The sweet potato (Ipomoea batatas) is a popular vegetable often confused with other underground crops regarding its growth habit. People frequently wonder if the edible part is a root or a tuber and whether the plant grows primarily underground or on a sprawling vine. To clarify its unique biology, it is necessary to examine the plant’s distinct structural components and growth patterns.
The Definitive Answer: Storage Root Structure
The edible sweet potato is botanically classified as a storage root, a specialized type of root tuber. This structure is a swollen, enlarged portion of the plant’s lateral root system designed for nutrient storage. Its primary function is to accumulate starches and sugars to sustain the plant, which also makes it a valuable food source for humans.
The sweet potato develops multiple storage roots that radiate outward from the main stem base beneath the soil surface. These roots are not formed from underground stems, correcting a common misconception based on other root crops. As the growing season progresses, each storage root thickens, concentrating the energy produced by the leaves above ground. This confirms that the vegetable itself grows entirely beneath the soil.
Understanding the Sweet Potato Vine
While the harvested portion is subterranean, the sweet potato produces extensive, fast-growing vines above ground. These herbaceous vines are the stems and foliage, often spreading across the soil as a dense ground cover. The vines are responsible for photosynthesis, capturing sunlight to create energy transported down to the storage roots below.
Trailing vines can reach lengths of 10 to 13 feet. They also help suppress weeds by shading the soil, contributing to the overall health of the crop. Although the vine does not yield the edible vegetable, its vigorous growth indicates the energy being channeled into the underground storage roots.
Key Differences from White Potatoes
The sweet potato is often compared to the common white potato, but they are fundamentally different botanically. Sweet potatoes belong to the morning glory family (Convolvulaceae), and the edible part is a true root. In contrast, the common white potato (Solanum tuberosum) is a member of the nightshade family and is classified as a stem tuber.
This difference is evident in their anatomy. White potatoes possess “eyes,” which are the nodes or buds of the stem from which new shoots emerge. Sweet potatoes, being roots, lack these nodes and instead form adventitious shoots called “slips,” which are necessary for propagation. The white potato’s tuber is a swollen underground stem, while the sweet potato’s storage root is a modified lateral root.
The internal structure also reflects this distinction. The common potato stores energy in a central pith, typical of stems. The sweet potato stores its starches throughout the modified root tissue. This botanical separation confirms they follow different developmental pathways.
Basic Growing and Harvesting
The sweet potato’s root-based biology dictates its unique cultivation process, which begins not with seeds or cut tubers, but with “slips.” A slip is a rooted cutting grown from a mature sweet potato, and this vegetative material is planted in the field. Slips are typically set into raised soil mounds or ridges to ensure good drainage and warm soil temperatures.
Planting usually occurs after the last spring frost when the soil temperature has warmed consistently to at least 65°F. The storage roots mature over a long growing season, generally requiring 90 to 120 days before harvest. To harvest the crop, the extensive vines are typically cut back and removed to clear the area. The roots are then carefully dug up with a fork to avoid bruising the delicate skin, usually just before the first expected fall frost.