Dioscoreaceae, the Yam Family
This is a deciduous, herbaceous vine, dying to the ground during the winter. The stems are twining, growing to 50 feet or more, with four often purplish wings (cross-shaped or square in cross-section). The tubers are borne in the axils of the leaves and are generally elongated, with a rough-textured surface, growing to as much as 4 inches long. The leaves are opposite, but often alternate at the base of the vine. The blades are heart-shaped, with rounded basal lobes, or arrowhead-shaped, with angular basal lobes; the five to seven arching, longitudinal veins are conspicuous. Leaf blades are as much as 8 inches long and 6 inches broad with petioles (leaf stalks) often as long as the blades. Flowering is rare in Florida, and the whitish flowers are inconspicuous. Fruits, also rarely produced, are three-winged capsules borne in pendent racemes.
This plant is widely cultivated as a food plant, with hundreds of named cultivars. Its area of origin is not known for certain, but is believed to be in Southeast Asia. In the United States, it is reported to be naturalized only in Florida, Georgia and Louisiana. Its naturalized range is expanding, and potentially could include most of the Southeast.
Although the winged yam was introduced into Florida by Spanish and Portuguese traders in the 1500s, it only recently has become widely naturalized. At present, it is found growing outside cultivation in counties scattered throughout the state, from Escambia to Miami-Dade.
Two species of Dioscorea are native to Florida, and an additional three exotic species are naturalized here. The native species, D. floridana and D. villosa, are more delicate, less rampant plants with alternate leaves that are seldom more than five inches long. In addition, these yams do not produce aerial tubers, and their stems are round in cross-section, without wings. The air potato, D. bulbifera, is the species most likely to be confused with the winged yam. The air potato, however, has alternate leaves throughout, and its stems may have low ridges, but they are never winged. Its aerial tubers are borne much more prolifically than those of winged yam. They are rounded in shape rather than elongate, and the surface of air potato tubers is smooth-textured rather than rough.
The winged yam seldom flowers, and fertile seeds have not been found in Florida. It reproduces primarily by means of aerial tubers, or bulbils, which are borne on the stems in the axils of the leaves. These bulbils are not produced as prolifically as in the similar Dioscorea bulbifera, but in sufficient numbers so as to effectively disperse the plant.
This is the most commonly cultivated of the edible yams, and it is an important food crop throughout the tropics. The massive tubers are deeply buried in the earth and may weigh several pounds, but in Florida they usually grow to a more modest size. Although it is not as common as the air potato, individual vines of the winged yam are as vigorous, smothering shrubs and growing high into trees. Both species are listed as noxious weeds by the state of Florida (Rule 5B-57.007, Florida Administrative Code) and the Florida Invasive Species Council as a Category I exotic invasive plant.