Why Do Potatoes Turn Green In Sunlight? (3 Ways to Prevent)


If you grow potatoes, you have probably seen some of them turn green.  If so, you might be wondering what the green color on potatoes means, and how to prevent it.

So, why do potatoes turn green in sunlight?  Potatoes turn green in sunlight due to the production of chlorophyll.   Chlorophyll is a natural chemical produced by plants to aid in photosynthesis (turning light into sugar).  Potatoes that turn green from chlorophyll may also contain solanine, which is a toxic substance.

Of course, there are things other than sunlight that can cause potatoes to produce the toxic substance solanine.  Sunlight exposure is not necessary, but it does make potatoes produce solanine faster.

In this article, we’ll take a look at what makes potatoes turn green in sunlight and why it happens.  We’ll also look at how to prevent it, along with some cautions about solanine.

Let’s get started.

Why Do Potatoes Turn Green in Sunlight?

Potatoes turn green in sunlight due to the production of chlorophyll.  Chlorophyll is a chemical produced by plants that makes them green.

green potatoes
Potatoes turn green when they produce chlorophyll after sunlight exposure.
Image courtesy of user Rasbak at Wikimedia Commons: https://commons.wikimedia.org/wiki/File:Groene_aardappels_%27Dor%C3%A9%27_(Solanum_tuberosum_%27Dor%C3%A9%27).jpg

Chlorophyll also plays a key role in photosynthesis, or the process of turning sunlight, water, and carbon dioxide into sugar.

When a potato tuber (the part of the plant that we eat!) is exposed to sunlight, it will begin to produce chlorophyll.  This is what causes the potato skin (and possibly the flesh underneath) to turn green.

There are a few different factors that determine how much green appears on a potato, and how fast it happens.  These factors include:

  • Light (Duration and Intensity)
  • Temperature
  • Variety of Potato

Let’s examine each of these factors, starting with light.

Light

According to the University of Nebraska Lincoln, even low levels of light exposure for a short duration can cause greening:

“Light durations as short as 12 hours can cause greening of a few potato varieties such as Kennebec.”

https://cropwatch.unl.edu/potato/greening

This means that potatoes still attached to the plant and exposed to strong sunlight can turn green in only one day.

sunlight through forest
Exposure to sunlight, even for a brief time, can cause potatoes to turn green.

Even potatoes that have already been harvested and left out in the sun to dry can turn green in a single day.

However, light is not the only culprit in making potatoes turn green.  Soil temperature also plays a role.

Temperature

Temperature makes a big difference in terms of whether a potato will turn green and how fast it happens.

At low temperatures, little or no greening of potatoes occurs.  At warmer temperatures, potatoes can turn green fast.

According to the University of Nebraska Lincoln:

“There is NO greening when the temperature is less than 40F, refrigeration temperature, and is most rapid at 68F, room temperature.”

https://cropwatch.unl.edu/potato/greening

This means that potatoes in cool soil are less likely to turn green (for example, during the early fall).

On the other hand, potatoes that mature during summer and stay above the surface of the soil will soon turn green.

green sprouted potato
A potato will turn green faster at warm temperatures around 68 degrees Fahrenheit (20 degrees Celsius).
Image courtesy Wikimedia Commons: https://commons.wikimedia.org/wiki/File:Evil_green_potato.jpg

Keep in mind that potato tubers can turn green whenever they are exposed to light – either outdoors or indoors.  This means that potatoes stored on a countertop where they are exposed to natural or artificial light will turn green.

According to North Dakota State University:

“Potato tubers turn green when they are exposed to sunlight during growth or storage. The green comes from the pigment chlorophyll. Potato tubers exposed to light will become green naturally as the plant seeks to harvest the light.”

https://www.ag.ndsu.edu/publications/lawns-gardens-trees/from-garden-to-table-my-potatoes-turned-green-now-what

In addition to light levels and temperature, the variety of potato also makes a difference in how fast potatoes turn green.

Variety of Potato

According to North Dakota State University:

“Potato varieties can differ in their sensitivity to light. In general, white-skinned varieties tend to turn green more easily than red- or russet-skinned varieties.”

https://www.ag.ndsu.edu/publications/lawns-gardens-trees/from-garden-to-table-my-potatoes-turned-green-now-what
potatoes
Lighter potato varieties turn green easier than red or russet potatoes.

Regardless of what type of potato you are growing, light is necessary for the production of chlorophyll to make the potato turn green.

However, light is not required for a potato to produce solanine!

What is Solanine?

According to Wikipedia, solanine is a glycoalkaloid produced by potatoes.  Solanine is also produced by other plants in the nightshade family, such as tomatoes and eggplants.

Most of the solanine in a potato tuber develops close to the skin.  Potatoes produce solanine to help protect potatoes against natural enemies, such as:

  • Insects
  • Animals
  • Disease

Solanine repels insects and makes animals sick if they eat parts of the potato plant.

Solanine is found in the stem and leaves of potato plants. It helps to protect the plant against insect and animal pests.

The leaves, stems, and shoots of a potato plant all contain high levels of solanine.  However, the potato tubers (the part we eat that grows underground) can also produce high levels of solanine.

According to Wikipedia:

“When potato tubers are exposed to light, they turn green and increase glycoalkaloid production. This is a natural defense to help prevent the uncovered tuber from being eaten.”

https://en.wikipedia.org/wiki/Solanine

This makes sense, since the tubers need to sprout in order to reproduce and make the next generation of potato plants.

As mentioned earlier, potato tubers produce chlorophyll when exposed to sunlight.  As it turns out, potatoes can produce solanine without light.

However, light does speed up the process of solanine production.  While chlorophyll is tasteless and harmless if eaten, solanine is colorless but bitter, and it is toxic if eaten in large amounts.

A potato can produce both chlorophyll and solanine at the same time.  However, potatoes can also produce one without the other.

So, a green potato may or may not contain solanine.  Similarly, a potato that is not green may still contain solanine and be unsafe to eat!

According to North Dakota State University:

“Solanine is a toxic substance that is produced naturally in potatoes and other plants to aid in resistance of insects and animals. Solanine is present in all parts of the potato plant, including the sprouts, roots, leaves, flowers and fruits.”

https://www.ag.ndsu.edu/publications/lawns-gardens-trees/from-garden-to-table-my-potatoes-turned-green-now-what

Solanine can also harm human beings if eaten in large amounts.  As mentioned earlier, potatoes can also create solanine without sunlight and without turning green.

This can happen due to the following factors:

  • Damage due to bruises, cuts, or punctures during harvest or handling
  • Warm storage temperatures
  • Frost if left underground late in fall after the first frost

How to Prevent Potatoes from Turning Green

There are several things you can do throughout the growing season to prevent your potato tubers from turning green.  One of the most important things is hilling, which is done during the growing season.

Use Hilling on Your Potato Plants

Hilling your potato plants means that you put extra soil at the base of the plant, where the stem meets the soil line.  The purpose of hilling is to keep the tubers covered with soil and out of the sunlight.

Potatoes produce tubers above the point where the seed potato was planted.  Without proper hilling, some of the tubers produced later in the season will grow above the surface of the soil.

After exposure to sunlight, these tubers will turn green and start to produce solanine.  To avoid this, simply use hilling a few times during the growing season.

All you need to do is pile a few inches of soil in a mound around the base of your potato plant as it grows.  When the plant gets taller, just add a little more soil to the mound.

When you start to hill potato plants, begin when the plant is 6 inches (15 centimeters) tall.  Add 4 inches (10 centimeters) of soil to cover all but the top 2 inches (5 centimeters) of the plant.

Repeat this process throughout the season.  By harvest, the hills around your potato plants could end up standing 12 inches (30 centimeters) above the surface of the soil!

When planting in containers or raised beds, I recommend a depth of 24 inches for potato plants.  This leaves enough room for both root growth and hilling during the season.

You need a container 16 to 24 inches (40 to 60 centimeters) deep to allow space for hilling potato plants later in the season.

A grow bag with a depth of 16 inches would be sufficient for growing potatoes.  This method would also make it more convenient to harvest the tubers at the end of the season.

You can learn more about the depth and space requirements for potato plants in my article here.

Proper Planting, Harvesting, and Storage

In order to avoid frost damage to potato plants, it is important to plant them on time so that they can mature before frost arrives in the fall.

You can learn more about when to plant potatoes in my article here.

frost
Harvest potatoes before frost in the fall to prevent damage that may lead to green tubers.

It is also important to harvest potato tubers before the first fall frost.  This avoids any cold damage that could cause tubers to produce solanine.

When harvesting your potato tubers, use your hands if possible.

Otherwise, use a pitchfork instead of a shovel.  It is too easy to cut into the potatoes and cause damage with a shovel.

You can also use a soil sifter to separate the potatoes from the soil (watch out for those pesky rocks though!)

Another idea to avoid injury during harvest is to grow potatoes in a bin with a door that lets you pull out the tubers easily without digging.

After you harvest your potato tubers, handle them carefully to avoid bruising.

You will also need to store the tubers properly to keep them from turning green.

Don’t leave them out in the sun to dry.  Instead, put them in a low-light or completely dark place.

Keep them at high humidity and cool temperatures of 45 to 50 degrees Fahrenheit (7 to 10 degrees Celsius) for optimal storage.

What to Do With Green Potatoes

If your potatoes are already green, there are a couple of things you can do with them.

One idea is to put your green potatoes in storage until next year (mark them off so you don’t cook and eat them by accident!).

Then, allow the tubers to sprout and plant them the following spring to produce more potato plants.

Let your green potatoes sprout and plant them next spring!

If you are interested in learning more about this, check out my article on planting sprouted potatoes.

Another option is to cut off the parts of the potato that are green.  If just a little bit of the skin is green, you might be able to salvage part of the potato for cooking.

However, make sure to try a little bit of the potato before eating the whole thing.  If it tastes bitter, don’t eat it!

Remember that cooking does not remove chlorophyll and solanine.  The potatoes will still be green after cooking them, and they will still contain toxic solanine that can make you sick!

What Happens if You Eat Green Potatoes?

Green potatoes can make you feel sick, but they can do more damage than that.  According to the Iowa State University Extension:

“Green tubers have a bitter taste when eaten.  They may also cause an upset stomach and more serious health problems.”

https://hortnews.extension.iastate.edu/faq/why-are-some-my-potato-tubers-green

When in doubt, avoid eating potatoes that taste bitter or look green.  Instead, put them in the compost pile and use the resulting compost to help you grow next year’s crop!

Conclusion

Now you know why potatoes turn green in sunlight.  You also know what to do to avoid green potatoes and solanine.

I hope you found this article helpful.  If so, please share it with someone who can use the information.

If you want to read some of my most popular posts, check out the “Best of GreenUpSide” page here.  Enjoy!

~Jonathon

jonathon.david.madore

Hi, I'm Jonathon. I’m the gardening guy (not guru!) who is encouraging everyone to spend more time in the garden. I try to help solve common gardening problems so that you can get the best harvest every year!

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