The potato (Solanum tuberosum) is a globally important crop, unique because the part consumed is the underground storage organ, the tuber. Energy is captured by the green foliage above ground and converted into starches stored in these subterranean structures. Growers often observe the eventual dieback of the leafy tops, leading to questions about what happens beneath the soil once the plant appears dead. While the tubers stop increasing in size, a necessary physiological process begins to prepare them for harvest and storage.
The Natural Cycle of Potato Foliage Dieback
The potato foliage undergoes a natural life phase known as senescence, marking the end of its active growing cycle. This process involves the plant shifting its metabolism to recycle and reallocate nutrients. Valuable starches and compounds previously used to maintain the leaves and stems are actively transported down into the developing tubers. This nutrient resorption causes the above-ground parts to yellow, wither, and eventually collapse. The natural dieback of the vine signals that the tubers have reached full physiological maturity.
Foliage dieback can also be triggered by external factors, such as early frost or disease, which can prematurely terminate the life of the vine. Commercial growers often intentionally induce dieback, known as vine killing, using mechanical means or chemical desiccants to control the timing of the final maturation phase.
Tuber Development and Skin Setting
Once the green foliage is gone, the tubers do not continue to grow in mass or size because the primary engine of growth, photosynthesis, has ceased operation. The tubers have reached their maximum bulking stage, and no new carbohydrates are produced or transported from the tops. Further increase in size is biologically impossible without the active leaf canopy.
The period immediately following vine death is when a transformation known as “skin setting” or periderm maturation takes place. This highly important physiological change determines the quality and storability of the final harvest. The skin of an immature potato is thin, delicate, and easily damaged, often “slipping” off when gently rubbed.
The potato’s skin, or periderm, consists of several layers, including the phellogen, a meristematic tissue that produces new skin cells as the tuber expands. When bulking stops and senescence begins, the phellogen’s activity slows down significantly. This slowdown allows the existing skin cells to stiffen and anchor themselves firmly to the underlying tissue.
During this time, a waxy, water-resistant substance called suberin is infused into the outermost phellem cells of the skin. This suberization process creates a thicker and tougher protective barrier. This hardened skin defends the tuber against scuffing, bruising, water loss, and the invasion of pathogens during storage.
Physiological maturity is not about the tuber getting bigger, but about its outer layer becoming durable. This curing period underground transforms the soft, vulnerable potato into a robust, storage-ready one. Without this process, the potato would rapidly spoil.
Optimal Timing for Potato Harvesting
The cessation of vine growth indicates that the tubers are ready for the final maturation step underground. To allow the skin setting process to fully complete, the recommended practice is to leave the tubers in the soil for a designated period after the tops have died back. This timeframe is typically between 10 and 14 days, though it can extend up to three weeks depending on the variety and soil conditions.
Determining whether the skin has fully set can be confirmed with a simple field test. Gently rub the skin of a sample tuber with your thumb. If the skin resists peeling or rubbing off easily, the periderm maturation is complete. If the skin is delicate and flakes away, the tubers require more time underground to finish curing.
This waiting period is the main difference between harvesting “new potatoes” and mature storage potatoes. New potatoes are intentionally harvested early, often shortly after the plants flower while the foliage is still green. This results in small, thin-skinned tubers meant for immediate consumption. Mature potatoes must go through the entire dieback and skin-setting cycle to achieve the thick skin and high starch content required for long-term storage.