Yams and sweet potatoes are in fact two different root vegetables. And unless you shop in a specialty store, it’s likely that you’ve only purchased sweet potatoes (even if they were labeled as yams).
Yams are the tuberous roots of the genus Dioscorea. They’re native to parts of Asia and Africa, and can grow to weigh over 100 pounds. The word yam is derived, via Portuguese or Spanish, from a West African language called Wolof. The Wolof word nyam means “to sample” or “taste.” Similar words in other African languages for yam mean “to eat” and “to chew.”
Like the yam, the sweet potato is grown for its edible root. But unlike the yam, it is not part of the Dioscoreaceae family. Sweet potatoes are native to South America, and they were the main source of nourishment for early Europeans in the Americas.
So, why do we get them confused?
In general, there are two kinds of sweet potatoes. The firm, white variety was cultivated first in the United States. But once a soft variety was developed for a commercial market, people wanted a way to distinguish between the two. Because of their resemblance to yams grown in Africa, African slaves in North America had already been referring to sweet potatoes as yams. So, the name stuck.
The U.S. Department of Agriculture tries to help confused consumers by requiring that sweet potatoes that are called “yams” are also labeled sweet potatoes. But whether you call them yams, sweet potatoes, or yams-that-are-actually-sweet-potatos is up to you!