After a recent conversation with a colleague regarding the difference between sweet potatoes and yams with no real conclusion, I felt compelled to get to the bottom of this.
First, in the United States, yams, an edible tuberous root vegetable, are actually sweet potatoes (Ipomoea batatas), a member of the bindweed or morning glory family, Convolvulaceae. Sweet potatoes can range in shape, size, and color, and will have a tapered end. On the other hand, botanically different, the true yam is the common name for some of the plants in the Dioscoreaceae family. This root has a very thick, bark-like skin with dense, dry flesh which can range from white/yellow/purple or pink - not orange, and can grow to over a hundred pounds. Completely different vegetables, neither are actually a "potato" which is in the nightshade family.
So how did this confusion begin? Very simply, as part of a marketing strategy. Producers and shippers branded the sweet, orange fleshed sweet potato being grown in the south as 'yams' to differentiate it from it's lighter relatives grown in the north. Often mislabeled in our fresh produce aisle, the USDA actually requires that 'Yam' is followed by 'sweet potato' on shipping box labels - most canned products do adhere to this requirement.
If seen in the market, true yams are typically imported from the Caribbean, Asia and Latin America and can usually be found in ethnic grocery stores. Note: Unlike sweet potatoes, these tubers can be toxic if not cooked thoroughly.
So next time you gather with family, pass the sweet potatoes please and enjoy.