep pink flower heads. Pitcher's thistle are unlikely to co-occur with other thistles, as Pitcher's thistle only persist in good quality, open dunes habitat. Vegetatively, all other thistles in Michigan lack the deep blue-green color of Pitcher's thistle and its usually dense covering of white woolly hairs.