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 1½" 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 1¼-2" 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
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
1½ 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.
Swamp Thistle is occasional in NE Illinois, otherwise it is uncommon or
absent elsewhere in the state (see Distribution
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.
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
), leaf-cutting bees (Megachile spp.
Swallowtail butterflies (Papilio
Fritillary butterflies (Speyeria
), Painted Lady butterflies
and day-flying moths. Most of these insects suck
nectar, although some bees also collect pollen. A long-horned bee,
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
(Swamp Metalmark). Other insect feeders of
thistles include caterpillars of Vanessa
caterpillars of Platyptilia
(Artichoke Plume Moth) and
(Northern Burdock Borer Moth), the leaf beetle
stem-boring larvae of Lixus
(Thistle Aphid) and Capitophorus elaeagni
(Artichoke Aphid), and
the treehopper Entylia
. See the Insect Table
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
A fen in NE Illinois. The photographs were taken
by Lisa Culp (Copyright © 2011).
Thistle (Cirsium muticum
resembles the more common Pasture Thistle
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
). A Eurasian
species that has not been found in Illinois, but appears to be
other areas of the United States, is the Marsh Thistle (Cirsium
). Like the Swamp Thistle, Marsh Thistle also
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.
common name of Cirsium
is Fen Thistle.