The short answer is no, potato tubers do not increase in size after the foliage above ground has died back. This natural dieback, called senescence, signals that the plant has completed its primary growth cycle. Although the tubers stop growing, this stage is necessary as the underground portion of the crop enters a crucial phase of maturation.
Why Tuber Growth Stops
The size of a potato tuber is directly dependent on the health and activity of the plant’s green tops. The leaves function as the plant’s energy factory, capturing sunlight to perform photosynthesis. This process converts light energy into carbohydrates.
These carbohydrates are then transported down through the stems and stolons to the developing tubers, where they are stored as starch, causing the tubers to increase in size. When the foliage naturally yellows, browns, and dies, this energy supply is cut off. Without the continuous flow of carbohydrates from the leaves, size growth ceases entirely.
The cessation of growth is a natural part of the potato’s life cycle, indicating that the plant has invested its maximum energy into the underground storage organs. If the tops die prematurely due to disease, frost, or mechanical damage, the growth stops early, resulting in smaller, immature potatoes. Natural dieback means the tubers have reached their full genetic potential for size under the season’s growing conditions.
The Maturation Process Underground
Although the tubers stop increasing in size, the period immediately following the dieback is when a necessary maturation process occurs underground. This phase is referred to as “skin set” or “curing,” and it is essential for the quality and storability of the harvest.
During skin set, the thin skin of the tuber thickens and toughens considerably. This physiological process involves the outer layer, the periderm, becoming firmly bound to the underlying tissue. The cells in the periderm become infused with suberin, a waxy, water-resistant substance.
This toughening of the skin provides a protective barrier that is highly resistant to mechanical damage, such as bruising and skinning, during harvest and handling. The suberin layer minimizes moisture loss and blocks the entry of pathogens. This maturation is why mature potatoes store well for months, unlike thin-skinned “new potatoes,” and requires a waiting period before the final harvest.
When to Harvest After Dieback
The timing of the harvest for mature, storage-ready potatoes is linked to the completion of the skin set process. Once the potato tops have died back and turned brown, a waiting period of two to three weeks is recommended before digging. This interval allows sufficient time for the periderm to fully mature and set.
To check if the skin is properly set, a simple test involves gently rubbing the skin of a freshly dug tuber with a thumb. If the skin easily rubs or slips off, the tubers are not ready for storage and need more time in the ground. If the skin remains firmly attached, the potatoes are mature and ready for digging.
This waiting period is for potatoes intended for long-term storage. If the goal is to harvest “new potatoes,” they are dug earlier in the season, often two to three weeks after the plant has flowered while the foliage is still green. New potatoes have thin, tender skins because they are harvested before natural dieback and skin set occur, meaning they must be consumed quickly as they do not store well.