The Okinawan sweet potato, a specific cultivar of Ipomoea batatas, is highly sought after for its creamy texture, sweet flavor, and vibrant purple flesh. This deep coloration comes from high concentrations of anthocyanins, potent antioxidants also found in blueberries, making the tuber a nutritional powerhouse. While commonly linked to the Ryukyu Islands of Japan, the plant’s botanical origins trace back to Central and South America. Successfully growing this tropical vine requires proper propagation material and dedicated effort.
Starting Slips and Propagation
Sweet potatoes are grown from vegetative sprouts called “slips,” which are stem cuttings grown from a mature tuber. Slips guarantee a clone of the parent plant, ensuring the signature purple flesh. Gardeners can purchase slips from a nursery or sprout them from a whole, healthy Okinawan sweet potato that has not been treated with sprout inhibitors.
To sprout your own, bury the potato in a shallow tray of moist soil or sand and place it in a warm location to encourage leafy shoots. Once shoots are six to twelve inches long and have developed small root systems, they are cut away from the parent tuber to become individual slips.
Start slips indoors four to six weeks before the last expected frost date. The goal is to have well-rooted slips ready for transplanting when outdoor temperatures are consistently warm. Before planting, acclimate the slips to outdoor conditions by gradually exposing them to sunlight over about a week.
Establishing the Ideal Growing Environment
Okinawan sweet potatoes are tropical plants requiring a long, hot growing season, typically 120 to 150 frost-free days. Soil temperature is the most important factor; it must consistently be 65°F or warmer before planting slips outside. Planting too early in cool soil will stunt the plants and may cause crop failure.
The location needs full sun, receiving at least six hours of direct sunlight daily. Tubers require loose, sandy, and well-draining soil. Heavy clay soil must be amended with compost and sand to prevent tubers from rotting or becoming malformed.
To maximize harvest, plant slips in raised rows or mounds, a practice known as hilling. Hilling allows the soil to warm faster and provides loose soil for tuber expansion. The ideal soil pH ranges from 5.5 to 6.8; a slightly acidic pH below 6.0 may enhance purple pigmentation.
When transplanting, plant the slip about four inches deep, covering the root node and lower leaves. Space slips twelve to eighteen inches apart within the rows to allow room for tuber and vine growth. Water the slips thoroughly immediately after planting.
Managing Growth and Ongoing Care
Once the slips are established, the focus shifts to maintaining conditions that favor tuber enlargement over excessive vine growth. Sweet potatoes need consistent moisture, especially during the 50 to 60 days of tuber initiation. They benefit from deep watering, aiming for about one inch of water per week, but avoid waterlogging, which causes root rot.
Fertilization must be carefully managed, as sweet potatoes require less nitrogen than many other garden vegetables. High nitrogen encourages lush, leafy vines at the expense of tuber development. The plants benefit instead from phosphorus and potassium.
Apply a fertilizer with a low first number in the N-P-K ratio, such as 5-10-10, at planting. Throughout the growing season, practice “vine lifting” or “pruning” to prevent vines from rooting at their nodes. If the vines re-root, the plant’s energy is diverted to forming small, secondary tubers, reducing the overall harvest.
Regularly inspect the foliage for common sweet potato pests, such as the sweet potato weevil, and watch for signs of fungal diseases. Addressing these issues early helps ensure the plant’s energy remains focused on producing the main crop.
Harvesting and Post-Harvest Curing
Harvest timing is crucial for maximum size and flavor, typically 120 to 150 days after planting. The ideal time is when the foliage naturally begins to yellow, signaling the end of the growth cycle. Tubers must be harvested before the first hard frost, as cold soil damages the roots and compromises storage life.
Before digging, cut back the dense vines to clear the area and locate the plant’s crown. Okinawan sweet potatoes have delicate skin, so excavate carefully to avoid nicks or bruises that lead to rot. Use a garden fork placed well away from the center of the plant to loosen the soil and gently lift the tubers.
Immediately after harvest, the unwashed tubers must undergo curing for long-term storage and flavor development. Curing requires a high-heat, high-humidity environment—ideally 85–95°F with 85–90% humidity—for five to seven days. This intense heat heals surface damage and converts starches into complex sugars, enhancing sweetness.
After curing, move the sweet potatoes to a cool, dry location maintained between 55–60°F for long-term storage. Proper curing allows the purple tubers to be kept for many months, providing a supply of this nutritious crop well into the winter.