Why Are My Potatoes Green Under the Skin?

You cut into a potato and find a green layer beneath the skin, or you notice that some of your stored tubers have a greenish tinge on the surface. This is not a disease or pest problem — it is a chemical response to light. When potato tubers are exposed to light, they produce chlorophyll (which turns them green) and, more importantly, solanine — a bitter, mildly toxic glycoalkaloid. Green potatoes are a real safety concern if consumed in significant quantities, so understanding why they green up and how to prevent it is genuinely important.

Why potatoes turn green

Potatoes are modified underground stems, and like all green plant tissue they respond to light by producing chlorophyll. The green colour is the visible marker, but the solanine that accompanies it is the real concern. Solanine is a natural pest deterrent — it tastes intensely bitter and causes nausea, cramps, and in very large amounts more serious symptoms. Any potato tuber that receives light — whether in the garden, during harvest, during storage, or on a shop shelf — will begin producing solanine within a day or two. Light exposure for even a few hours can start the process.

Greening in the garden

Tubers green in the garden when they are not adequately covered with soil. Potatoes naturally push toward the soil surface as they swell, and if not earthed up regularly, the tops of tubers break through the surface where they are exposed to light. Earthing up — mounding soil over the emerging shoots and around the base of the plant — is the critical preventive technique. You should earth up when the shoots are 15–20 cm tall and again as the season progresses. The mound should cover any potential tuber zone by at least 10–15 cm of soil at all times.

Greening in storage

Potatoes stored in any light — even dim light — will slowly green up over weeks. Any translucent storage container, a window, a light-coloured bag, or even reflected light in an otherwise dim store will cause greening over time. Potatoes must be stored in complete darkness in paper bags, hessian sacks, or opaque boxes. Never store potatoes in clear plastic bags or on a kitchen worktop where any light reaches them. Check stored potatoes regularly and use any that have the slightest hint of greening immediately, cutting away and discarding the green parts generously before eating.

Are green potatoes safe to eat?

Cut away all green flesh and any nearby areas generously — solanine penetrates the flesh beyond the visible green zone. A small green patch, thoroughly peeled away with 5–10 mm of flesh removed around it, leaves a safe potato. However, if a potato is extensively green, deeply green, or has a strong bitter smell, discard it entirely. Never feed green potatoes to children in any amount. Cooking does not destroy solanine, so cooking a green potato does not make it safe.