Amberique Bean (Trailing Wild Bean) is a native twining annual vine, reaching 3 to 8 feet in length, that by maturity may have a reddish-purple coloration on the stems and leaf stalks.
Stems branch, are circular in cross section or may be slightly angled.
Leaves are alternate, stalked, mostly smooth and three-parted - up to three inches long. The underside of the leaflets is much paler in color with whitish hair on the ribs. Leaflets are ovate in shape. The two lateral leaflets have unequal lobes toward the base, the terminal leaflet usually has equal lobes. There are small stipules at the base of the petiole.
The inflorescence is a single flower or a long stalked loose cluster (a raceme) of 3 to 10 flowers that are each stalkless, with only 1 or 2 open at one time. Clusters rise from the leaf axils.
Flowers: By mid July there are pink to lilac 5 part pea-type flowers. The calyx is green initially, has 5 unequal pointed teeth with fine hair. The upper pair of lobes are usually united. A pair of small green hairy bracts subtend the calyx and are about the same length as the calyx. The corolla has an upper petal (called a standard or banner) that is rounded, about 1/2 inch wide with a slight notch resembling a fold at the top. There are two lateral petals that are slender and project forward partially enclosing two petals that form the keel, which is curved forward and upward toward the inside of the standard. The keel holds the reproductive parts which consist of 10 stamens united at their bases into two groups and a long style the extends into the underside of the banner petal from an extension of the keel.
The fruit matures to a rounded pod up to 3 1/2 inches long that contains 4 to 8 woolly seeds. Pods are green during development and brown to brownish-black when mature. The tip of a pod has the persistent bent style.
Habitat: Amberique Bean (Trailing Wild Bean) prefers dry sandy soil with sun. Being a legume it is nitrogen fixing.
Names: The genus Strophostyles, is derived from two Greek words - strophos, meaning 'twisted' or 'a band' and stylos meaning 'style' and together referring the twisted or 'curved' style of the flower of this genus. The species name, helvola, means 'pale brownish yellow' which may refer to the color of the extension of the keel petals, but that is unclear. The author names for the plant Classification are, first, '(L.)' refers to Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy. His work was updated by ‘Elliot’ which refers to Stephen Elliot (1771-1830) American botanist and collector whose herbarium was the largest in America at the time. He is noted for A Sketch of the Botany of South Carolina and Georgia.
Comparisons: Only one other bean is similar and present in our area - see note at bottom of the page.
Above: A group of Trailing Wild Bean vines. Drawing from Britton, N.L., and A. Brown. 1913. An illustrated flora of the northern United States, Canada and the British Possessions. 3 vols. Charles Scribner's Sons, New York.
Below: In the pea family the 5 petals of the flower are arranged with one usually upright and much larger - called the banner or standard. There are two smaller lateral petals and two that form what is called a keel. The keel encloses the reproductive parts. In Trailing Wild Bean, the keel is curved upward toward the inside of the banner petal (1st photo). In the 2nd photo we see the 5 pointed unequal lobes of the outer calyx. These are hairy, green initially, turning purplish as the flower opens. One mature flower has already formed a green pod (extending left out of the photo.
Below: The calyx is green in these immature flowers. A pair of green bracts with pointed tips are visible subtending the calyx and about as long as the calyx lobes.
Below: 1st photo - As the stem emerges from the ground (left center) it branches considerably. Note the slight angles on the stem circumference. 2nd photo - The three part leaf. Note the lobes of the two lateral leaflets are not symmetrical toward the base, whereas the terminal leaflet is usually symmetrical.
Below: 1st photo - Note the twining stem and the small stipules at the base of the leaf stalk. 2nd photo - you see a mature stem and leaf stalk with stipules. Mature stems turn reddish in color. The leaf stalk can be angled as seen here. 3rd photo - The underside of the leaf is very pale with long whitish hair on the ribs and margins.
Below: 1st photo - Without other plants to climb on, the stems sprawl along the ground inter-twining with each other. 2nd & 3rd photos - The seed pods look like regular green bean pods without the curves, green initially and then turning dark brown at maturity with a persistent style at the tip and finally spliting and coiling to open and release seeds.
Notes: Amberique Bean was not listed by Eloise Butler as indigenous to the Garden. It was listed on Martha Crone's 1951 census of plants in the Garden. She reported planting seeds of the species in 1947. It is native to Minnesota, but in the wild, it has only been found in 11 scattered counties, most in the southern part of the state; these are Hennepin, Scott, Ramsey, Washington, Goodhue, Wabasha, Houston, Lac Qui Parle, Nicollet, Brown and Blue Earth. The plants native range in the U. S. is from the Central states eastward. The beans may have been gathered for food but there is little in the literature about them.
Amberique Bean (Trailing Wild Bean) is one of two species of Strophostyles found in Minnesota. The other is S. leiosperma, Slick-seed Fuzzybean (also called Trailing Pea). It is more widely spread but still only known in 17 counties. S. leiosperma has densely hairy leaves, a much smaller flower and a shorter seed pod that is very hairy when green.
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.
Identification booklet for most of the flowering forbs and small flowering shrubs of the Eloise Butler Wildflower Garden. Details Here.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"