Purple sweet potatoes, known scientifically as Ipomoea batatas, offer an appealing combination of vibrant color and nutritional density in the home garden. Their deep purple flesh is a result of high levels of anthocyanins, the same antioxidants found in blueberries and red cabbage. These warm-season crops thrive in heat and sun, rewarding the patient gardener with a harvest that is both beautiful and healthy. Cultivating this distinct variety follows the same general principles as growing their orange-fleshed relatives, beginning with securing the correct planting material.
Sourcing or Growing Sweet Potato Slips
Sweet potatoes are not grown from seeds but from “slips,” which are rooted vine cuttings sprouted from a mature tuber. Gardeners can choose to purchase these slips directly from a nursery, ensuring they receive disease-free stock of a specific purple variety. If you buy them, they may arrive wilted but will quickly recover once given water and planted.
A popular and economical alternative is to grow your own slips at home from a store-bought purple sweet potato. This process involves placing a tuber, or a section of a tuber, partially submerged in water or nestled in moist potting soil. This should be started indoors approximately six to eight weeks before the last expected frost date in your area.
The warm, bright conditions encourage the development of leafy sprouts. Once these sprouts reach about six inches in length, they are gently twisted or sliced away from the parent potato. These newly separated slips are then placed in water for a few days to develop their own root system before being hardened off for planting outside.
Optimal Soil Preparation and Planting Time
Sweet potatoes require a long, warm growing season and are highly sensitive to cold, making planting timing essential. The soil temperature must consistently be above 60°F, and ideally closer to 65°F, with all danger of frost passed before transplanting the slips outdoors. Planting too early can stunt growth and damage the developing roots.
The structure of the soil is important, as the tubers need loose material to expand properly. Sweet potatoes grow best in light, sandy, or loamy soil composition that drains water rapidly. Avoid heavy clay soils or areas with poor drainage, as excessive moisture can lead to root rot and poor tuber development.
To ensure optimal drainage and provide room for the tubers to bulk up, prepare the planting bed by creating hills or raised rows. These raised mounds, typically about eight to ten inches high, also help the soil warm up faster in the spring. Select a location that receives a minimum of six hours of direct sunlight each day, as this crop thrives in intense heat and light.
Transplanting the Slips for Success
Once the soil is sufficiently warm, the rooted slips are ready to be moved into the prepared garden mounds. Transplanting requires attention to spacing and depth to maximize the final yield. Plant the slips so that the entire stem is buried, leaving only the top one or two leaves above the soil line.
This method ensures that multiple nodes, which are the points where roots and eventually tubers will form, are in contact with the soil. Space each slip about 12 to 18 inches apart within the row to give the roots ample room to grow large. Rows should be spaced a generous three feet apart to accommodate the sprawling nature of the vines.
Immediately following the transplanting, the slips must be watered deeply to settle the soil around the buried stem and minimize transplant shock. Consistent daily watering is necessary for the first week until the young plants begin to establish themselves. Planting on a cloudy day or in the late afternoon can also help reduce stress.
Seasonal Care and Harvesting the Tubers
After the initial establishment period, sweet potatoes require less frequent watering, typically about one inch per week, as they are drought-tolerant. The plants should be fertilized sparingly and only with a low-nitrogen formula, since too much nitrogen encourages excessive vine growth at the expense of tuber formation.
As the season progresses, the vigorous vines will spread, naturally shading out weeds and reducing the need for cultivation. Vine management, or “pinning,” prevents the vines from rooting at their nodes along the ground. Allowing these vines to root can result in many small, stringy potatoes rather than a few large ones, so periodically lift the vines to break any new root connections.
The tubers are ready for harvest between 90 and 120 days after the slips were planted, often signaled by the yellowing of the foliage. Harvest must occur before the first hard frost, as cold damages the tubers and leads to decay. When digging, start about 12 to 18 inches away from the main stem and use a garden fork to gently loosen the soil, carefully lifting the purple sweet potatoes to avoid bruising the delicate skin.
After harvesting, the sweet potatoes require a process called curing to sweeten their flavor and harden their skins for long-term storage. This involves keeping the roots in a warm, high-humidity environment (85°F with 85% humidity) for about five to ten days. Following the curing period, they can be stored in a cool, dry place (55°F to 60°F) where they will keep for several months.