The sight of a potato tuber turning green is a common concern for both home cooks and gardeners. This discoloration signals a change in the potato’s internal chemistry, leading many to question its safety for consumption and viability for planting. Greening is a defense mechanism triggered by light exposure. While an inedible potato might seem destined for the compost pile, its viability for growing a new crop is a different matter.
The Physiology of Greening
Potato tubers, which are modified stems, change when exposed to light from the sun, artificial lights, or fluorescent bulbs. This exposure initiates greening in the peripheral cell layers beneath the skin. The green color comes from the production of chlorophyll, the pigment used for photosynthesis.
Chlorophyll is harmless and is not the source of the potato’s toxicity. However, its presence serves as a visible marker that a more concerning chemical process is occurring simultaneously. Both chlorophyll synthesis and the production of toxic compounds are activated by light exposure.
Why Green Potatoes Are Toxic to Eat
The danger in a green potato comes from the concurrent production of steroidal glycoalkaloids (SGAs), primarily \(\alpha\)-solanine and \(\alpha\)-chaconine. These compounds are the plant’s natural defense against pests and herbivores. When exposed to light, the tuber increases the synthesis of these bitter toxins alongside the chlorophyll.
The highest concentrations of solanine are found in the peel, the sprouts, and the area directly beneath the greened skin, often exceeding the safe limit for consumption. A greened potato can have glycoalkaloid levels five to fourteen times higher than a healthy potato. Consuming elevated solanine can cause symptoms ranging from gastrointestinal distress, such as vomiting and abdominal pain, to neurological issues like headache and confusion. Since cooking does not effectively destroy solanine, removing the green parts and sprouts is the only way to mitigate the risk, or the potato should be discarded if greening is widespread.
Viability for Planting
Despite being unsafe to eat, green potatoes are viable for planting. The solanine produced does not damage the buds, or “eyes,” which are the parts of the tuber that sprout new growth. The potato’s ability to grow relies on the viability of these dormant stem buds.
Although solanine is concentrated in the outer layer, the core tissues supporting the new plant’s initial growth remain sound. When planted, the tuber serves as the initial food source for the developing sprout. The resulting new potatoes will not be contaminated by the solanine in the parent seed piece.
New tubers develop on underground stems called stolons, and if covered by soil, they will be white and safe to eat. Intentionally exposing seed potatoes to light before planting, known as chitting, is a positive step that produces short, sturdy sprouts less likely to break off during planting.
Preparing Green Seed Pieces
To use a green potato for planting, it must first be prepared into seed pieces, which are typically chunks of the tuber. Use a sharp, clean knife to cut the potato into pieces, ensuring each section is roughly two inches square and contains at least one or two healthy eyes. Small potatoes, about the size of a golf ball, can be planted whole.
Trimming away any heavily greened skin and flesh is prudent, though this is primarily a precaution and not strictly necessary for the plant’s health. Once cut, the seed pieces must be allowed to “cure” for two to five days in a warm, dry area away from direct sunlight.
This curing process allows the cut surface to dry out and form a protective, leathery layer called a callus. The callus prevents the seed piece from rotting in the soil, especially in cold or damp conditions, ensuring a strong start for the new sprout. When planting, ensure the seed pieces are buried deep enough, typically three to four inches of soil initially, and plan to “hill” soil up around the growing stems to prevent the new crop of potatoes from turning green.