Tag Archives: Dioscorea

Sweet Potato Awareness Month

November is Sweet Potato Awareness Month (SPAM), and I do my part by reminding people that yams are something else entirely. As a foodie and an evolutionary biologist, I feel obliged to be a nudge about this. So here are three images to help.

First, a photograph of a white yam (Ipomoea rotunda) in a bin of sweet potatoes.

Colin Purrington Photography: Blog photos &emdash; A white yam (Dioscorea rotundata) in a bin of sweet potatoes (Ipomoea batatas)

Second, a photograph of three cultivars of sweet potato (all Ipomoea batatas) next to a yellow yam (Dioscorea cayennensis).

Photograph of sweet potatoes and yam

Third, an illustration of how yams and sweet potato are related (they aren’t). As a bonus, I’ve also indicated the position of potato.


Please share this page with your family prior to Thanksgiving dinner. It will be one less thing to bicker about. If you need more details, here’s my “Yams versus sweet potatoes” page. Read it if you want to know why the slave trade caused the whole “yam” confusion problem.

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Sweet potatoes and yams

Some photographs to get you excited about starches on Thanksgiving.  If you’re curious about the difference between yams, sweet potatoes, and potatoes, please refer to my page, “Yams and sweet potatoes are not potatoes.”

colin purrington photography: plants &emdash; yam-display-at-ethnic-grocer
The bin of “#1 American yams” is filled with sweet potatoes. The labeling mistake is a common one. The white yam and yellow yams are actual yams.
colin purrington photography: plants &emdash; purple-yams
Okinawan sweet potatoes label as yams. They are not yams.
colin purrington photography: plants &emdash; oriental-sweet-potato
Oriental sweet potatoes. Note that sweet potatoes are not potatoes.
colin purrington photography: plants &emdash; potatoes
Potatoes are unrelated to sweet potatoes.


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Yams and sweet potatoes are not potatoes

If you’re celebrating Thanksgiving today, you’ll soon be having the annual argument about whether yams and sweet potatoes are different.  They are.  They really are.  Sweet potatoes are dicots and closely related to morning glories (the vine with pretty flowers and hallucinogenic seeds).  And yams are monocots, like grass.  Yams are vines, too, by the way, and the original source of birth control pills (fyi, they don’t work if you just eat lots of them).

Yams versus sweet potatoesIn case you slept through your evolution class, monocots and dicots diverged 150-200 million years ago — that’s during the Mesozoic, when dinosaurs were diversifying.  A bonus fact to wow your family with is that neither yams nor sweet potatoes are potatoes, which are in the deadly nightshade family (wild potatoes, pre-domestication, are toxic). The phylogeny (“family tree”) based on their chloroplast genomes is shown above. There are, of course, lots of other plant species, but you should get the picture — sweet potatoes are grouped with kin-folk, and yams are grouped with their kin-folk.  They look and taste a lot like each other but are completely different beasties.  Yams are more expensive to produce, so sweet potatoes in stores are often labeled as yams.  If you think you are eating yams today, chances are you’ve been duped.  But with enough marshmallow and butter, you probably won’t notice.

Of course, if you kin-folk don’t accept an Old Earth or evolution, your conversation at the dinner table is probably going to be fundamentally different (so to speak).  Can’t help you there.

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