This perennial wildflower forms arching stems about 3-8' long that
are unbranched or sparingly so. The base of each stem is spongy and
swollen if it is submerged in water, otherwise it is more constricted
and angular. Each stem is pale green to red and either glabrous or
pubescent; it tends to be slightly woody at the base, but dies down to
the ground each winter (at least in the Midwest). Both opposite leaves
and whorls of 3 leaves can occur along the stems; they are up to 6"
long and 1½" across. The leaves are elliptic in shape and smooth along
their margins. The upper leaf surface is medium to dark green and
glabrous, while the lower surface is pale to medium green and either
glabrous or pubescent. The leaves taper gradually into short petioles
(¼" or less) and long narrow tips.
Clusters of non-terminal
flowers occur in the axils of the leaves. Individual flowers are up to
1" long and 1" across, consisting of 5 wrinkled purple petals, a short
calyx with 5 primary teeth alternating with 5 secondary teeth, 10
stamens of varying lengths, and a pistil with a style. Sometimes
there are more calyx teeth and fewer stamens. The bell-shaped calyx is
light green to cream-colored with rose
tints; its primary teeth are ovate, while its secondary teeth are
elevated above the primary teeth and they are linear. The slender
teeth are often contorted or crooked and they are ciliate along their
The pedicels of the flowers are light green, glabrous or pubescent, and
short (about ¼" in length). There are 2 or more leafy bracts
underneath each cluster of flowers; they are up to ¾" in length
lanceolate to ovate in shape. The blooming period occurs from
mid-summer to early fall, lasting about 1-2 months.
typically lasts only 1-2 days. During the autumn, the flowers are
replaced by globoid seed capsules about ¼" across. Each capsule
contains many chunky seeds; the seeds probably float on water.
This wildflower reproduces clonally whenever its stem tips touch
moist ground, where new plants will take root. As a result,
colonies of clonal plants often form.
preference is full or partial sun, wet to moist conditions, and soil
that is mucky, peaty, or sandy (or some combination of the preceding).
Swamp Loosestrife is usually an emergent aquatic plant in stagnant or
slow-moving water. However, it also colonizes damp ground near
Swamp Loosestrife is an
uncommon wildflower that is found in scattered areas of Illinois. It is
apparently absent in the NW and east-central
areas of the state. Habitats include marshes and sandy marshes, swamps
and sandy swamps, shorelines along ponds and small lakes,
calcareous fens, and peaty bogs. Sometimes Swamp Loosestrife occurs on
floating mats of vegetation in fens and bogs. It is usually found in
high quality wetlands.
The flowers are
cross-pollinated by honeybees, bumblebees, Swallowtail butterflies, and
probably other insects. These visitors obtain primarily nectar
from the flowers. The caterpillars of Darapsa versicolor
Sphinx) and Eudryas unio
(Pearly Wood Nymph) feed on the foliage, while
the caterpillars of Papaipema
(Burdock Borer Moth) and
(Decodon Borer Moth) bore through the stems. The range of the rare
Decodon Borer Moth is restricted to northeastern United States. The
seed capsules of Swamp Loosestrife are eaten by several ducks:
the Mallard, Black Duck, Blue-Winged Teal, Green-Winged Teal, and Wood
Duck. Muskrats like to feed on the swollen and spongy underwater stems.
Shoreline of a small lake at the Indiana Dunes
National Lakeshore in NW Indiana.
Swamp Loosestrife is both large in size and attractive, especially when
it is in bloom. Its two closest relatives in Illinois, the native Lythrum alatum
(Winged Loosestrife) and introduced Lythrum salicaria
Loosestrife), prefer somewhat drier areas of wetlands. Compared to
Swamp Loosestrife, both of these species have very similar purple
flowers, but they are both erect in their habit of growth, rather than