Yams are vines cultivated for ornament or for their edible tubers. They are native to the South Pacific islands, but their culture has spread to other tropical areas. Japan, China, Australia, India, Africa, the West Indies, South America, and the southern tip of Florida all grow yams. In many of these places, yams provide an important part of the diet. The sweet potato is sometimes called a yam, but it is of an entirely different genus (Ipomoea).
Most yams do best in near-tropical climates. Their tubers may be planted any time of the year in warm, sandy soil. Place them two to three feet apart in rows about five feet apart. Some species produce their tubers above ground in leaf axils; others produce them so far underground that they are difficult to dig. For optimum yields, stake the vines.
The Chinese yam or cinnamon vine (D. Ba-tatas) bears cinnamon-scented flowers and aerial tubers which are used for propagation, as well as large edible, deep-growing under-ground tubers. It is grown for ornament as well as for food and is hardy as far north as New York although it will not always produce edible tubers. The air potato (D. bulbif era) has no big underground tubers, but is grown in the South and in greenhouses for the odd tubers borne in the axils of the leaves which are some-times eaten like potatoes. Yams contain more protein and less starch than potatoes.
The yampee (D. trifida) is another southern vine with small underground tubers, prized for their flavor, while the wild yam (D. villosa) grows along the Atlantic and Gulf coasts and has a woody rootstock. There are many other edible species, mostly tropical, some of which have tubers weighing up to 100 pounds.