Marsh Pea is a native, perennial, climbing vine with a much branched smooth sprawling stem of 1 to 4 feet in length. It climbs by means of tendrils at the end of the leaves. The stem is often winged making it appear angled and flattened.
The leaves are alternate and pinnately compound with 2 to 4 pairs of leaflets (usually 3 in our area). What would be the terminal leaflet and the final pair are modified into 3 tendrils. Each leaflet is oval, sharply pointed at the tip with a rounded asymmetrical base. Veins are prominent on the underside which is slightly paler color. Both surfaces of the leaflet are smooth and margins are entire. The entire leaf is about 5 inches by 3 inches with each leaflet up to 1-1/2 inches long. At the base of each leaf are a pair of leaf-like stipules, sharply pointed at each end.
The inflorescence is a stalked raceme rising from the upper leaf axils. Stalks are often to 6 inches long, smooth and pinkish-red colored. The raceme usually has 2 to 6 stalked flowers.
The flowers are in common with the bean/pea family with a pinkish to purplish corolla of five irregular petals, one held upright forming the larger 'banner' or 'standard'. Darker colored veining is often visible on this petal. Two others are laterals and jut out in a forward position, mostly concealing the two joined petals which form the keel. The reproductive parts are enclosed within the keel and consist of 10 stamens and a single carpel with a single style. Stamen filaments are greenish-white with yellow anthers at maturity. The calyx of the flower is a slightly deeper color with 5 unequal lobes.
Seed: Flowers are fertilized by long tongued insects such as bees and produce an elongated flattened green pod with 4 to 6+ round but flattened peas, green initially turning dark brown at maturity. The pod splits into two sections at maturity to release fruit. Much like your typical garden pea pod.
Habitat: Marsh Pea grows from long thin rhizomes which allow the plant to spread vegetatively. As the common name implies, it is found in wet to moist areas where it can receive adequate sun and other plants to climb on. The flowers are attractive and slightly fragrant.
Names: The genus Lathyrus, is from the Greek word lathyros, which referred to a pea plant of olden times. The species palustris, means 'of the marshes'. The author name for the plant classification - 'L.' is for Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy. In past years several varieties were named for this species but most authorities today group them all. An older species name now consolidated into L. palustris is L. myrtifolius.
Comparisons: Other members of the pea family found in Minnesota that are similar with tendrils at the leaf tip and pink flowers are Vicia americana, American Vetch and Veiny Pea, Lathyrus venosus. But in Veiny Pea the leaflets are not opposite each other and in American Vetch there are 4 to 8 pairs of wedge shaped leaflets. Cream pea, Lathyrus ochroleucus, has a leaf-tip tendril but the flowers are white.
Above: The Inflorescence usually has from 2 to 6 stalked flowers rising on a long stalk from a leaf axil.
Below: The large upper petal (the 'banner' or 'standard') is nicely veined and notched at the top. The short calyx is green shading to a darker reddish-pink with five unequal pointed lobes.
Below: The pinnate leaf has 2 to 4 pairs of narrow leaflets arranged slightly sub-alternate, with detailed veining on the underside. The pair of stipules at the leaf base are sharply pointed - detailed in second photo.
Below: Looking like a typical garden variety pea pod, the pod usually contains 4 to 6+ flattened peas.
Marsh Pea is found in most counties of Minnesota with widely scattered exceptions. In North America is is found in all the lower Canadian Provinces and most states of the U.S. except in Rocky Mountain area and a few states in the deep south.
There are six species of Lathyrus found in Minnesota: L. maritimus, Beach Pea; L. ochroleucus, Cream Pea or Pale Vetchling; L. palustris, Marsh Vetchling; and L. venosus, Veiny Pea are all considered native. Two others are introduced: L. latifolius, Everlasting Pea; and L. tuberosus, Tuberous Vetchling. Only L. venosus and L. ochroleucus are found in the Garden.
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"