True yams (Dioscorea) are geophytes that produce large, starchy tubers, distinguishing them botanically from the sweet potato. These tubers are a major staple crop in West Africa, the Caribbean, and parts of Asia. Unlike mechanized root crops, harvesting yams is an intensive, physically demanding task due to the deep and often irregular growth habit of the tuber. The process requires precision and careful handling to protect the tuber’s integrity for long-term storage.
Indicators of Harvest Readiness
The primary signal that a yam tuber is ready for harvest is senescence, the programmed aging and death of the plant’s above-ground foliage. This is visually apparent when the green vines and leaves begin to yellow, wilt, and eventually die back. This physiological change indicates that the plant has ceased vegetative growth and mobilized starches and nutrients into the underground storage organ.
The growing season typically ranges from six to ten months after planting, varying by species and local conditions. Farmers often time the harvest to coincide with the end of the rainy season or the onset of the dry season, when soil moisture is lower. Harvesting too early results in low starch content, while waiting too long increases the risk of the tuber sprouting, which depletes its stored energy.
Techniques for Excavating Yams
Removing the yam tuber from the soil is a delicate operation, as the thin skin is easily bruised or cut, creating entry points for rot-causing microorganisms. Since tubers grow deep and in irregular shapes, mechanized harvesting is not widely feasible, making manual labor the dominant method. Workers use specialized tools like digging sticks or pointed hoes to carefully loosen the soil around the base of the vine mound.
The excavation must be slow and deliberate, working outward and downward to expose the tuber without striking its surface. Farmers often bend or squat to maintain the necessary control and precision during this process. The goal is to fully detach the tuber from the main stem and roots while inflicting minimal damage to the skin, ensuring suitability for extended storage.
A traditional technique known as “yam milking” or “double harvesting” is sometimes employed for certain varieties, such as Dioscorea rotundata. This method involves a partial, early harvest where the mature portion of the tuber is carefully cut and removed, usually five to six months into the growing cycle. The top part of the tuber, known as the head, is intentionally left in the soil with the roots intact.
This preserved head regenerates, producing smaller “seed yams” for the following planting season. This achieves a dual purpose: an early yield for consumption and the production of seed material for the next crop.
Post-Harvest Handling and Curing
Once excavated, immediate post-harvest handling focuses on minimizing damage and preparing the tuber for storage. Freshly harvested yams are highly susceptible to sunscald caused by intense sunlight, so they must be shaded quickly. Any remaining soil is gently brushed off, and the tubers are inspected for cuts or bruises.
Curing is applied to extend the shelf life of the yam by healing minor wounds and thickening the skin. Curing is achieved by holding the tubers in a warm, high-humidity environment, ideally at temperatures between 29°C and 32°C (84°F and 90°F) for four to eight days. This promotes suberization, where a protective, cork-like layer forms over surface injuries.
This protective layer prevents excessive water loss and blocks the entry of pathogens that cause rot. After curing, the yams are ready for long-term storage in cool, well-ventilated structures, often called yam barns, which protect them from heat, pests, and chilling injury. Curing is a critical step in preserving the bulk of the harvest.