Wild Yam is a plant of much variability depending on where in its range it is found, whether in the understory or in alluvial areas, in full sun or full shade, and in the various soil types. it is a native perennial herbaceous vine growing from 3 to 18+ feet long on twining stems the are cylindrical to slightly tapering in cross section and usually with a longitudinal groove. The lower stem portion is somewhat rigid, ridged and woody but the upper portions and smooth and flexible. Stem color can vary from greenish-yellows to much deeper color. Twining is counter-clockwise.
Leaves on the lower portions of the stem can be alternate to sub-opposite or in whorls of 3 to 7 leaves (particularly in an understory). Upper stem leaves are always alternate. The long leaf stalk is usually ridged and frequently longer than the leaf blade which is ovate in shape, margins entire, 9 to 11 prominent veins, the tip pointed, the base heart-shaped. The upper surface is usually smooth and lustrous green, but the underside can have fine hair to dense short hair. Young leaves have a golden-green cast.
The inflorescence: Wild Yam is dioecious, that is, it has male (staminate) and female (pistillate) flowers separated on different plants. The staminate flowers occur in a solitary inflorescence from the leaf axils, in the form of a drooping panicle that is widely branched, each cyme on the panicle having 1 to 3 flowers. Pistillate flowers occur in a drooping spike-like raceme, 2 to 4 inches long, with 4 to 18 flowers.
Flowers: Staminate flowers have a greenish-white perianth, bell to funnel shaped, very small - about 2 mm in diameter. Petals and sepals are combined as 6 tepals, ovate to elliptic in shape with the tips rounded to slightly pointed, united only at their bases. Stamens are erect but in two whorls of unequal length. with the anthers about 1/2 the length of the filaments. The pistillate flowers are similar in design to the staminate and have 6 staminodes (sterile stamens) with the anthers less than 1/2 the length of the filaments. The ovary is 2 to 3x the length of the perianth and has a 3-branched style.
Seed: Fertile flowers form a greenish-gold papery capsule, ovoid to more broad at the apex in shape, varying much in size, from 1 to 3 cm wide by 1 to 3.5 cm long. The capsule is 3-sectioned with usually 2 thin winged seeds per section. The capsule sections looking like wings. The capsule turns dark brown in the fall and opens along the seams of the section to release the seeds. The remains of the perianth and style usually still attached. Seeds require 60 days of cold stratification for germination.
Habitat: Wild Yam grows in partial sun to shade with wet-mesic to mesic moisture conditions. Shade all the time may not produce many flowers or capsules. Even in the full shade, the leaves are the decorative part of the plant. The root system is rhizomatous, the rhizomes brown in color, varying in shape from linear to highly contorted. The rhizomes branch and grow longer allowing the plant to put up multiple stems in an enlarging area, but it does not aggressively spread. Only a short 2 to 3 inch section is needed to start a plant. There is much literature on the medicinal effects of the root. See notes at bottom of the page. In the wild, the plant is found in woods and river-bottom forests and roadsides.
Names: The genus Dioscorea, is named for the 1st century Greek physician and herbalist Pedanios Dioscorides. The species name, villosa, means 'covered with soft hairs' which refers to the underside of the leaf. The author name for the plant classification - 'L.' refers to Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy.
Comparisons: The Dioscorea are somewhat distinct with the drooping inflorescences of very small flowers on a vine with long-stalked leaves and 3-sectioned seed capsules that are usually winged. Other native species of Dioscorea are only found in the SE states of the U.S. and most introduced species of the genus are found in that same area. Several plants of the Smilax genus may have a similar appearance to Wild Yam, particularly Smilax tamnoides, Catbrier. Look also at the two Carrion Flowers, Smilax ecirrata, Upright Carrion Flower and Common Carrion Flower, S. lasioneura. Leaves and stems are somewhat similar, but not the flowers and seeds. A comparison drawing is given below.
Above: A web of vines with young leaves and staminate inflorescences forming. 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: A young section of the vine with the branching staminate inflorescences just forming.
Below: 1st photo - A staminate inflorescence with flowers open. 2nd photo - A pistillate inflorescence. Pistillate flowers have six false stamens.
Below: A branching section of the root.
Below: 1st photo - Typical leaf showing the quilted pattern of the main and secondary veins. 2nd photo - the underside is pale in color with fine hair.
Below: 1st photo - Stem sections can be rigid and woody with a deep coloration. 2nd photo - Mature seed capsules. Each wing is 1/3 of the 3- sectioned capsule. Photo - Derek Anderson, Wisconsin Flora.
Below: A drawing comparing Dioscorea villosa with a similar looking vine - Smilax tamnoides. Drawing ©Flora of North America.
Wild Yam is not indigenous to the Garden. Eloise Butler introduced it in Sept. 1907 with plants obtained at Mahtomedi MN. She added more in 1909 and 1910. Martha Crone planted it in 1947. The species is found in Minnesota in only about 20 counties on the east central to southeast section of the State from the Metro area to the Iowa border. It is the only species of Dioscorea in the state.
In North America it is native to the eastern half of the U.S. excepting Maine and New Hampshire, and is found in Ontario in Canada. There are six species of Dioscorea in North America, the others restricted to the Gulf Coast States, but there are about 600 species world-wide.
Lore and Uses: The roots of this species are not edible as they are acrid but medicinal uses have been found from folk medicine to laboratory drugs from certain root chemicals. The root chemicals dissipate after a year so fresh roots are needed. The most common folk use was to treat nausea and spasms during pregnancy and treatment for colic. The root contains the saponin 'diosogenin' which is used in the lab to make progesterone and other steroids. This article from the National Institutes of Health explains part of this.
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"