Sweet potatoes are storage roots, distinct from true yams, that offer a rich source of nutrients and natural sweetness. Determining the precise moment to harvest is important for maximizing flavor, overall quality, and long-term storage potential. Harvesting too early results in small, starchy roots, while delaying too long risks damage that compromises storage. Knowing how to read the plant and the calendar ensures a rewarding yield of mature, flavorful sweet potatoes.
Determining the General Harvest Window
Planning the harvest begins with noting the planting date, as sweet potatoes generally require a long growing season to reach their full potential. Most sweet potatoes mature between 90 and 150 days after planting the slips, depending on the variety and local climate conditions. While this timeframe provides a helpful guideline, the roots will continue to grow and size up until the plant’s growth is halted by external factors.
The absolute deadline for harvest is dictated by the weather, as the roots are highly sensitive to cold temperatures. Sweet potatoes must be fully dug up before the first hard frost, which can penetrate the soil and damage the roots near the surface. Exposure to soil temperatures below 50–55°F causes chilling injury, leading to internal decay and a hard core when cooked, significantly reducing the eating and storage quality.
Reading the Signs from the Vines
The plant itself offers a clear visual signal that the roots below ground are nearing maturity and ready for harvest. The primary sign is the natural yellowing and subsequent dieback of the dense, sprawling foliage and vines. This process, known as senescence, indicates that the plant has completed its above-ground growth cycle.
When the leaves begin to yellow, the plant is no longer prioritizing energy for foliage production. Instead, the plant directs its final energy reserves and starches down into the storage roots, which helps them bulk up and develop their characteristic sweetness. This natural dieback is a sign of readiness, not a disease, and suggests that the tubers have stopped expanding and are beginning to thicken their protective skin.
How to Conduct a Test Dig
Since the roots develop underground, visual cues from the vines should be confirmed by physically inspecting a sample tuber before harvesting the entire crop. This process, called a test dig, helps confirm both the size and the maturity of the roots. Begin by using a garden fork to gently loosen the soil about 12 to 18 inches away from the main crown of a plant to avoid piercing the roots.
Carefully lift one or two roots to check their size and the quality of the skin. A mature sweet potato will have a thick, corky skin that is firm and resistant to damage. If the skin is thin and easily rubs or “slips” off when gently scratched, the roots are still immature and require a week or two more underground. If the sampled roots are already the desired size and possess tough skin, the entire crop is ready to be harvested.
Preparing Sweet Potatoes for Eating and Storage
After harvest, sweet potatoes are not immediately ready for long-term storage or optimal flavor, requiring a mandatory two-step post-harvest process: curing and storage. Curing is a short-term, high-heat, and high-humidity treatment that converts starches into sugars, developing the roots’ sweetness. This process also heals any minor cuts or abrasions sustained during the digging process, which is important for preventing rot in storage.
For a successful cure, the roots should be held at a high temperature, ideally between 85°F and 95°F, with a relative humidity of 80 to 95 percent for four to fourteen days. Once curing is complete, the roots should be moved to a cooler, dry location for long-term storage, ideally between 55°F and 60°F. Maintaining this range prevents chilling injury from lower temperatures and discourages sprouting caused by higher temperatures.