Swamp Thistle
Cirsium muticum
Aster family (Asteraceae)

Description: This wildflower is a biennial that forms a rosette of basal leaves during the first year, followed by a flowering plant about 2-7' tall the following year. The basal leaves are up to 16" long and 6" across, although they are normally closer to about one-half that size. Aside from a tendency to be larger in size, they are similar to the alternate leaves of a flowering plant (as described below). A typical flowering plant is unbranched below and occasionally branched above. The central stem and any lateral stems are light green, terete, and longitudinally furrowed; they are finely hairy above, becoming more glabrous below. Alternate leaves are up to 10" long and 4" across, becoming gradually smaller in size as they ascend the stems. In outline, these leaves are lanceolate-oblong to elliptic-oblong; they are also pinnatifid with several pairs of lanceolate to linear-lanceolate lobes that have spiny tips. The lobes of the alternate leaves usually have smaller secondary lobes or sharp teeth that may also have spiny tips. The spines are golden yellow and up to 1/8" (3 mm.) long. The upper leaf surface is medium to dark green and glabrous to sparsely short-pubescent. The lower leaf surface is usually whitened from a dense coating of short white hairs while young, but it becomes more pale green with age from a thinning of such hairs.

The central stem and any upper stems terminate in either individual flowerheads or open corymbs of 2-5 flowerheads. The peduncles of these flowerhead are 1-6" long, light green (less often pale purple), terete, finely grooved, and finely hairy to nearly glabrous. There is usually a single leafy bract (up to 3" long) at the base of each peduncle and, less often, there may be a whorl of a few leafy bracts (up to 1" long) underneath the flowerhead. Otherwise, the peduncle is naked. The leafy bracts resemble the alternate leaves, except they are smaller in size and their spiny lobes are more narrow. Each peduncle terminates in a flowerhead that is about 1-2" across and similarly in length. At the flattened top of the flowerhead, there is a dense cluster of numerous disk florets; there are no ray florets. The disk florets are perfect and fertile. The corollas of the disk florets are narrowly cylindrical below, becoming slightly wider above with 5 narrow lobes. These corollas vary in color from pink to pinkish purple. The sides of the flowerhead consist of overlapping floral bracts (phyllaries) that are appressed together in several series. These floral bracts are lanceolate to ovate in shape; their tips taper into erect minute spines, or such spines may be absent. The sticky floral bracts have narrow mid-ribs that are pale green, otherwise they are blackish green; appressed woolly hair usually occurs between the floral bracts. The blooming period occurs from mid-summer to early fall, lasting about 1 months. Afterwards, the ray florets are replaced by small achenes with tufts of white hair. Individual achenes are about 5 mm. long, oblongoid, and dark brown to black with bands of yellow at their upper ends. They are distributed by the wind. The root system consists of a stout taproot.

The preference is full sun, wet to moist conditions, and calcareous sand, although partial sun and other soil types are tolerated. The size of individual plants can be highly variable, depending on moisture level, soil fertility, and competition from other plants.

Range & Habitat: The native Swamp Thistle is occasional in NE Illinois, otherwise it is uncommon or absent elsewhere in the state (see Distribution Map). This plant is more common further to the north. Habitats include wet sand prairies, sandy swamps, marshes, fens, interdunal swales near Lake Michigan, limestone bluffs (where some seepage of ground water occurs), pastures, and abandoned sandy fields. Swamp Thistle is found primarily in sandy wetlands and rocky areas where seepage through limestone occurs. It is perhaps most common in fens.

Faunal Associations: The flowerheads of Swamp Thistle and similar Cirsium spp. (thistles) attract many insects, especially long-tongued bees and butterflies. Typical floral visitors include honeybees, bumblebees, long-horned bees (Melissodes spp.), leaf-cutting bees (Megachile spp.), Cuckoo bees (Triepeolus spp.), Swallowtail butterflies (Papilio spp.), Fritillary butterflies (Speyeria spp.), Painted Lady butterflies (Vanessa spp.), and day-flying moths. Most of these insects suck nectar, although some bees also collect pollen. A long-horned bee, Melissodes desponsa, is an oligolege (specialist pollinator) of thistles. Other insect feed destructively on these plants. Swamp Thistle is the preferred host plant for the caterpillars of an uncommon butterfly, Calephelis mutica (Swamp Metalmark). Other insect feeders of thistles include caterpillars of Vanessa cardui (Painted Lady), caterpillars of Platyptilia carduidactyla (Artichoke Plume Moth) and Papaipema arctivorens (Northern Burdock Borer Moth), the leaf beetle Cassida rubiginosa, stem-boring larvae of Lixus concavus (Rhubarb Weevil), Melanoplus borealis (Northern Grasshopper), Brachycaudus cardui (Thistle Aphid) and Capitophorus elaeagni (Artichoke Aphid), and the treehopper Entylia bactriana. See the Insect Table for a more complete listing of these species. Because Swamp Thistle has some protection from its spines and it occurs in less accessible wetland habitats, it is usually ignored by mammalian herbivores. Some small songbirds, however, eat the seeds of thistles, especially the American Goldfinch. This bird also uses the hair tufts of thistle seeds in the construction of its nests. Another small bird, the Ruby-Throated Hummingbird, occasionally visits the flowerheads of thistles for their nectar.

Photographic Location:
A fen in NE Illinois. The photographs were taken by Lisa Culp (Copyright 2011).

Comments: Swamp Thistle (Cirsium muticum) resembles the more common Pasture Thistle (Cirsium discolor) and other tall-growing thistle species. It can be distinguished from these other species by the lack of significant spines on its floral bracts (phyllaries) and its preference for wetland habitats. While the leaf undersides of Swamp Thistle tend to be somewhat whitened, they are usually less whitened than either the Pasture Thistle or Tall Thistle (Cirsium altissimum). A Eurasian species that has not been found in Illinois, but appears to be spreading in other areas of the United States, is the Marsh Thistle (Cirsium palustre). Like the Swamp Thistle, Marsh Thistle also prefers wetland habitats, but it has smaller flowerheads (1" across or less) and its floral bracts have outward-pointing fine spines. This Eurasian thistle also tends to be somewhat smaller in size than the Swamp Thistle. Another common name of Cirsium muticum is Fen Thistle.