Yams are not sweet potatoes, but sweet potatoes are sometimes “yams.”

Though most people think of sweet potatoes as only being orange, there are also some white varieties, like the one seen here.

Photo Credit: Megan Bame

What’s in a name? This can of “yams” is actually full of sweet potatoes. Be sure to read the “fine print” for full veggie disclosure!

Photo Credit: Megan Bame

Confused yet?

Botanically speaking, sweet potatoes are completely different from yams, but if you recall, horticulture allows for different common names – and that’s what often gets us in trouble.

The confusion between sweet potatoes and yams apparently arose when producers in the US were looking for a way to distinguish the Southern-grown, moist-fleshed sweet potatoes (which were generally orange) from the Northern-grown, dry-fleshed sweet potatoes (which were traditionally white). The English word “yam” came from the African word “nyami,” which referred to the starchy, edible root of the Dioscorea species (the true yam) – and that’s generally yellow- or white-fleshed. But oddly enough, somehow the word “yam” became associated with the orange sweet potato.

The US Department of Agriculture allows sweet potatoes to be sold as “yams,” recognizing that consumers use the two terms interchangeably; but the “yam” label must also include the words “sweet potato” to ensure truth in labeling. (Follow that?)

Let’s take a closer look at the differences between these two garden veggies.

For starters, they come from different plant families. The sweet potato, Ipomoea batatas, is a cousin of the morning glory in the Convolvulaceae family. The yam, Dioscorea spp., belongs to the Dioscoreaceae family. In the sweet potato, the storage organ we eat is the root, as opposed to the edible tuber that’s produced by the yam. A tuber is actually a swollen, underground stem. (The Irish potato is also a tuber.) A yam plant yields only 1-5 tubers per plant, while the typical sweet potato yield can be in the range of 4-10 per plant (in different sizes).

If you know the characteristic appearances of these two garden yummies, then looks alone should distinguish the two. Sweet potatoes have smooth, thin skin and a blocky shape with tapered ends. Yams have rough, scaly skin and are usually cylindrical shaped (though some have “toes”). To a discerning palate, sweet potatoes are moist and sweet, while yams have a dry feel and a starchy taste.

Culturally, the sweet potato originated in Peru and Ecuador and is considered a prehistoric plant. The yam is thought to be younger, originating in 8000 B.C. in West Africa and Asia. Both prefer tropical climates for growth, but the sweet potato also produces well in temperate climates. Today’s sweet potatoes are primarily grown in the US, while true yams are imported from the Caribbean.

I think the differences are becoming quite clear, but if you’re not convinced yet, let’s take a closer look at science and genetics: The sweet potato has a chromosome number of 2n=90, whereas the yam’s chromosome number is 2n=20. The two can’t even interbreed!

Now see if you can keep this straight: the sweet potato is a monoecious dicot, and the yam is a dioecious monocot. What this means is the sweet potato has male and female flowers on the same plant (monoecious) compared with the yam species, in which male plants produce only male flowers and female plants produce only female flowers (dioecious, like humans). As a dicot, the sweet potato produces two cotyledons (first leaves) when its seed germinates. The yam, being a monocot, only produces a single leaf at germination, much like grass, lilies or palms.

So you can see, in fact, that yams and sweet potatoes just aren’t the same. Heck, they aren’t even close. So when someone asks you to pass the candied yams at Thanksgiving, you might correct the mistaken identity and enlighten your family members with your knowledge and bit of horticultural trivia! (I know I yam.)