Most people don't grow thistles as ornamentals but you have to admit the flower is nice and is very attractive to bees and butterflies.
Stems: Swamp Thistle plants grow erect, 4 to 7 feet in height with little to usually much branching in the upper section - the floral array. Stems are coarse, ridged, green initially with purplish tinges developing with age. Young stems have fine white hair, thinning as the season matures. The plant is biennial, forming a basal rosette the first year and sending up the tall stem the second year.
Leaves: Leaves are alternate, pinnatifid, (divided usually more than halfway to the mid-veins, sometimes almost to the vein). Leaf lobes vary from linear to more lance shaped, with lobe tips not sharply pointed. Basal leaves are largest, up to 20 inches long in some areas, but typically under 10 inches in Minnesota. Stem leaves are much smaller with very short stalks becoming stalkless in the upper stem. Basal leaves have few marginal spines, while the upper stem leaves have the most, but no parts of the plant are heavily armed. The more basal leaves are usually dropped by flowering time. The upper side of the leaves is green with fine hair and the under appears more whitish due to fine hair but some plants may have little underside hair on older leaves. Leaves are sparse at the top of the stem and do not crowd the flower heads.
The floral array consists of one to many stalked flower heads in an open panicle or corymb type array.
Flowers: The flower heads are up up to 1-1/4 inches high and wide and are densely filled with small 5 lobed disc florets that have lavender to purple corollas (rarely white) rising from a tubular base with the corolla noticeably longer than the tube and the throat of the corolla noticeably wider than the tube, similar to the Field Thistle, C. discolor. Florets are bisexual. Like most florets in the Aster family, there are five stamens, tightly surrounding a single style.
The outside of the flower head has 8 to 12 series of phyllaries that look like fish scales, are dull green in color with a lighter central stripe (a glutinous ridge which produces some stickyness) and a darker patch near the tip. The outer and middle phyllaries are appressed to the flower head. The outside of the head is not very prickly except where the tips of the upper phyllaries curl outward. Fine cob webby hairs appear between the phyllaries. The base of the head lacks a ring of floral bracts, instead there may be at least one, sometimes two small leaf-like bracts and also a small upper leaf that is sometimes longer than the flower head stalk may obscure part of the head. Flowers do not have a fragrance.
Seed: The flowers mature to a dry 4.5 to 5.5 mm cypsela that has attached some whitish to tan fuzzy pappus for wind dispersion. There is a yellow collar where the pappus attaches. The pappus is 12 to 20 mm long and is shorter than the corolla itself. Seeds require at least 30 days of cold stratification for germination.
Habitat: Swamp Thistle prefers full sun in the mosit soils of marshes, wet meadows, bogs and other moist open areas. It grows from a fleshy tap root. It does not spread vegetatively but re-seeds by wind dispersion of the seeds. The flowers provide an attraction for bees and butterflies. C. muticum will hybridize with C. flodmanii and C. discolor, both present in Minnesota.
Names: An older scientific name for the plant is Carduus muticum. The genus Cirsium is from the Greek word kirsion - for a particular thistle and has been adopted for a number of plants generally not appreciated by gardeners. The species muticum means 'blunt- without a point, referring to relatively few spines on this thistle. The author name for the plant classification, ‘Michx.’ refers to Andre Michaux (1746-1802), French botanist who made many exploring expeditions in the U.S. collecting and cataloging many species. Two important works of his are the Histoire des chênes de l'Amérique septentrionale (1801 - Oaks of North America), and the Flora Boreali-Americana (2 vols., 1803). His son Francois, traveled with him and the father’s notes were later used for the 3-volume North American Sylva, for which Thomas Nuttall provided some supplements.
Comparisons: Here is a comparison sheet of the 7 thistles found in Minnesota.
Above: The flower heads of Swamp Thistle are well liked by pollinators. Multiple heads occur in the floral array. Drawing from Britton, N.L., and A. Brown. 1913. An illustrated flora of the northern United States, Canada and the British Possessions. 3 vols. Charles Scribner's Sons, New York.
Below: Heads are about 1-1/4 inch wide with pinkish-purplish tubular disc florets. The style and stamens are greatly exserted from the corolla tube when in flower. The darker color stamens tightly surround the style. There is usually one or two small leaf-like appendages ascending from directly under the head and tightly embracing it are the phyllaries, each a dull green with a lighter central ridge and surrounded by a fine cob-webby hair.
Below: This photo shows more clearly the small bract at the base of the head with one of the small upper leaves rising below it.
Below: The basal leaves are long and deeply cut into lobes but with few spines.
Below: The stem leaves are smaller but also deeply lobed with spines on the edges.
Below: 1st photo - The leaf underside has a dense mat of fine hair. 2nd photo - growth stage of the stem.
Below: 1st photo - the young stem is green, ridged, with fine whitish hair. 2nd photo - the mature stem loses most of the longer hairs and takes on purplish tones. 3rd photo - the cypselae with papus attached.
Notes: Swamp Thistle is considered indigenous to the Wildflower Garden as Eloise Butler noted it in bloom on August 5, 1914. Martha Crone noted it in bloom in 1939 and also planted it in 1946. It was also listed on Martha Crone's 1951 inventory of plants in the Garden at that time. Swamp Thistle is native to Minnesota in most counties in the northern 2/3rds of the state. In North America it is found in the eastern half of the continent, from Saskatchewan and the Dakotas eastward, including the northern parts of the southern Gulf Coast states.
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Identification booklet for most of the flowering forbs and small flowering shrubs of the Eloise Butler Wildflower Garden. Details Here.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"