The bitter yam or air potato (Dioscorea bulbifera Linnaeus) [di os kor' ee a bul bif' er a] has a native range from tropical East Africa across southern Asia. Early on, the Polynesians introduced the bitter yam wherever they settled in the Pacific. It came to the New World from Africa with the slave trade. In 1905, the USDA sent a sample to Henry Nehrling in Florida. (See Plant of the Week, July 26, 2004) Planting it, Nehrling quickly learned of its aggressive growth and recommended against its cultivation. But somehow it escaped.

During warm weather, the vines grow at a rate of twenty centimeters (8 inches) a day reaching lengths of twenty meters (65 feet). The vines climb by twining counterclockwise. In under a century, the yam became a major nuisance species capable of invading and overwhelming even established forests.

The broadly cordate (heart-shaped) leaves belie the fact that yams are monocots (one seed leaf) in the class, Liliopsida, along with lilies, grasses, onions, and orchids. Nor should yams be confused with sweet potatoes (Ipomoea batatas) which are dicots (two seed leaves) in the class, Magnoliopsida.

Like sweet potatoes, yams are known for their tubers. Tubers arise from roots. In the bitter yam, the tuber grows from the hypocotyl that part of the seed's embryo that develops into the radicle or first root. The bitter yam, though, is not known for its tuber, but for its specialized branches called bulbils.

Each node along the bitter yam's vine produces a single leaf and has the capability of growing a bulbil. The bulbil is a storage unit for starches and a propagule, a means of asexual reproduction. In winter, the vine dies back to the tuber and the bulbils drop. Should they land in water, they will float to a new location. In spring, the bulbils root and send up vines.

Since each vine can produce up to two hundred bulbils, the bitter yam's population explodes exponentially. Each tuber and bulbil can produce vines for years. The vines cover other plants starving them of sunlight. Sun starved plants decline and eventually die and animal species dependent on those plants have a choice, leave or also starve, since bitter yams are toxic to most animals.

(Compiled from: "That Infernal Air Potato", Marie Harrison, Master Gardener, IFAS, University of Florida Extension Service, Okaloosa County; "Dioscorea bulbifera L.", Florida Exotic Pest Plant Council; "Dioscoreaceae", Guide to Flowering Plant Families, Wendy B. Zomlefer, University of North Carolina Press, Chapel Hill, 1994; Guide to the Vascular Plants of Florida, Richard P. Wunderlin, University Press of Florida, Gainesville, 1998)