Pasture Thistle

Thistle flowers have to rank among the most attractive of all blooms. The bloom of the Pasture Thistle, Cirsium pumilum, presents a kaleidoscope of color when viewed from above. The early season blooms mark the beginning of the summer pasture blooming season. Being a short statured plant with a maximum height of about 18 inches, the Pasture Thistle presents its flowers before the taller growing prairie species cover it over.

A biennial species, the Pasture Thistle’s first year is spent as a basal rosette of prickly leaves. In the second year, a flower stalk shoots up from the center of the rosette. The thickness of the stalk often makes it appear mismatched to the basal portion.

Several spiny leaves grow from the stalk. A dense mass of soft hairs cover the stalk itself. The plant seems armed to defend itself against a wide array of predators. Spines could help discourage large browsers from nibbling the plant and the hairs may make it difficult for small insects to ascend the stalk.

Rows of spine tipped phyllaries, situated below the flower head, protect the receptacle where the seeds will eventually develop.

Another apparent defense against insects making their way upward to the flowers is the presence of a glutinous ridge running the length of the phyllary. Glutinous means sticky and several small insects have been trapped by this sticky strip. Pasture Thistles don’t always develop the glutinous ridge and its occurrence can vary greatly within a population. A similar thistle species, Cirsium hillii, always displays a strong glutinous ridge. Whenever I find one of these plants that is exceptionally sticky, I begin to wonder if I might have found one of the rarer C. hillii. After years of searching, I’m pretty sure that C. hillii does not occur here. C. hillii is a perennial species and would survive after flowering. Every one of the thistles that I’ve seen has died after producing seed. It’s still exciting to imagine that Blue Jay Barrens might host both species.

This has been an exceptional year for the Pasture Thistles. Normally producing only a single bloom, this year’s plants are showing two and sometimes three flower buds.

Like their larger relatives, Pasture Thistles are butterfly magnets. At this time of year, the thistles are the most abundant nectar source on the prairie. About a third of the flowers held at least one Great Spangled Fritillary.

Swallowtail numbers are also on the rise. This beauty is a Spicebush Swallowtail.

Bumblebees are frequent visitors to the thistle flowers. This fellow appears to be really embracing his floral feast.