The potato we eat is not a root vegetable like a carrot or a beet. Botanically, the potato is a modified underground stem known as a tuber, which is the energy storage organ of the Solanum tuberosum plant. This distinction is fundamental to understanding its growth because the edible part forms along specialized stems, not the root system. This process begins with planting a small piece of a previous harvest and culminates in the maturation of new tubers, typically taking a full season of carefully managed development.
From Seed Potato to First Shoots
The journey of a new potato plant begins not with a true seed, but with a piece of an existing tuber, commonly called a “seed potato.” This seed piece contains at least one “eye,” which is a vegetative bud capable of sprouting a new stem. Gardeners often perform a pre-sprouting technique known as “chitting” several weeks before planting to give the crop an early advantage. This involves placing the seed potatoes in a cool, light location to encourage the growth of short, sturdy, pigmented sprouts from the eyes.
Once the danger of hard frost has passed and the soil temperature reaches at least 45°F (7°C), the seed pieces are planted roughly 6 to 8 inches deep. The seed potato provides the necessary stored energy to fuel the initial growth of the plant’s root system and the first above-ground stems. Within a few weeks, the sprouts emerge from the soil, initiating the vegetative phase where the plant begins to produce leaves for photosynthesis.
Stolons and the Underground Transformation
As the main stem grows above the soil, the plant begins to develop specialized structures underground called stolons. These are thin, horizontal stems that branch out from the buried portion of the main stem. Stolons look like roots initially, but they possess nodes and internodes, confirming their identity as stems. The formation of these stolons is a prerequisite for the potato tuber itself to develop.
The entire biological transformation hinges on a process called tuber initiation, where the tips of these stolons begin to swell. This swelling is triggered by a combination of factors, including shorter daylight hours and optimal soil temperatures, ideally between 60°F and 70°F (15.5°C and 21°C). The plant redirects the carbohydrates produced by the leaves downward, converting the stolon tips into the dense, starchy tubers we harvest.
Hilling Essential for Tuber Development
A gardener’s work is required to ensure the plant can successfully transform its stolons into edible tubers, a process called hilling, or earthing up. This involves periodically mounding soil, compost, or straw up around the base of the growing plant stems. The first hilling is typically performed when the shoots reach about 6 to 8 inches in height, covering all but the top few inches of foliage. This technique is repeated every two to three weeks as the plant grows taller.
Hilling serves two major functions. First, it protects the developing tubers from exposure to sunlight, which can cause them to turn green. This greening indicates the production of solanine, a toxic compound. Second, covering the lower stem with soil encourages the plant to produce more stolons from the newly buried stem tissue. This increases the total number of sites where tubers can form, directly leading to a larger yield per plant.
Signs of Maturity and Harvesting
The final stage of the potato’s life cycle involves maturation and harvest. The most visible sign that the tubers are reaching full size is the natural yellowing and eventual dieback of the above-ground foliage, a process known as senescence. Once the vines have completely died back, the plant has finished directing energy to the tubers, and the skins begin to thicken. For maximum storage potential, it is best to wait approximately two weeks after the foliage has died before digging the potatoes.
This waiting period allows the thin skin of the developing tubers to “set” or toughen up. Potatoes harvested early, often around 7 to 8 weeks after planting, are called “new potatoes” and have delicate, thin skins, making them unsuitable for long storage. Immediately following harvest, potatoes intended for storage should undergo a curing process for 7 to 14 days in a dark, humid environment with moderate temperatures, ideally 45°F to 60°F (7°C to 15.5°C). Curing heals minor cuts and bruises, further toughens the skin, and prepares the tubers for storage.