“In the U.S., there is no difference between a sweet potato and a yam,” says Sue Johnson-Langdon, executive director of the North Carolina SweetPotato Commission. What Americans commonly refer to as a yam is simply a variety of sweet potato with a particularly moist, bright orange flesh, explains Johnson-Langdon.
It’s all the result of a marketing ploy on the part of Louisiana sweet potato growers in the 1930s who wanted to differentiate their variety of sweet potatoes from the drier, white-fleshed varieties that were being grown on the East Coast, says Tara Smith, a sweet potato extension specialist at the Louisiana State University Agricultural Center. The growers chose yam, the English word that derives from West African words meaning either true yams or “to eat.” (The USDA requires that the vegetables still be identified as sweet potatoes though, so the label will say both yams and sweet potatoes.)
True yams have rougher, scalier skin than sweet potatoes and are often pale-fleshed. They are generally starchier in texture, and are hard to come by in the United States (though Chowhounds have tried). True yams belong to the family Dioscoreaceae, whereas sweet potatoes are in the Convolvulaceae family. Sweet potatoes originate from Peru and Ecuador, while true yams are from West Africa and Asia.