Botany
Weed of the Month
July, 2010: Dioscorea alata, the white or winged yam
An important food crop in the tropics, this vine is becoming a serious weed of natural areas in Florida..
Dioscorea alata, female flowers
Photograph courtesy of Forest & Kim Starr, Hawaiian Ecosystems at Risk project (HEAR)
Distinguishing Characteristics: 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; they grow 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 eight inches long and six inches broad; the petioles are often as long as the blades. Flowering is rare in Florida, and the whitish flowers are inconspicuous. Fruits, also rarely produced, are 3-winged capsules borne in pendent racemes.
Dioscorea alata, young growth showing winged stems
Photograph courtesy of Forest & Kim Starr, Hawaiian Ecosystems at Risk project (HEAR)
Distribution: 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 Puerto Rico, and possibly in Louisiana. Its naturalized range is expanding, and potentially could include most of the Southeast.
Occurrence in Florida: Although it was introduced into Florida by Spanish and Portuguese traders in the 1500s, it only recently has become widely naturalized. At present, it is known from 15 counties scattered throughout the state, from Escambia County to Miami-Dade County.
Similar Species in Florida: 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 seldom more than five inches long. In addition, they 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 white yam. It, 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, and they are rounded in shape rather than elongate and their surface is smooth-textured rather than rough.
Dioscorea alata, aerial tubers on the vine
Photograph courtesy of Forest & Kim Starr, Hawaiian Ecosystems at Risk project (HEAR)
Means of Dispersal: The white 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.
Comments: 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 white yam are as vigorous, smothering shrubs and growing high into trees. Both species are listed as noxious weeds by the State of Florida (DPI Rule 5b-57.007, F.A.C.), and as Category I invasives by FLEPPC.
Dioscorea alata, aerial tuber producing roots
Photograph courtesy of Forest & Kim Starr, Hawaiian Ecosystems at Risk project (HEAR)
Further Information: Raz, Lauren. 2002. Dioscoreaceae in Flora of North America 26: 479 – 485; http://www.fleppc.org/ID_book/Dioscorea%20alata.pdf (Accessed: June 30, 2010)
Dr. Richard Weaver, Botanist
Florida Department of Agriculture and Consumer Services
Division of Plant Industry
1911 SW 34 Street
Gainesville, FL 32614
