Why Potatoes Turn Green

What the green color means

A potato tuber is a swollen underground stem, and like any stem it turns green in the light. The green itself is chlorophyll, which is harmless. The problem is that light also increases glycoalkaloids, mainly solanine and chaconine, which are concentrated in the skin, the eyes, the sprouts and any green parts. Chlorophyll and solanine build up separately, but they're triggered by the same thing, so green skin is a warning sign that solanine is likely higher too.

Eaten in quantity, solanine can cause stomach upset. Potatoes that taste bitter also tend to be high in it. The usual advice is to cut away green parts and the skin around them before cooking, and to throw away potatoes that are green all over or taste bitter. Never eat potato sprouts.

Where light gets in

• Gaps around the basket: the most common leak, where the basket sits in the hole in the lid.
• Translucent walls or lids: many white buckets and some totes let light through.
• Tubers pushing up: tubers near the top of the basket swell up into the light if the medium is too shallow.
• Medium settling: perlite and coco coir settle and shift as the plant grows, exposing the upper tubers.
• After harvest: potatoes left in daylight or under room lights for a few days turn green in storage too.

How to find the leaks

Put the lid on, take the container into a dark room and shine a flashlight inside. Any glow on the outside walls or around the lid is a leak. Check the basket too: lift a little medium at the top and look for tubers that are already tinged green. Algae in the solution, or green slime on the walls, is another sign that light is reaching the container.

How to keep tubers dark

• Use an opaque container, or paint or wrap the outside. See best containers for Kratky potatoes.
• Close the gap between the basket and the lid with tape or a ring of foam.
• Keep adding medium around the stems as the plant grows, which is the Kratky version of hilling.
• Choose the deepest basket the container allows, so there's room for medium above the top tubers.
• Check the top of the basket every week or so, and cover any tuber you can see.

Greening on red and purple potatoes

Greening is harder to spot on red, blue and purple potatoes, because the colored skin hides it. Scrape a little skin away with your thumbnail or a knife and look at the flesh just underneath. A green layer there means the potato has been in the light, whatever color the skin is.

Checking without causing greening

Lifting a little medium to look at the tubers for a minute or two does no harm. Greening builds up over hours and days of light, so the problem is a gap that lets light in all day, not a quick check. Cover the tubers again as soon as you've looked.

Green after harvest

Greening doesn't stop at harvest. Cure and store potatoes in the dark, in a paper bag or a covered box rather than a clear bag or an open bowl near a window. Potatoes on a kitchen counter can start to green within days.

Is a green potato a failed crop?

Usually not. Light green patches on one side of a tuber are common, and the rest of the potato is fine once the green part is cut away. Treat greening as a sign to fix the container before the next crop rather than as a reason to give up. For the rest of the setup, see how to grow potatoes without soil.