Tag Archives: botany

Yams versus sweet potatoes

Photograph of cut Okinawan purple sweet potato, Beauregard sweet potato, oriental sweet potato, and a yellow yam.This page is for people bickering over whether there’s a difference between sweet potatoes and yams. There is. Although both taste great, yams (on right in photo) are usually bigger, often asymmetric, and have rough, bark-like skin.

And they are completely unrelated. Sweet potato (Ipomoea batatas, in the family Convolvulaceae, with morning glory) is a dicot, whereas yams (many species, all in the family Dioscoreacea) are monocots, like onions and grass. In case you skipped high school biology class, monocots and dicots diverged some 200 million years ago. Neither is related to the potato (Solanum tuberosum, in the Solanaceae, with tomatoes and eggplants). If you teach biology, you can prove it to your students by trying to align DNA sequences; phylogeny estimation will place them as distantly related. Their locations on an evolutionary tree of all plants is shown at right.

yams-and-sweet-potatoes-treeUnless you live in West Africa, Asia, or the Caribbean (the places where yams are grown) chances are good that you haven’t eaten one. The exception would be foodies who eat at unusual restaurants and shop at places like H-Mart.  If you’re a typical American, you’ve probably never even seen a yam in your life, even on TV. The only real exposure most Americans have to yams is when females pop progesterone-containing birth control pills — the progesterone is made from a yam saponin (diosgenin).

Yam festival celebrations

Photograph of yams by Jeff Haskins

I mentioned above that yams are usually larger than sweet potatoes. As proof, below is a photograph (by Jeff Haskins, for Global Crop Diversity Trust) from a yam festival (apparently before it got festive) showing a nice array of large yams.

But they can get much bigger. Here’s a 304 lb yam in Fiji. And a 606 lb one in India.

In case you’re curious, the confusion about yams and sweet potatoes started several hundred years ago. Yam (the word) is West African in origin (anyinam means yam; nyami means “to eat”). That “yam” is from a West African language rather than from one of the 1000s of other language elsewhere in Africa or in Asia is a grim reminder that slaves were taken from Western Africa, largely. When Africans were taken to areas planted with sweet potatoes (like on George Washington’s farm) most likely the slaves called them yams, even though they weren’t. As a result, some varieties of sweet potatoes were sometimes called yams (e.g., see this book from 1900), especially in the South and especially in regards to sweet potato cultivars that were soft and extra sweet. This caused confusion early on, of course, since most people knew they weren’t really yams. Indeed, after weeks of looking I dug up a 1921 publication (The Sweet Potato: A Handbook for the Practical Grower) that advised that the habit of referring to sweet potatoes as yams “best be dropped.”

Unfortunately, the marketing of sweet potatoes as yams was formally adopted by growers in Louisiana:

The Louisiana industry coined the term ‘yam’ in 1937 as part of a national marketing campaign …” — La Bonte and Smith [pdf]


Photograph of box of sweet potatoes, not yamsThe USDA eventually started to require that growers indicate, somewhere, that the tubers are actually sweet potatoes. But growers usually shrink the font of the actual ingredient so that is it all but invisible next to the huge “YAMS” text. Or they place the text on a place on the box that is effectively hidden. As a result, many US citizens don’t know what a yam actually is.

Who cares? Allowing companies to label sweet potatoes as yams is like selling prunes as dates (dates are more expensive and taste better). Moreover, mislabeled sweet potatoes are a health problem for people who are allergic to them. So if you end up dying from anaphylaxis after eating a sweet potato marketed as a yam, in your last gasps please direct your family to sue Louisiana (or the USDA). That would motivate the USDA to ban the mislabeling that it currently tolerates.

Labels for sweet potatoesProduce managers can help eliminate the confusion over sweet potatoes and yams by making bin labels more informative. For example, add the variety name and, ideally, photographs and cooking tips. The image above shows how this could work for ‘Nancy Hall’ and ‘Beauregard’ sweet potatoes. I’d wager that customers will buy more sweet potatoes if you give them useful information.

Sign for informing customers that sweet potatoes will no longer be sold as yamsProduce managers might also put up a permanent sign near the sweet potato display that informs customers what a yam is and that you don’t sell them (unless you do sell them, of course). The sign at right is actually ready to be printed, if you like it. Having an informational sign saves produce managers from the agony of having to explain the difference between yams and sweet potatoes all the time, just like speleologists have to do their “stalagmites and stalactites” 10x a day during the cave tour.

Of course, if your local store has a produce manager from Louisiana, you might be out of luck. But you can take control of the situation by making your own bin labels and discreetly swapping them with the ones that are misleading. Kids love guerrilla action like this, so consider involving your whole family. Or report the store (or product) to the USDA’s Misbranding and Misrepresentation Office by emailing Cathy Hance. Just send her a photograph along with a store or company address. The label doesn’t need to be intentionally misleading (deceptive), by the way — it just needs to claim the product is a yam.

You can also print out and distribute wallet-sized reference cards (image at right; click to enlarge). These cards also make thoughtful Thanksgiving gifts if you have a laminator in the house; just leave them on everyone’s plate just in case they are needed during dinner. In the graphic I allude to a white sap can sometimes be seen bleeding out of a freshly cut sweet potato (photo below, on right). Sometimes you’ll need to squeeze it, but often the droplets will just appear (depends on the variety and the age of the tuber). If you see the white latex, it’s definitely a sweet potato: yams don’t do that, ever.

White latex from sweet potatoPlease consider sharing this page to spread awareness about sweet potatoes and yams. For the highest impact, share this page in November, Sweet Potato Awareness Month (SPAM). You get bonus points for sharing this page with your family on Facebook prior to Thanksgiving — doing so will make the gathering 20% less confrontational. Except, of course, if one of your family members is a “yam” farmer in Louisiana. If that’s the case, ask everyone beforehand to say, “would you please pass the …. ‘yams’ …” with the requisite pauses that indicate that they are not actually yams. And do that “air quotations” thing with your hands in a really annoying way. That will drive him/her nuts and will be really funny for everyone else. [NB: Louisiana has the highest gun homicide rate in the country, so if s/he starts reaching for a concealed weapon, hide under the table.]

Yam producing nations in AfricaAnd in August, don’t forget about Yam Awareness Month (YAM). OK, I’ll be honest — I just made that up. But with an acronym like that, I’m sure it will catch on. August is when some African nations have New Yam Festivals, so choosing August is not entirely stupid. By the way, 95% (ish) of the yams in the world are grown in Africa (Nigeria, Ghana, Ivory Coast, Benin, Togo, Cameroon, Central African Republic, and Chad). Nigeria alone grows about 75% of the yams on the planet. Note that you can plant yams in the United States (zones 8 and 9), but you’ll rarely get anything to harvest. That’s useful information in deciding whether something is a yam or a sweet potato: if you have a box that says it’s the product of the United States, it’s not full of yams. (The only exception might be several species of yams that escaped from cultivation in the South; not sure if these are ever harvested though.)

Finally, if you ever find yourself using the roasted sweet potato emoji, you can always get identification assistance by hovering your mouse over the icon. The computer will tell you what it is.

Sweet potato (not) yam emojiStill want more? The Botanist in the Kitchen has it.