The potato plant, Solanum tuberosum, is unique because the part consumed is not a root, but a specialized underground stem known as a tuber. The development of this staple crop involves distinct visual stages both above and below the soil line, signaling different phases of growth and nutrient storage. Understanding these phases requires observing the bushy foliage and recognizing the biological mechanisms that lead to the formation of harvestable tubers.
Above-Ground Appearance and Growth Stages
The potato plant initially emerges from the soil as thick, angular, green shoots that quickly develop into a dense, bushy plant up to three feet high. The stems themselves are upright and branching, often displaying a smooth or slightly hairy texture. As the plant matures, the foliage becomes the primary engine for energy production, transferring starches downward to the developing tubers.
The leaves are compound, divided into several smaller leaflets arranged along a central stalk. A single compound leaf typically consists of seven to eleven oval to heart-shaped leaflets, with a terminal leaflet at the tip. This dense canopy provides the necessary photosynthetic surface area, and also serves a practical purpose for growers.
Later in the growth cycle, the plant may produce small, star-shaped flowers that belong to the nightshade family, Solanaceae. These flowers vary widely in color, ranging from white and pink to shades of blue or purple, often featuring a distinct yellow center. Following flowering, the plant sometimes produces small, green, tomato-like berries, which are inedible and contain seeds.
Growers often mound soil around the base of the growing stems, a practice known as hilling, to maximize underground yield and protect the tubers. Hilling shields the subterranean parts of the stem and developing tubers from sunlight. Light exposure causes the formation of chlorophyll and the toxin solanine, leading to green, bitter potatoes.
How Tubers Form Underground
The edible potato tuber is a modified stem structure that develops underground, differentiating it from true root vegetables. Potatoes form on specialized, horizontal stems called stolons, which grow outward from the lower nodes of the main stem. These stolons are typically pale, lack chlorophyll, and grow horizontally through the soil.
The actual formation of the potato, known as tuberization, begins when the tips of these stolons start to swell. This swelling is initiated by the plant partitioning starch produced by the leaves into the stolon tips, causing them to bulk up into the familiar tuber shape. This process is largely driven by environmental factors, such as decreasing day length and cooler temperatures, which signal the plant to transition from vegetative growth to energy storage.
The surface of the mature tuber is marked by “eyes,” which are dormant buds arranged in a spiral pattern. These eyes indicate that the potato is a stem, as roots do not possess such nodes or buds. When soil is mounded around the emerging stems through hilling, it encourages the plant to form more stolons along the buried stem, leading to a greater number of tubers and a higher yield.
Visual Signals for Harvesting
Determining the ideal time to harvest the underground tubers relies almost entirely on observing the above-ground foliage. For mature storage potatoes, the primary visual cue is the natural process of “dieback,” where the plant stops putting energy into the leaves. This process begins with the foliage yellowing, browning, and eventually collapsing completely to the soil surface.
Vine dieback signals that the plant has finished its main growth cycle and starches have been fully deposited into the tubers. The decline of the plant is also when the potato skins begin to “set,” or toughen. This setting is important for improving storage life and reducing damage during harvesting. Harvesting too early, while the vines are still green, results in thin-skinned potatoes that bruise easily and do not store well.
In contrast, “new potatoes” are small, immature tubers favored for their tender skin and fresh flavor. These can be harvested much earlier, often shortly after the plant has flowered and while the foliage is still green and vigorous. The plant’s overall health and the final collapse of the canopy are the most reliable visual indicators for the farmer, determining whether the harvest yields a small, tender potato or a fully mature, storage-ready tuber.