Why Do Potatoes Turn Green?

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Potatoes turn green because light reaches the tuber. The green colour can appear on a plant in the garden or on a bag sitting under store lights. The short answer to why do potatoes turn green is chlorophyll, which the sources describe as harmless. The longer answer involves solanine, a bitter compound that rises with light and can cause illness in large amounts. This guide uses four university extension pages to explain the cause, when a green potato should be cut or discarded, and how to prevent greening.

Quick answer

Light causes the green colour, which comes from chlorophyll. The sources call chlorophyll tasteless and not toxic.
Light also raises solanine, which is the reason green potatoes are a concern. Solanine is highest in the skin and just beneath it.
Cut away small green areas and any sprouts before cooking. The sources advise discarding potatoes that are largely green.
Hill soil over garden tubers and keep harvested potatoes dark, cool and well ventilated.

How we chose these sources

We used four extension pages from four separate institutions: Iowa State University, Purdue University, the University of Alaska Fairbanks and Penn State. Each was opened on 2026-10-29 and read in full. Because the question has a health side, we used more than the minimum of three sources that a health claim needs. None of the pages is a retail page.

The sources agree on the cause and on the main prevention steps. They differ in tone on the health risk, and in how they describe what to do with a potato that has only some green. The draft reports both views instead of choosing one. The Penn State page is a canning article. Only its section on green and on storage is used here, and its canning instructions are not relied on.

What makes a potato turn green

Purdue explains that the tuber is a modified stem structure that grows underground, and that its eyes are buds that sprout into shoots. For the basics of why plant tissue turns green, see why are leaves green. When light reaches the skin, the tuber produces chlorophyll, which gives the skin its green colour. Iowa State says that when potato tubers are exposed to light, either in the garden or in storage, their skin turns green because chlorophyll forms.

Light can reach a potato in several places. Purdue says that potatoes turn green when they grow too close to the soil surface, and also when they are stored under even low-light conditions. The University of Alaska page describes the same routes. It says exposure can happen when the soil does not cover the tubers well during growth, or when the potato is exposed to artificial light during storage or on the retail shelf. Penn State adds that commercially packaged potatoes in clear plastic bags may turn green in the store.

Packaging has changed the picture. The Alaska page explains that potatoes used to be packed in burlap and other opaque materials. Consumers now want to see what they are buying, so packaging has moved toward transparent bags that let light in during display and storage. Penn State says that potatoes packed in heavy paper with a viewing window underneath are less likely to show green beneath the skin.

Why the green matters: solanine

The colour and the concern are separate. Purdue says that chlorophyll is not toxic, but that light also triggers the tuber to make a colourless alkaloid called solanine. The amount of solanine rises with the length and intensity of the light. The Alaska page makes the same point in plainer terms: chlorophyll is tasteless and harmless, and the health concern is solanine, which develops in the same area as the chlorophyll.

The bitter taste is the main warning. The Alaska page says that the bitter taste comes from solanine, and that solanine can irritate the gastrointestinal tract. Purdue says that a large quantity of solanine can cause illness or even death in extreme cases. Both pages add that most people are unlikely to eat enough of a bitter tuber to cause harm, because the taste stops them first. Iowa State uses a different name for the same family of compounds. Its page says that green tissue can contain higher levels of glycoalkaloids, and that green tubers may cause an upset stomach and more serious health problems.

Because the tuber is a stem that stores food underground, not a leaf, the greening happens in a plant part that is normally hidden. For a general guide to underground plant storage organs, see what is a bulb in plants. Green colour is not a precise measure of the toxin. The Alaska page says that the amount of greening is not a direct measure of solanine content, because the two are produced by separate processes. The page also says that some greening without solanine production is not unusual, and that excessive greening may indicate solanine is present. A bitter taste is what the page says will verify it.

When to cut away and when to discard

The advice is consistent at the two ends of the scale. Iowa State says that tubers with small green areas can be safely eaten if the green portions are cut off and discarded, and that it is best to discard potatoes that are largely green. Penn State says that if there is slight greening, the green portions should be cut away before cooking and eating. Purdue says that removing the green portion removes most of the toxin, because the highest concentration is in the skin.

The Alaska page is more relaxed about the middle ground. It says that greened potatoes do not necessarily need to be discarded, because the solanine is greatest in or just beneath the skin and peeling is an effective way to remove the affected tissue. The same page says that cooking in steam or water reduces solanine levels by 30 to 40 percent. It does not say that cooking makes a largely green potato safe, and the draft does not claim that it does.

Sprouts need the same attention. Purdue says the sprouts of the eyes are also high in solanine and should be removed before cooking. Penn State says that sprouting shows the potato is trying to grow, and that the sprouts should be cut away before cooking. A potato with both green skin and sprouts needs both steps.

Prevent greening in the garden

In the garden, the main fix is to keep the tubers covered. Iowa State advises hilling soil around the base of the plants so the developing tubers are not exposed to light. Purdue recommends mulching the plants to prevent greening. The Alaska page lists hilling to cover exposed tubers and reducing exposure to natural light as two of the main steps. Its list also includes selecting suitable varieties and planting at the right depth.

Variety matters too. The Alaska page says that white-skinned varieties often green more readily than red or russet varieties. It also says that the red and russet types can still green, but that the colour change is easier to miss.

Immature and recently dug potatoes are another risk. The Alaska page says that these green more readily because they have a thin outer skin or have not completed skin set. Harvest timing and handling therefore affect how much green shows up after digging.

Prevent greening in storage

Storage is where many green potatoes start. Iowa State advises storing potatoes in a dark location after harvest. Purdue says that storing harvested potatoes in complete darkness will prevent greening. The Alaska page advises storing them at home in a cool, dark place with good air circulation, and it says that greening increases with higher air temperatures, citing 68 degrees Fahrenheit against 41 degrees Fahrenheit as an example.

Penn State gives the most detail on conditions. It says potatoes should be kept in a cool, humid but not wet environment that is dark and well ventilated, and that the ideal storage temperature is 45 to 50 degrees Fahrenheit. It also warns that potatoes stored below 40 degrees Fahrenheit, as in a refrigerator, will develop a sweet taste. The page adds that cold temperatures change starch into sugar, which causes darkening when potatoes are cooked at high heat.

A simple storage plan follows from these notes. Keep the potatoes out of light, in a cool place with air moving around them, and check them regularly for green skin and sprouts. Remove any that are greening before they reach the kitchen. Those are the points the sources agree on.

What we could not verify

The sources answer the main question, but several details are missing. The draft does not supply them.

What the sources do not say

None of the four pages gives a solanine amount for a potato or a safe limit for a person or a child.
Iowa State says “largely green” and Purdue says “large quantity” without defining either one.
The Alaska page says greening is not a direct measure of solanine. None of the pages gives a colour or size rule that predicts the toxin level.
The pages do not describe a home test for solanine beyond the bitter taste that the Alaska page mentions.
The Alaska page says that varieties differ, but it does not name low-greening varieties, and the other pages do not either.
The sources do not describe what to do if someone has eaten a large amount of green potato. Anyone worried about symptoms should contact a health professional.

Frequently asked questions

Is the green on a potato poisonous?

The green colour itself is chlorophyll, which Purdue and Alaska both call not toxic. The concern is solanine, which light also raises. The bitter taste is the usual warning that solanine is present.

Can I eat a potato that has a little green on it?

The Iowa State and Penn State pages say yes, as long as the green parts are cut away before cooking and eating. Iowa State adds that it is best to discard potatoes that are largely green.

Does peeling remove the solanine?

The Alaska page says peeling is an effective way to remove solanine-affected tissue, because the concentration is greatest in or just beneath the skin. Purdue says that removing the green portion also removes most of the toxin.

Why do potatoes in clear bags turn green in the store?

Penn State says that commercially packaged potatoes in clear plastic bags may turn green from light exposure in the store. The Alaska page says transparent packaging lets light in during display and storage.

Does cooking remove all the solanine?

No source says that. The Alaska page reports that cooking in steam or water reduces solanine levels by 30 to 40 percent, and it does not say that cooking removes all of it.

Sources

Iowa State University Extension and Outreach, Yard and Garden, “Why are some of my potato tubers green?” (last updated July 30, 2024): https://yardandgarden.extension.iastate.edu/faq/why-are-some-my-potato-tubers-green (opened 2026-10-29)
Purdue University Extension, Indiana Yard and Garden, “Shining a light on why potatoes turn green”: https://www.purdue.edu/hla/sites/yardandgarden/shining-a-light-on-why-potatoes-turn-green/ (opened 2026-10-29)
University of Alaska Fairbanks Cooperative Extension Service, “Green Potatoes: Causes and Concerns” (FGV-00337, reviewed March 2022): https://www.uaf.edu/ces/publications/database/food/files/pdfs/FGV-00337-GreeningofPotatoes.pdf (opened 2026-10-29)
Penn State Extension, “Canning Potatoes” (updated April 15, 2024), green and storage sections: https://extension.psu.edu/canning-potatoes (opened 2026-10-29)