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Yams are sprawling tropical vines with edible tubers, widely cultivated as a primary food source in Africa, Polynesia, and South America. Americans sometimes erroneously call sweet potatoes yams, because some sweet potato varieties are marketed as yams, but in fact, true yams are difficult to find in the United States unless they are sought out in a specialty store. Over 150 species of Dioscorea, the yam genus, are grown worldwide, and they vary widely in shape, size, and color, although all of them have sprawling vines with heart shaped leaves and dainty flowers.
The name “yam” comes from a Wolof word, nyam, meaning “something to eat,” or “taste.” When the Portuguese asked the Africans about the large edible tubers which made up a large part of the African diet, the Africans misunderstood the question and responded that the tubers were nyam. The word was adopted by the Portuguese and picked by up other nations as well when the tuber was introduced to them.
True yams come in a range of colors including off-white, yellow, pink, and purple. They can range in size from very small tubers to impressive specimens that measure over seven feet (a little over two meters) in length. Many of them have starchy flesh which must be cooked in order to be palatable, and some species will actually cause severe stomach upset if eaten raw or undercooked. Most African species must be laboriously treated through a series of boilings and pounding in order to be edible, and true yams also have a somewhat bland flavor, with a few exceptions.
Although yams, potatoes, and sweet potatoes are all edible tubers, none of them are actually related. Yams are in the Dioscorea genus, while potatoes are found in the Solanum genus. Sweet potatoes are in yet another botanical group, Ipomoea. All are edible, and all do provide valuable nutrition and starch, but they are entirely separate plants with different growth habits and flavors. Americans, especially, often find themselves dismayed by the flavor of true yams.
Yams are hardy through USDA zone five, depending on the species. They prefer sun or partial shade, and do well with lots of water as long as they are provided with drainage. For this reason, yams are usually planted and staked in large mounds, which prevents them from sitting in standing water. The yams are planted when the tubers begin to make sprouts, usually around February, and are ready to harvest when the vines have died back entirely. They should be stored in a cool dry place, and edible specimens will be unwrinkled and firm to the touch.