What Is the Difference Between Yams and Sweet Potatoes?

Sweet potatoes and yams are distinct crops from different plant families with different nutritional profiles and growth patterns, according to researchers cited by Live Science. While often used interchangeably in U.S. supermarkets, sweet potatoes belong to the morning glory family (Convolvulaceae), while true yams belong to the Dioscoreaceae family.

The confusion stems from historical dietary substitutions and modern marketing. Phillip Wadl, a research geneticist at the U.S. Vegetable Laboratory in Charleston, South Carolina, told Live Science that most products labeled as yams in typical U.S. supermarkets are actually soft, orange-fleshed sweet potatoes.

Botanical and Biological Differences

The two vegetables differ in their basic plant structure and how they store nutrients. Sweet potatoes form storage roots, which are enlarged parts of the root system, while yams form tubers, which are enlarged underground stems, according to Wadl.

Jeffrey Cole Denis Gregorie, a sweet potato specialist at the Sweet Potato Research Station in Chase, Louisiana, noted that they belong to different cotyledon types. Yams are monocots, meaning their embryo has one seed leaf, similar to lilies and grasses. Sweet potatoes are dicots, possessing two embryonic seed leaves, a trait shared with tomatoes and squash.

Growth cycles and yields also vary between the two crops. Craig Yencho, a professor of horticultural science at North Carolina State University, stated that sweet potatoes generally mature in three to five months, whereas yams require six months to a full year. A single sweet potato plant typically produces four to 10 sweet potatoes, while a yam plant yields one to five.

Nutritional Profiles and Safety

The nutritional value of the two vegetables differs significantly, particularly regarding vitamin A. Orange-fleshed sweet potatoes are a primary source of beta-carotene, a precursor to vitamin A, as well as vitamin C, potassium, and dietary fiber, according to Wadl.

Yams also provide dietary fiber, potassium, vitamin C, and manganese, but Wadl noted they typically contain less beta-carotene than orange sweet potatoes. In terms of taste and texture, sweet potatoes are generally sweeter and creamier when cooked, while yams are starchier, drier, and have a more earthy flavor.

Safety requirements differ based on the species. Wadl stated that sweet potatoes are generally safe to eat both raw and cooked. However, many wild varieties of true yams contain compounds like dioscorine, which can be toxic to humans if consumed raw. These must be thoroughly cooked to remove toxins.

Visual Identification

Visual cues can distinguish the two crops. Sweet potatoes are typically shorter and blockier with tapered ends and thin, smooth skin. Their flesh can be orange, yellow, purple, or white.

Yams are usually long and cylindrical, sometimes featuring branching shapes or toes. Wadl described yam skin as often being rough and bark-like, with flesh that can be white, purple, or reddish. One specific purple yam, ube (Dioscorea alata), is frequently used in desserts like ice cream and cakes.

Origins of the U.S. Naming Confusion

The tendency to call sweet potatoes yams in the U.S. has historical roots in the transatlantic slave trade. Judith Carney, a professor of geography at UCLA, explained that yams were a key provision for enslaved people from West Africa. In North American plantation societies where African yams could not grow, sweet potatoes served as a substitute dietary staple.

Because the two crops shared similar characteristics, the term yam became a vernacular label for both tubers in mainland America, according to Carney.

This linguistic overlap was later adopted by the U.S. sweet potato industry for marketing purposes. Gregorie told Live Science that the industry leaned into the confusion to differentiate products. Wadl added that orange-fleshed, moist sweet potatoes are often mislabeled as yams to set them apart from drier, white-fleshed varieties.

The University of California, Davis, confirms that common supermarket labels such as jewel yams and garnet yams refer to softer, sweeter varieties of sweet potatoes rather than true yams.