perennial wildflower is a herbaceous vine about 1½-4' long that
occasionally. This vine climbs adjacent vegetation for support using
its tendrils. The stems are light green, yellowish green or reddish
green, glabrous or nearly so, and winged.
Alternate compound leaves occur at intervals along these stems; they
are even-pinnate with 3-4 pairs of leaflets. At the end of each
compound leaf, there is a branched
The leaflets are ¾-2" long and about ¼" (6 mm.) across; they are
narrowly elliptic in shape, smooth along
their margins, and sessile or nearly so. The upper blade surface is
medium green, while the lower blade
surface is pale green. All parts of the compound leaf are glabrous or
nearly so. The petioles and rachises of the compound leaves are light
green, yellowish green, or reddish green; they are glabrous or nearly
so. At the base of each petiole, there is a pair of stipules about
¼-1" long. Each stipule is half-sagittate or half-hastate in shape; it
has a basal lobe that tapers to a point, a terminal tip that tapers to
and an outer
margin that is mostly smooth, although it may be slightly undulate or
toothed. Sometimes the foliage of this wildflower
is sparsely and minutely pubescent (puberulent).
Individual racemes of
2-8 flowers develop from the axils of compound leaves; the
peduncles of these racemes are about as long as, or a little shorter
than, the length of the compound leaves. Each flower is about ½-¾"
long, consisting of 5 petals with a pea-like floral structure, a
tubular calyx with 5 teeth, several stamens, and a pistil with a single
style. The petals consist of an upright banner and a pair of projecting
wings that enclose the keel. These petals are mostly reddish purple;
although the petals forming the keel and the base of the remaining
petals are more pale. The banner has a network of purple veins. The
calyx is reddish purple and mostly glabrous, although its teeth may be
slightly ciliate along their margins. The blooming period occurs from
late spring to mid-summer,
lasting about 1 month. Afterwards, the flowers are replaced by
elongated seedpods about 1-2½" long, ¼" across, and flattened.
Each seedpod splits open into two valves to release its seeds. The
small seeds are globoid in shape and somewhat flattened. The root
system is fibrous and rhizomatous. At favorable sites, clonal
colonies of plants often develop from the rhizomes.
The preference is full or partial sun, wet to moist conditions, and
soil containing loam or sand. Standing water is tolerated if it is
occasional, rather than permanent. In the absence of some kind of
structural support, this vine will sprawl across the ground.
The native Marsh Pea is occasional in
the northern half
of Illinois, while in the southern half of the state it is rare or
absent (see Distribution
). More broadly, it has a circumboreal
distribution, occurring in both North America and Eurasia. Habitats
include wet prairies, interdunal swales, borders of marshes, fens,
sedge meadows, low areas along streams, soggy thickets, and
swamps. This wildflower occurs in both sandy and non-sandy habitats.
The flowers attract primarily bumblebees and
long-tongued bees that feed mostly on nectar. Other insects feed on
foliage, flower tissues, or plant juices of Marsh Pea and other
These species include Acyrthosiphon
caterpillars of the butterflies Everes
(Eastern Tailed Blue)
and Leptotes marina
(Marine Blue), and Cerotoma
Beetle). In the past, the seeds of Lathyrus
were eaten by the
extinct Passenger Pigeon.
Moist sandy ground near a river at Illinois
Beach State Park in Lake County, Illinois.
The typical variety of Marsh Pea (Lathyrus palustris palustris
as described here, has winged stems, narrow upright leaflets, and
relatively dark reddish purple flowers. Mohlenbrock (2014) also
describes another variety of Marsh Pea (Lathyrus palustris
that has stems without wings. Some authorities do not recognize
distinct varieties, preferring to lump them together as a single
species. This variety of Marsh Pea can be distinguished from other Lathyrus
(Vetchling species) by the number leaflets per compound leaf (typically
6 or 8), the shape and size of its stipules, the color of its flowers,
and the number of flowers per raceme. Another species, Veiny Pea (Lathyrus
differs from the typical variety of Marsh Pea by having more leaflets
per compound leaf, wider leaflets, and more flowers per
raceme. Non-native Lathyrus spp.
(Vetchling species) in Illinois are quite distinct because they have
only 2 leaflets per compound leaf.