Hog-peanut is a native annual twining vine growing up to 60 inches long on slender light green to reddish stems. Tendrils are absent; the stem twines in a left to right orientation. Stems have either appressed white hair or spreading light brown hair (as in an example shown here).
Leaves are alternate, on long stalks and divided into 3 leaflets, each of which is ovate with smooth margins. Each side leaflet has a short stalk; the terminal leaflet, with a much longer stalk, is also the largest leaflet. At the base of the main leaf stalk is a pair of small grooved stipules. Upper leaflet surface is green to dark green, the underside lighter with fine hair.
The inflorescence is a cluster rising from the leaf axils, consisting of up to 15 5-parted tubular flowers of the pea type. The cluster elongates as the first flowers start to open.
Flowers are of two types: Those that form an open flower on the upper stem are called "chasmogamous" and those that never open at ground level are called "cleistogamous." Both types are bisexual (perfect) and set seed. The 1/2 inch long chasmogamous flowers have 5 petals, white to lavender in color. One petal forms the upright banner (which folds backward), 2 form lateral wings and 2 form a keel, which curves upward between the two laterals. The calyx tube is usually also white or very light green and is formed of 4 fused sepals with 4 pointed teeth on the upper edge. There are 10 stamens in 2 groups; one group of 9 with fused filaments and one singly. There is a single pistil with style. Style and stamens are visible in the mouth of the corolla, the keel petals do not hide them as in some Pea family species. Usually only the banner petal and the two laterals pick up a tint of color, the keel remaining white. Each flower has a short stalk, subtended by small oval bracts. The cleistogamous self-fertile flowers are produced on stolons along the ground. These are without petals and inconspicuous.
Seeds: Fertile stem flowers from the vine produce a flat oblong pod like a garden pea, pointed at the ends, containing 1 to 4 (usually 3) seeds. These are black, shaped like a bean, have a small curved beak and are not edible. The cleistogamous flowers produce a fuzzy brown tough skinned, pear-shaped fruit containing one bean-like seed, that buries itself just below ground level. These are edible. Seed from the upper pods disperse by the dry pod twisting to open. These are scattered a bit from the plant.
Varieties: Two varieties are known, both of which are found in Minnesota: var. bracteata, called "American Hog Peanut" and var comosa, called "Lowland Hog Peanut." The main differences is that comosa has spreading stem hair rather than appressed hair, the leaflets are larger and the flowers show more color.
Habitat: Hog peanut grows in moist soils in woods, meadows and prairies in full sun to partial shade. Other vegetation is helpful to support the vine. It is a nitrogen fixing legume. Propagation is from the scattered upper stem seeds and from the buried seed pods.
Names: The genus, Amphicarpaea, is from two Greek words referring to two kinds of seed (the upper pods and the ground level pod). Amphi meaning 'both' and karpos meaning 'fruit'. The species, bracteata, Latin for 'bracted', refers to having 'bracts' like the small oval bracts subtending each flower. As to the common name, Hog-peanut, the fruits of the self-fertile flowers mature just at or below the ground level or under a mulch if the area is mulched. These provide a good protein source for hogs and other rooters.
The author names for the plant classification are: First to classify in 1753 was '(L.)' which refers to Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy. He established two species - Glycine bracteata and Glycine comosa. His work was amended by ‘Fernald’ which is for Merritt Lyndon Fernald (1873-1950) American botanist, Harvard Professor, scholar of taxonomy, author of over 850 papers, editor of the 7th & 8th editions of Gray’s Manual of Botany. Fernald assigned the plants to Amphicarpaea bracteata and separated the two species into the current varieties in 1933 and 1937 respectively.
Comparisons: Similar species to this are Groundnut, Apios americana, which has 5-part leaves; Trailing Wild Bean, Strophostyles helvola, which has 3-part leaves also, but much more pointed, pink flowers, and the seed pod is a long bean-like pod.
Above: A typical flower cluster and leaf. Note how the banner petal curls backward at the edges.
Below: Flowers of Hog-peanut occur in clusters rising from the leaf axils. Each flower has a short stalk subtended by oval bracts.
Below: The banner petal's edges turn backward, exposing the stamens and the style. Note the pointed teeth on the calyx tube.
Below: 9 of the 10 stamens have fused filaments for most of their length. Anthers are deep yellow at pollen maturity.
Below: 1st photo - Each flower is subtended by a small oval bract. 2nd photo - The leaf stalk ends with a pair of grooved stipules.
Below: 1st photo - The vines are usually a reddish-brown with tawny colored hair. Hair can be white and appressed to the stem also - this is a difference between varieties. 2nd photo - The above ground flowers produce a garden pea-like pod that turns to brown at maturity.
Below: 1st photo - The 3-part leaflet, note the longer stalk on the terminal leaflet. 2nd photo The pods contain 1 to 4 (usually 3) bean-like flattened seeds. Pods have a hairy edge.
Below: The underground tubers of Hog Peanut. Photo ©Merle R. Black, Wisconsin Flora.
Notes: Hog-peanut is indigenous to the original Garden area. Eloise Butler catalogued it on May 25, 1907. It was present at the time of Martha Crone's 1951 Garden census and probably was there when the upland was added to the Garden in 1944. Hog-peanut is found in North America from the Great Plains eastward to the coast. Within Minnesota it is widespread with only about 10 counties not reporting it - most of those in the South Central part of the state. The DNR county census report does not list the varieties separately. This is the only species of the genus Amphicarpaea found in Minnesota.
Uses: Hog-peanut is useful as a nitrogen fixing ground cover. It will climb to a height of about 3 feet and cover most of what it climbs over. As a food source there are a number of references to native use, particularly in the central states area. The tough skin of the subterranean 'peanut' was removed by boiling during which it would crack, or by rubbing after the pod had been soaked in warm water. (Ref. #6).
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"