The sweet potato, Ipomoea batatas, belongs to the morning glory family, Convolvulaceae, and grows on a vine. This vigorous plant is cultivated globally as a major vegetable crop for its underground storage organs. Its characteristic growth pattern involves a sprawling network of stems that allow the plant to maximize its exposure to sunlight and cover a large area of ground.
The Vining Growth Habit
The above-ground structure of the sweet potato plant consists of long, trailing stems, often called runners or vines, which spread horizontally along the soil surface. These vines can be quite extensive, commonly reaching lengths of between 3 and 10 feet in a single growing season. This prostrate, running growth habit is a survival mechanism, allowing the plant to rapidly colonize and shade out competing weeds.
The leaves along the vine are typically heart-shaped or lobed, and they vary in color from deep green to chartreuse or even purple, depending on the cultivar. While some commercial varieties have been bred to be more bushy or semi-erect, the majority of sweet potato plants exhibit this spreading, vining habit. The extensive surface coverage allows the plant to efficiently capture solar energy, which is then used to synthesize the sugars stored in the underground parts.
What We Eat: The Storage Root
The edible part of the sweet potato is not a true tuber, but rather a specialized structure known as a storage root or tuberous root. This botanical distinction is important because the storage root is modified root tissue, unlike a true potato which is a modified underground stem. The storage roots develop directly from adventitious roots, which form from non-root tissue, specifically the nodes along the vining stem.
When a sweet potato cutting, or slip, is planted, root primordia at the nodes quickly develop into adventitious roots. Under favorable conditions, certain adventitious roots begin to thicken due to the activity of specialized vascular cambium. This cambial activity produces thin-walled parenchyma cells, which are primarily tasked with storing starch and sugars. The final product is the familiar, energy-rich root that serves as the plant’s nutrient reservoir.
Sweet Potato vs. Potato vs. Yam
The confusion surrounding the sweet potato is often amplified by its common comparison to potatoes and yams, which are botanically distinct. The sweet potato (Ipomoea batatas) is a dicot in the morning glory family, with its edible part being a storage root. In contrast, the true potato (Solanum tuberosum) is in the nightshade family (Solanaceae) and produces a stem tuber, which is a thickened underground stem.
Yams, belonging to the genus Dioscorea and the family Dioscoreaceae, are monocots that produce a stem tuber. They are genetically distant from both potatoes and sweet potatoes. Yams are typically starchy, with a rough, bark-like skin, and are a staple crop primarily grown in West Africa. The term “yam” in North America is generally a misnomer, applied to certain soft, moist-fleshed sweet potato varieties.
The US Department of Agriculture requires that any sweet potato labeled as a “yam” also include the term “sweet potato” on the label. The culinary differences are noticeable; sweet potatoes are sweeter and moister due to their higher sugar content, while potatoes and yams are starchier and drier. This botanical separation places the three in entirely different plant families, despite their shared role as starchy root vegetables.