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What’s the difference between yams and sweet potatoes? 3 things to know

Paul Cappiello Ph.D.

Paul Cappiello Ph.D.

Yew Dell Botanical Gardens Executive Director

Ah . . . it’s Thanksgiving time. As someone who loves to cook – actually, as someone who loves to hang out in the kitchen wherever I go – this is a wonderous time of year. Thanksgiving morning I’m up early, making tea and breakfast for the early risers, starting the turkey in the oven, and stealing a few quiet moments with my own cup of coffee and a comfy chair. Later, the sleeper-inners among the season’s house guests will stumble downstairs well after the whole house has started to smell . . . well . . . like Thanksgiving. 

But of course, the serenity of a slow-motion unfolding of the day inevitably devolves into the annual discussions, debates and diatribes dedicated to the food of the day. A day that started all warm and quiet and cozy has now spiraled into screams of heresy and hearsay punctuated by occasional wrestling matches on the carpet. Stuffed bird or not . . . Sausage in the dressing?! What do you mean there’s no French-fried onion rings atop the green bean casserole?! If there’s one thing you can count on, it’s that a house with 10 guests will generate something like 12 million opinions . . . all firmly held . . . and most often, based on nothing resembling a fact. We do hold our traditions near and dear!

One of those annual debates typically revolves around the correct terminology for the venerable sweet potato . . . or is it a yam? “Of course it’s a yam! It says so on the big, orange can!” you might here from one not-so-neutral corner. “Sure. And I bet you believe that those individually wrapped, square, yellow slices of . . . stuff . . . are actually cheese, just ‘cause the package says so?” you might hear from the other. 

So, in the spirit of the season, allow me to remove one contentious topic to help the family dinner stay keel down. Exactly what is a sweet potato and what is a yam?

What is a sweet potato?

Native to the tropical parts of Central and South America, this dicot (two seed leaves, like tomatoes and maples) is a vining plant that forms a large, dense, and often orange-fleshed tuber. It is related only quite distantly to our white potato (Solanum tuberosum). According to legend, it was none other than Christopher Columbus and his crew who were the first Europeans to sample the New World sweet potato (Ipomea batatas). 

Of course some gardeners may cross paths more commonly with the ornamental sweet potato varieties (Marguerite, etc.) that are essentially, sweet potato varieties selected for fancy leaves than for basal tuber flavor and texture. They’re edible but not overly tasty.

True Ipomea batatas has bright orange flesh (although modern varieties have much more intense colors than older forms), occasionally white and rarely purple. Their skin is relatively smooth. When cooked, the flesh is smooth and gooey and in need of nothing but a generous pat of butter . . . but more on that later. 

The big, orangy things in the large bin at the grocery store, labeled “Sweet Potato” are in fact sweet potatoes. No problem there, so let’s move on.

What is a yam?

True yams (Discorea species) are actually monocots – seedlings having single seed leaves – which means they are more closely aligned with lilies and corn than they are with true sweet potatoes. They are native to Africa and parts of south Asia. Yams tend to be white fleshed with a rough, brown exterior. Individual yam tubers can grow to be from 6 inches to several feet long. When cooked (and some yams are poisonous when raw – although generally, not the types grown commercially) their texture is a bit grainer and starchier than the true sweet potato. 

The name “yam” seems to track to one of two paths. One traces the name “yam” to enslaved Africans bringing the name nyam that gradually morphed into the name, yam. The second possibility is Portuguese and Spanish trade with west Africa as the source for the names inhame or name. Similar nomenclatural slide may have brought us the yam name that way. 

But here’s the big bit – those “yams” in the instantly recognizable can that only shows up on store shelves this time of year . . . I hate to be the bearer of bad news for half of you out there – these “yams” are actually not even close. They’re sweet potatoes. 

So what's the difference?

If you read this column even occasionally, you know my aversion to common plant names. They’re so regional and inherently so imprecise. And the sweet potato/yam debacle is no exception. See if you can follow this:

The “yam” in America may refer to either a member of the species genus Discorea or the true sweet potato (Ipomea batatas). Asking a grocer in Australia, Malaysia or Singapore for a yam might get you taro (Colcoasia esculenta) instead. Even worse – in some parts of Africa and Pacific islands, elephant foot yam is Amorphophallus paeonifolius – that’s right, a relative of the big stink, corpse flower!

To introduce further confusion there’s “purple yam” that can be either a variety of Discorea or an actual purple fleshed true sweet potato (‘Okinawan Purple’) and so on. Discorea alata (the air potato) is actually a true yam relative that produces aerial “potatoes” at each leaf node, above ground! And if you’re really looking for an argument, ethno-taxonomists out there are in heated debate about whether it’s sweet potato or sweetpotato! Dive into that one if you like.

Either way, the Thanksgiving sweet potato has a magical flavor and texture that requires little more than a bit of butter to bring out all the essence it has to offer. Now if you’re like my wife and son who insist on a baking dish with sweet potatoes, brown sugar, and orange juice (really!?) with a layer of marshmallows on top – you can join the two of them Thanksgiving morning, making that mess out on the driveway – not in my kitchen. That just ain’t proper. And you can trust me on this one . . . it’s a fact!

Happy Thanksgiving!

This article was originally submitted to the Courier Journal on November 14, 2023.

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