Japanese sweet potatoes, often sold as Murasaki or Satsuma Imo, offer a distinct culinary experience compared to orange-fleshed varieties. They have reddish-purple skin and a pale interior that transforms into fluffy, nutty, and richly sweet flesh upon cooking. Their high starch content and slightly drier texture are enhanced through a specific post-harvest process. Understanding the specialized needs of this unique cultivar is the first step toward a successful harvest.
Preparing the Ground and Starting Slips
Successful Japanese sweet potato cultivation begins with preparing the right environment, as the plant needs loose soil to form well-shaped tubers. The best soil is sandy, loose, and well-draining, preventing the storage roots from becoming stunted or misshapen. It is important to avoid soils that are overly rich in nitrogen, as this nutrient encourages excessive leafy vine growth at the expense of developing large, edible potatoes underground.
Planting should occur once the threat of the last frost has passed and the soil temperature is consistently warm, ideally above 65°F (18°C). Sweet potatoes are grown from “slips,” which are rooted sprouts taken from the parent potato. Slips can be started indoors roughly six to eight weeks before the intended planting date by suspending a tuber partially in water or laying it in a tray of moist soil.
Once the slips have developed a healthy stem and roots, they are carefully removed from the mother potato and transplanted directly into the prepared garden bed. Planting them in raised rows or hills can help the soil warm up faster in the spring, improve drainage, and give the expanding tubers more room to develop. The young slips should be planted deep enough so that at least two nodes are buried in the soil.
Ongoing Maintenance During the Growing Season
After planting, the slips require consistent moisture to establish their root systems during the first few weeks of growth. Once the plants are established, their watering needs become moderate, but they should still receive about one inch of water per week during dry spells for optimum production. However, it is beneficial to reduce watering in the final three to four weeks before the anticipated harvest, as excessive moisture late in the season can cause the developing tubers to crack.
The rapidly spreading vines of the sweet potato naturally shade the ground, which helps to suppress weed competition. A maintenance task is vine management, as the long vines can develop secondary roots at the nodes where they touch the soil. These secondary roots divert the plant’s energy into forming many small, unusable potatoes. To prevent this, the vines should be lifted or flipped regularly to break any new root attachments.
Knowing When and How to Harvest and Cure
Japanese sweet potatoes typically require a long growing season of 90 to 120 days. The most important consideration for harvest timing is to dig the potatoes before the soil temperature drops below 50°F (10°C) or before the first hard frost. Exposure to cold soil temperatures can damage the tubers, leading to a bitter taste and reducing their storage life.
To harvest, use a garden fork or shovel to gently loosen the soil in a wide circle, about 18 inches away from the center of the plant, to avoid piercing the fragile tubers. Handle the lifted roots with care to prevent bruising, as nicks or cuts can lead to rot during storage. Do not wash the harvested potatoes; instead, carefully brush off any large clumps of soil.
Curing is necessary to develop the characteristic sweetness and ensure a long storage life. Curing involves holding the potatoes at high temperature and humidity to convert starches into complex sugars and allow small wounds to heal. Optimal curing requires a temperature between 85°F and 95°F (29–35°C) and 80% to 95% humidity for four to fourteen days. After curing, store the roots in a cool, dark, and well-ventilated location that maintains 55°F to 60°F (13–16°C).