Know Your Natives – Forest Pea

Forest pea or bushy vetch (Lathyrus venosus) of the Bean or Legume (Fabaceae) family is a perennial deciduous vine with pea-like flowers. The genus name is based on the Greek Lathyros, the ancient name for a leguminous plant. The specific epithet is from the Latin for “conspicuously veined,” in reference to the banner, the large, uppermost petal of the legume flower. In the U.S., forest pea occurs primarily in three areas: 1) an area from far-east Texas and northwestern Louisiana to southern Missouri, 2) an area from the central Great Lakes region that extends westward into the eastern Dakotas northward to the Canadian border, and 3) an area along the Appalachian Mountains with a concentration in central Virginia and West Virginia. In Arkansas, the species occurs primarily in the Arkansas Valley, Ouachita Mountains, and higher elevations of the West Gulf Coastal Plain. Preferred habitat is moist to dry slopes in open deciduous woodlands.

Forest pea has shallow ropy rhizomes terminating with a knobby vegetative bud that produces new growth the following year. Fibrous roots are mostly limited to year-old rhizomes. Like most legumes, forest peas have a symbiotic relationship with bacteria which “fix” atmospheric nitrogen in root nodules, converting it to a form the plant can use.

Photo 1: This rhizome produced three stems (greenish) and four new rhizomes (white). Lowermost rhizome terminates in a knobby vegetative bud. Note the small clusters of nitrogen-fixing root nodules. Photo – May 23.

Stems, to 2-3 feet long, are fairly stout and ascending; however, unless anchored by tendrils at the leaf tips, they lack the strength to remain upright and tend to sprawl. They are prominently angled and glabrous to finely pubescent. New stems appear in mid-winter.

Photo 2: In this natural setting, an open colony of forest pea has developed. Photo – April 18.

Ovate to elliptic leaflets, to 2 inches long and ⅞ inch wide, are rounded at the base and narrowed to a blunt, sometimes mucronate (pin-point) tip. Margins are entire. 

Photo 3: Single large compound leaves with butterfly-shaped stipules grow from swollen stem nodes. Note the axillary inflorescence also growing from the node.
Photo 4: This actively growing stem shows two new leaves with rolled-up leaflets (the younger leaf at the tip of the stem subtended by its stipules). The dangling raceme is growing from the base of the larger rolled-up leaf.

The inflorescence, in late March to early May, consists of a single erect raceme to 6 ½ inches long with 10-20+ closely spaced, pea-like flowers along the upper third of a slender peduncle. Racemes arise at the swollen leaf bases of several upper leaves. Flowers are about ⅝ inch long and ⅜ wide with a short pedicel.

The dangling flowers bear colorful, tubular calyxes (to ¼ inch long) with three larger acutely pointed lobes on the lower side and two smaller in-turned, pointed lobes on the upper side. The corolla exhibits the typical structure of the so-called papilionaceous flower of the legumes, comprising a large banner petal (flared laterally and apically), a free (unattached) pair of smaller wing petals, and a fused pair of keel petals. The projecting wing petals (opening downward) form a hood which encloses the beaked keel (opening upward). The keel encloses 10 stamens and a pistil, all well hidden. Nine of the staminal filaments are fused most of their length into a tube that encloses the pistil. One filament––the uppermost––is usually free from the staminal tube. The pistil (of a single carpel) comprises a greenish, elongate, straight, flattened ovary tipped by a sharply up-turned style which tapers to an elongate stigma. Flower color is a blending of pink and white with the banner more strongly colored and prominently veined.

Photo 5: Early in the raceme’s growth, the three larger lobes of the calyx, positioned below the flower, are a dominant feature. Note the stipules and the flat sides of the angled stem.
Photo 6: Along with the prominent banner, flowers have a prominent “nose” composed of two free wing petals covering the keel. In this photo, the keel is not visible. Note the colorful calyx with two short upper lobes.
Photo 7: Display of: 1) complete flower, 2) lower portion of calyx, 3) upper portion of calyx, 4) banner, 5) free wing petals, 6) fused keel petals, and 7) stamen tube surrounding pistil with protruding stigma and style.

With fertilization, long, flat, yellowish green pods develop to a mature length of about 3 inches. When mature and dry, pods dehisce and the smooth sides (valves) twist back to expose the seeds. The brown globoid seeds, up to a dozen per pod, are smooth with a lighter hilum. Seed dispersal by small mammals and birds.

Photo 8: As racemes transition from flowering to fruiting, the corollas disintegrate.
Photo 9:  Display showing immature pods and a tendril at the end of a leaf. Lowest pod is 3¾ inches long. Photo – May 29.

Forest pea would be a good choice for a woodland garden or natural area, with its loose growing habit and leafy character. Flowers and fruits are interesting, but not especially showy. It is not aggressive by rhizomes or seed. It is also apparently not eaten by deer.

In Arkansas, other species of the genus are non-native yellow vetchling (L. aphaca), non-native singletary pea (L. hirsutus), non-native everlasting pea (L. latifolius), and the native yet uncommon low vetchling (L. pusillus). Forest pea is readily distinguishable from these species based on its large compound leaves which have five to seven pairs of leaflets and terminal tendrils. The prominent veins of the banner also distinguish the species.

Forest pea may be confused with species in the vetch (Vicia) genus (nine of which occur in the state). Flowers of Lathyrus species have free wing petals whereas wing petals of Vicia species are adnate (fused) to keel petals. Also, leaflets of Lathyrus species are larger than those of Vicia species.

Article and photographs by ANPS member Sid Vogelpohl

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