Know Your Natives – Snoutbean

Snoutbean (Rhynchosia latifolia) of the bean or legume (Fabaceae) family is a perennial herbaceous plant with large trifoliate leaves. The genus name is from the Greek for “beak,” referring to the shape of the keel petals. The specific epithet is from the Latin for “broad leaves.” Snoutbean occurs from eastern Texas and Oklahoma, through southern Missouri, Arkansas, and Louisiana to western Tennessee and Mississippi. In Arkansas, it occurs across much of the state except for the Mississippi Alluvial Plain and Crowley’s Ridge. Habitats are sandy, partially shaded to sunny pinelands, savannas, and prairies.

Snoutbean has a woody, knobby, vertical root which may be a foot or more long. Current year’s stems grow from the roughened swollen crown at the soil surface.

Photo 1: The woody vertical roots are distorted. The center root, with lower portion removed, is 8 inches long.
Photo 1: The woody vertical roots are distorted with constrictions and knobby swellings. The center root, with lower portion removed, is 8 inches long.

Plants may have one to several, densely hairy, yellowish green stems, often with axillary branches along their lower portions. Initially erect, they become reclined and trailing, while the actively growing tips are twining. They grow to 4 feet long, supporting each other and sprawling against other vegetation. 

Leaves are densely short-pubescent, alternate, and pinnately trifoliate (the terminal leaflet is stalked but the laterals are subsessile), with a pair of small, lanceolate stipules at the base of the petiole. The lowermost leaves are simple. Leaflets are broadly ovate-orbicular with rounded bases and sides. Larger leaves may be 5¼ inches long and 5½ inches wide, with a terminal leaflet 3 inches long and 1¾ inches wide and a pair of opposite lateral leaflets 2½ inches long and 2 inches wide. Leaf size gradually decreases distally.

Photo 2: In spring, stems have simple lowermost leaves and trifoliate leaves higher up. The beginning of axillary branches can be seen, along with a stipule at upper left. Photo - April 21.
Photo 2: In spring, stems have simple lowermost leaves and trifoliate leaves above. The beginning of axillary branches can be seen, along with a stipule at upper left. Photo – April 21.
Photo 3: The trailing to twining stems become attached to each other and sprawl against other vegetation. The ferny plant at left if yarrow (Link: Feb 13, 2020). Photo - June 8.
Photo 3: The trailing to twining stems become attached to each other and sprawl against other vegetation. The ferny plant at left is yarrow. Photo – June 8.

Photo 4: This actively growing stem is 3½ feet long. The pinnate-veined leaves have three main veins (midrib and two secondary veins) which connect at leaflet base.
Photo 4: This actively growing stem is 3½ feet long. Note the distinctive venation of the leaflets. 

The inflorescences consist of elevated, axillary racemes of up to 15 or more flowers along the upper portion of the stems. The densely short-pubescent calyx, less than a half inch long, consists of three broadly lanceolate free-standing sepals below the corolla and a fused pair of similarly shaped sepals above. 

Photo 5: Positioning of flowers along the erect racemes varies from alternate to whorled. The persistent calyxes, when flowers are in bud, are snout-like. Photo - June 17.
Photo 5: Positioning of flowers along the erect racemes varies from alternate to whorled. In bud, the calyx is snout-like. Photo – June 17.

The bright yellow corolla is “papilionaceous,” a butterfly-like structure typical of many legumes, comprising a large, erect banner petal (flared laterally and apically) at the top, a free pair (not fused together) of smaller oblong wing petals projecting forward, and, within and below the wings, a pair of cupped-ovoid petals that together form a V-shaped keel. The wing petals (opening downward) form a hood over the keel (opening upward). The keel encloses 10 stamens with white filaments and light yellow anthers as well as a white pistil. Nine of the staminal filaments are fused into a tube, along their lower portion, which encloses the pistil. One filament, the uppermost, is free from the staminal tube. The pistil consists of a straight ovary tipped by a sharply up-turned style and terminal stigma. The corolla is about ½ inch long with a banner about ½ inch wide. The loosely attached wing and keel petals project ¼ inch beyond the banner. Flowers have short (less than ⅛ inch) pedicels. Flowering may extend over a month or more, primarily in June.

Photo 6: Flowers at center and right are whole while flower at left is shown without wing and banner petals. The free stamen and the staminal tube of the flower at left can be seen (pistil hidden the staminal tube). The position of the five sepals can be seen on flower at center.
Photo 6: Flowers at center and right are whole while flower at left is without wing and keel petals––note the tube of 9 fused stamens and, above it, the single free stamen; the pistil remains hidden within the staminal tube. The position of the five sepals can be seen on the flower at center.

Fertilized ovaries develop into flattened green, densely pubescent pods which become dark brown at maturity. Pods, with persistent calyxes and short down-turned terminal beaks, are ½ to ¾ inch long and ¼ inch wide. They split from base to tip to release two irregularly rounded, smooth, shiny, dark brown seeds. Diameter of seeds is about ⅛ inch.

Photo 7: The green pods become brown at maturity. Pods are ascending. Twining stems can be seen. Photo - July 18.
Photo 7: The green pods become brown at maturity. Pods are ascending. Twining stems can be seen. Photo – July 18.
Photo 8: The densely pubescent, beaked pods retain the calyxes. Pods contain 1 or 2 seeds.
Photo 8: The densely pubescent, beaked pods retain the calyxes. Pods contain 1 or 2 seeds. Scale – squares equal 1/4 inch.

In considering snoutbean for a garden, with its deep roots and good seed germination, it is probably best suited for a natural area––it has a somewhat untidy growth habit. Large shamrock-shaped leaves and showy flowers make it a decorative and interesting plant. Snoutbean prefers sandy and rocky well-drained soil and partial to full sun. It is not eaten by deer.

At least two other species in the genus have been reported from Arkansas. Least snoutbean (Rhynchosia minima) has only been documented in Chicot County. It can be distinguished as a smaller plant with more rhombic leaflets, descending flowers, very short calyxes and curved pods. Double-form snoutbean (Rhynchosia difformis) may be widely scattered in central and southern Arkansas. It can be distinguished by its shorter, compact inflorescences and shorter calyxes.

Article and photographs by ANPS member Sid Vogelpohl

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