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 can be pink, bluish purple, or white; sometimes they are bicolored with a veiny pink banner and white wings. The calyx is pale green or yellowish green and glabrous. 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, vegetative colonies of plants often develop from the rhizomes.
Cultivation: 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.
Range & Habitat: 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 Map). 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.
Faunal Associations: The flowers attract primarily bumblebees and other long-tongued bees that feed mostly on nectar. Other insects feed on foliage, flower tissues, or plant juices of Marsh Pea and other Lathyrus spp. These species include Acyrthosiphon pisum (Pea Aphid), caterpillars of the butterflies Everes comyntas (Eastern Tailed Blue) and Leptotes marina (Marine Blue), and Cerotoma trifurcata (Bean Leaf Beetle). In the past, the seeds of Lathyrus spp. were eaten by the extinct Passenger Pigeon.
Photographic Location: An interdunal swale at the Indiana Dunes National Lakeshore in NW Indiana. The photographed plant is Lathyrus palustris myrtifolius.
Comments: Marsh Pea is a variable species that has been divided into several varieties, subspecies, or species in the past. As described by Mohlenbrock (2008), the typical variety of Marsh Pea has winged stems and narrow leaflets, while var. myrtifolius has stems that are angular and leaflets that are more broad. At one time, this latter variety was even considered a distinct species, Lathyrus myrtifolius (see Britton & Brown, 1913/1970, Vol. 2). Today, most authorities no longer recognize these varieties, subspecies, or species, lumping them all together under Lathyrus palustris. Regardless of its variations, Marsh Pea can be distinguished from other Lathyrus spp. in Illinois by the number of leaflets per compound leaf (typically 6), the number of flowers per raceme (2-8), and the shape of its individual stipules (half-sagittate to half-hastate). For example, Lathyrus maritimus (Beach Pea), which is found on beaches along Lake Michigan, has individual stipules with a complete sagittate to hastate shape. Another species, Lathyrus venosus (Veiny Pea), has more leaflets per compound leaf and more flowers per raceme than Marsh Pea. In contrast to the native species, non-native Lathyrus spp. within the state have only 2 leaflets per compound leaf.