Why Do Potatoes Go Green?

Step 1 of 5

The eyes give it away.

Each eye on a potato is a bud, and buds only grow on stems. A root has no buds — which is why a potato sprouts and a carrot does not.

Step 2 of 5

Stems turn green in light.

Any stem exposed to light starts making chlorophyll, because that is what stems do. A potato left in the light is simply following the instruction.

Step 3 of 5

The green itself is harmless.

Chlorophyll is not poisonous, and eating it does nothing. It is a warning sign rather than the problem, because something else is made at the same time.

Step 4 of 5

The plant's defence is what to avoid.

Potato plants make a bitter substance that protects them from being eaten. Light and damage both trigger it, and most of it sits in the skin and the few millimetres beneath, so a green potato tastes bitter and is best cut away.

Step 5 of 5

Which is why they are stored in the dark.

Cool, dark and dry keeps a potato dormant: no light means no greening, and cool means the buds do not sprout. Not too cold, though, or the starch turns sweet.

The rest of the potato plant is poisonous throughout, leaves and berries included. Only the tuber is safe, and only when it is not green.