The question of whether to “hill” sweet potatoes is a common one, likely stemming from the similar practice used for Irish potatoes. Hilling involves progressively piling soil around the base of a growing plant to encourage more underground storage formation or protect developing tubers from light. For the sweet potato, however, this practice is generally unnecessary and can even be counterproductive, primarily because of a fundamental difference in how it forms its edible part. Sweet potatoes produce storage roots, not stem tubers, which dictates a completely different approach to soil management throughout the growing season.
Understanding Tuber Formation
The reason sweet potatoes do not require hilling lies in their botany, as they are not true tubers. Sweet potatoes (Ipomoea batatas) are classified as thickened adventitious roots, which grow downward and outward from nodes along the buried stem. These storage roots develop directly from the root system, unlike Irish potatoes (Solanum tuberosum), which are stem tubers that form along underground stems called stolons.
Hilling is performed on Irish potatoes to cover the newly forming tubers, protecting them from sunlight that would cause them to turn green and become toxic. Since sweet potatoes form their storage organs deeper in the soil and from the root structure, adding extra soil does not encourage more storage roots to form above the initial planting depth. Excessive hilling can sometimes divert the plant’s energy into producing more vine growth rather than increasing the size of the roots themselves. The goal of sweet potato cultivation is to maintain a loose, well-drained soil environment where the storage roots can expand unimpeded.
Establishing Sweet Potato Slips
Since continuous hilling is not the method for sweet potatoes, attention must be focused on the initial planting technique using “slips.” When planting these vine cuttings, the aim is to bury the lower portion of the stem deep enough to ensure multiple nodes are covered, as these nodes are where the adventitious roots will develop. A planting depth that buries four to six inches of the slip is recommended to establish a strong start.
The slips should be spaced approximately 12 to 18 inches apart within the row, with rows spaced about three feet apart to accommodate the sprawling vines. Growers often plant slips into raised beds or ridges that are prepared before planting. These initial mounds, which can be eight or more inches high, improve drainage and allow the soil to warm faster, which is beneficial for these heat-loving plants. This initial soil manipulation is a one-time preparation, not a progressive addition of soil throughout the season.
Ongoing Soil Management
Once the sweet potato slips are established, ongoing soil management shifts away from adding more soil and focuses on maintaining optimal conditions for root development. Consistent moisture is necessary, especially during the initial establishment phase and when the storage roots begin to swell. Sweet potatoes prefer a steady, moderate moisture level, and they are tolerant of dry weather once the plants are mature.
A thick layer of organic mulch, such as straw or compost, is often applied around the plants to suppress weeds and conserve soil moisture. Mulching is a direct alternative to hilling, preventing the need for soil disturbance while also helping to regulate soil temperature. Once the vines begin to sprawl and form a dense canopy, they naturally shade the soil, further reducing moisture evaporation and suppressing weed growth. Aside from weeding until the canopy closes, the root zone is largely left undisturbed to allow the storage roots to expand without interruption.