Amid the predictable fare of the Thanksgiving table there usually sits a bright orange dish - perhaps topped with marshmallows and brown sugar - that adds some sweetness to an otherwise savory meal. Southerners often refer to this as sweet potato casserole; Northerns might say it's candied yams. In this case, the Southerners win. The orange-fleshed tuber on American plates and in pies is a sweet potato, regardless of what some traditions - and grocery store label - say.
For starters, sweet potatoes and yams aren’t even related. Sweet potatoes are from the morning glory family and yams are related to lilies and grasses. Yams - a staple in some West African countries - are native to Africa and Asia, while the sweet potato hails from South America. True yams can’t be found in most American grocery store chains at all, but they are stacked in piles in West African and Caribbean ones.
The tuber confusion traces its roots back at least over a century, explains Joss Fong for Vox, although no one knows exactly when or how the it began. But as Fong explains, it could be that Africans forced into the American slave trade first referred to sweet potatoes as yams because of those roots' similarities. By the late 19th century, the nomenclature ambiguity was widespread enough that Robert Henderson Price, a professor of horticulture at Texas A&M, wrote in his book about the sweet potato: "It would greatly clear up the confusion if this little word 'yam' were dropped in reference to sweet potatoes."
But that never happened. Sweet potatoes are still an important part of Southern cuisine, and they are gaining popularity in the broader U.S. as well. For NPR, Dan Charles reports that production has doubled during the past 15 years. So assuming that sweet potatoes appear on your Thanksgiving plate, we reckon that "Do you know that these yams aren’t yams?" is probably good conversation fodder - in addition to good eating.