My dear friend and colleague at Alien Plantation has a post up about the frustrations of vernacular names: when people call the same organism by different terms, or mean different organisms when using a particular name. And one classic example of this frustration, one that I seem to find myself explaining often, is yams versus sweet potatoes.
What’s the difference, you ask?
The orangey (but sometimes yellow or purple) root vegetable that you may have eaten topped with marshmallows, or nibbled in french fry form at a pub with that not particularly good but ubiquitous “chipotle mayo”, is called “sweet potato” in most places, but also “yam” in many parts of North America. (Including where I grew up, so, for the record, I call them yams.) This yam/sweet potato is properly called Ipomoea batatas. (Ipomoea is usually pronounced “eye-poe-MEE-ah”.) Also in the genus Ipomoea are the morning glories, those vines with wide, trumpet-shaped flowers in many gardens and roadsides.
There is another root vegetable commonly called “yam” in English that’s a dietary staple in Africa, Asia, and Latin America. These yams belong to several species in the genus Dioscorea, distant relatives of arrowroot. Like Ipomoea tubers, Dioscorea tubers can range in colour from yellowish to purple, but they (at least the cultivated varieties) tend to be much larger and have a thicker, rougher skin. Also like Ipomoea, Dioscorea species are usually vines. However, they have tiny, inconspicuous flowers, often arranged on an inflorescence or flower stem.
Dioscorea and Ipomoea may have some superficial similarities, but they’re only distantly related. One of the fundamental divisions among flowering plants is that between monocots (including grasses, palms, lilies, and onions) and dicots (including most non-coniferous trees, roses, and sunflowers); Dioscorea is a monocot while Ipomoea is a dicot.
If you’re in North America (and if you have the yam/sweet potato problem you almost certainly are), unless you’re at a particularly fancy/”exotic”/”ethnic” grocery store*, anything labelled “yam” or “sweet potato” is almost certainly Ipomoea batatas. (I have seen different varieties of the plant sold as “yam” and “sweet potato” right next to each other in the same store!) So if someone asks you if they’re eating yam fries or sweet potato fries, the answer is “yes”. If you’re at an African or Asian restaurant, though, there may be Dioscorea yams in your meal. One of the more common varieties, ube (Dioscorea alata), is used in the Filipino dessert halo-halo. (Sweet potato rolls and tempura at sushi restaurants in North America are Ipomoea.)
Wikipedia tells me that there’s another tuber, that of Oxalis tuberosa, native to the Andes but also grown elsewhere, that’s called “yam” in New Zealand. The local variety of Ipomoea batatas there is called kumara, so you shouldn’t have too many problems.
Here I present, in convenient, printable, wallet-sized form, a brief guide to yams and sweet potatoes:
*Stop the presses! After I finished the draft of this post I went grocery shopping, and lo and behold there were Dioscorea tubers.