Sweet potatoes are a warm-weather vegetable grown worldwide. These roots are not true potatoes but are the storage roots of a tropical vine, Ipomoea batatas, a member of the morning glory family. Because of their origins, they demand a long, warm growing season, typically spanning three to four months, and are extremely susceptible to cold temperatures. Even a brief exposure to frost or cold soil can damage or kill the plant, making the timing of every step from preparation to harvest crucial.
Starting the Growing Cycle: Preparing Sweet Potato Slips
The sweet potato growing cycle begins indoors long before the last frost date, focusing on the production of specialized cuttings known as “slips.” A slip is a sprouted shoot taken from a mature sweet potato, and these must be started approximately six to eight weeks before the anticipated outdoor planting date. This indoor propagation phase ensures the plants are ready for immediate growth when the outdoor soil reaches the required warmth.
To start slips, gardeners typically place a healthy, whole sweet potato in a container of moist soil or suspend it partially in water in a jar. Ideal indoor conditions for this process include a bright location and temperatures consistently maintained in the 75–85°F range to mimic the necessary tropical heat. In this warm environment, the sweet potato tuber will begin to sprout new shoots from its “eyes.”
Once these shoots grow to about six inches long and develop a few small roots, they are gently detached from the mother potato. These rooted cuttings are the slips used for transplanting. Starting slips too late significantly reduces the available growing time. Starting them too early can result in leggy, overgrown slips that become stressed while waiting for proper outdoor conditions.
The Critical Outdoor Planting Window
The decision of when to transplant the prepared slips into the garden is governed by two environmental metrics: the final frost date and the soil temperature. Planting must be delayed until all danger of frost has definitively passed, often three to four weeks after the last expected frost. Planting too early when the soil is still cool will severely stunt growth, lead to a poor harvest, or cause the young slips to rot outright.
The most reliable indicator for successful transplanting is a consistently warm soil temperature, which must be 60–65°F (15–18°C) or higher, measured at a depth of four inches. Many seasoned gardeners wait for the soil to reach 70°F for optimal, rapid establishment. Nighttime air temperatures are also a factor, ideally remaining above 55°F, as prolonged exposure below this threshold can cause chilling injury.
Because this necessary warmth arrives at different times across geographical zones, the planting window varies considerably, from late May in warmer climates to early June in cooler regions. Using a soil thermometer to confirm the sustained temperature is a more reliable practice than relying solely on the calendar date. Delaying planting until the soil is sufficiently warm ensures the slips immediately enter a period of active root and tuber formation, maximizing the use of the available warm season.
Timing the Harvest for Optimal Yield and Curing
The final timing decision involves the harvest, which must occur after the tubers have reached maturity but before the onset of cold weather. Sweet potatoes generally require a long growing period of 90 to 120 days from planting the slips until they are ready to be dug. Varieties with shorter maturity times, such as ‘Beauregard’ or ‘Georgia Jet,’ are often chosen in regions with shorter summers.
The most important timing constraint is harvesting before the first hard, or killing, frost of the autumn. A hard frost will travel through the soil and damage the tubers near the surface, causing chilling injury that compromises their flavor and ability to store long-term. Visual cues that indicate readiness include the yellowing or bronzing of the foliage, signaling that the plant has largely completed its energy transfer to the roots.
Immediately following harvest, sweet potatoes require a specific, short-term curing process that develops their characteristic sweetness and ensures storage longevity. The tubers must be held at a high temperature, typically 85–90°F (29–32°C), with high humidity (around 85–90%), for four to ten days. This controlled environment allows the skin to heal any nicks or bruises sustained during digging. More importantly, it triggers the conversion of starches into sugars, which makes the sweet potato palatable and sweet.