Potatoes are one of the most widely consumed crops globally. Many people assume the potato is a root vegetable, but botanically, it is a stem tuber—a swollen, underground modification of the plant’s stem that stores energy as starch. This distinction dictates how the plant grows and develops its harvestable portion beneath the soil surface.
The Visible Potato Plant
The part of the potato plant seen above ground is a herbaceous perennial that typically grows one to three feet tall. The main stems are sturdy, supporting expansive foliage. The leaves are compound, divided into multiple smaller leaflets, creating a fern-like appearance.
This dense, green canopy captures sunlight to produce sugars transported underground for storage. Potato plants, which belong to the nightshade family (Solanaceae), also produce five-petaled flowers four to ten weeks after planting. These flowers range in color from white, pink, or blue to purple, often featuring yellow stamens.
The health and color of the above-ground growth serve as a direct indicator of the plant’s overall vigor and its capacity to produce tubers below.
The Hidden Process of Tuber Formation
The potato’s development begins below the soil with specialized underground stems called stolons. These are lateral shoots that grow horizontally away from the main plant stem. Tuberization, the process of potato formation, starts when environmental signals, such as specific photoperiods and temperatures, trigger a shift in the stolon’s growth.
The tip of the stolon stops elongating and begins to swell rapidly due to increased cell division and the massive accumulation of starch. This swelling transforms the slender runner into the tuber itself. The familiar “eyes” on the potato surface are nodes (dormant buds), which is a definitive characteristic of a stem structure.
To ensure the developing tubers remain edible, growers practice “hilling,” which involves mounding soil around the plant base. This prevents light from reaching the tubers, as sunlight exposure causes the skin to turn green and produce solanine, a toxic compound. As the season progresses, the tubers expand in size, maturing beneath the protective earth.
Visual Cues for Harvesting
Knowing when to harvest the hidden crop relies on observing the visible parts of the plant and understanding the final stage of tuber development. The most reliable visual cue for maturity is the senescence, or “die-back,” of the foliage (vines or haulms). When the leaves and stems naturally begin to yellow and turn brown, it signals that the plant has stopped channeling energy above ground and has completed the bulking of the tubers.
After the foliage dies down, maturity is confirmed by “skin set.” This is when the tuber’s outer layer, the periderm, thickens and adheres firmly, developing the durable skin needed for long-term storage.
Growers test for skin set by gently rubbing the skin with a thumb. If the skin easily “slips” or rubs off, the tubers are immature and require a waiting period, typically two to three weeks, before harvest. Harvesting “new potatoes” is an exception, as this occurs earlier, often one to two weeks after flowering, prioritizing flavor over storage potential when the skin is still thin and delicate.