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Ginkgo's autumn yellow was hiding in green summer leaves. So where was the maple's red all summer?

Video published November 4, 2026 (7 min 20 sec) · Checked November 2, 2026 · 5 min read

In autumn, ginkgo trees turn yellow and maples turn red.

But the ginkgo's yellow isn't a new color born in autumn. It was inside the green summer leaves all along, just hidden by the green.

So where was the maple's red all summer?

First guess

“Just like the yellow, hiding inside the green summer leaf?”

Only partly right. There is a little red pigment in the green summer leaf, but most of it isn't there at all. So where does all that autumn red come from?

Answer

Most of a maple's red (anthocyanin) wasn't anywhere yet in summer; the leaf makes it new in autumn.

In autumn the veins carrying sap in and out of the leaf gradually narrow, so sugar made on sunny days can't easily flow out on cold nights, and that trapped sugar, plus sunlight, makes the red.

That's why plenty of sunlight helps a leaf make red, and why how bright the red gets changes a lot with each year's weather.

Watch the video (7 min 20 sec)

Chapters (15)

Full transcript

This video was made before we renamed things, so the Chief is called the “Director” in it.

0:00Yellow was hiding in summer. So what about red?

NaruAutumn. Ginkgo trees turn yellow. Maples turn red.

NaruBut that yellow isn't a new color born in autumn.

NaruIt was already there, inside the green summer leaves, all along. It was just hidden by the green.

NaruSo was the maple's red hiding in the summer leaves, too...?

NaruBut look at a Japanese maple, the fullmoon maple. Its outer leaves, out in the sun, turn bright red.

NaruThe inner leaves end up a yellowish color with little red.

ChiefDifferent colors on the same tree!? Where was the red hiding? Go find out!

NaruThe clue was something apple farmers already knew.

NaruMarusuke's Naruhodo Lab!

0:46Yellow is the hidden color

NaruFirst, the yellow.

NaruThe green in a leaf is a pigment called chlorophyll. It catches sunlight and makes food. It's the star of the leaf.

NaruThe yellow pigments, carotenoids, are in the summer leaf too. But there's far more green, so the yellow stays hidden.

NaruIn autumn, as it gets colder, the green chlorophyll is broken down.

NaruAs the green fades, the yellow underneath shows through.

NaruIt's like peeling off green wallpaper and finding a yellow wall underneath.

1:22Was red hiding too?

NaruI've got it! The red was hiding under the green wallpaper, too!

ChiefRejected!

Naru...He says: was it hiding, or made in autumn? Check the pigments before you say that.

1:36Checking the red pigment

NaruAll right, let me check the red pigment. It's called anthocyanin.

NaruAnd it turns out most of this red isn't in the green summer leaf at all.

NaruThere's a little. But most of it is a color the leaf makes brand new, in autumn.

NaruThere was almost no red wall under the wallpaper. In autumn, the leaf goes out of its way to paint itself red.

NaruAnd plenty of sunlight helps the leaf make this red.

2:07Apple farmers turn the fruit

NaruApple farmers know this well.

NaruThe red skin of an apple gets its color from anthocyanins, the same red pigments as the maple.

NaruIn autumn, farmers give each apple a twist, so the side that was in the shade gets sunlight too.

NaruThey call it turning the fruit, and it helps the color spread evenly all the way around.

2:28Quiz: what is red made from?

NaruQuiz time!

NaruWhat does the leaf make its red from? A: iron drawn up from the soil. B: sugar the leaf made from sunlight. C: the broken-down green pigment.

NaruThe answer is B: sugar.

2:51How sugar builds up in the leaf

NaruIn the daytime, the leaf uses sunlight to make sugar, its sweet food. Normally it flows out through the veins, down into the branches and trunk.

NaruBut come autumn, the veins that carry sap in and out of the leaf gradually narrow.

NaruSugar made on a sunny day can't easily flow out on a cold night.

NaruInside the leaf, with its exits narrowing, the sugar piles up.

NaruLet's watch it happen in a single leaf.

NaruThe green fades. The sugar builds up. Sunlight pours in. And the red grows.

NaruUsing the stored sugar, bathed in sunlight, the leaf makes red. That's what the maple's red really is.

3:34Red changes with the weather

NaruSo an autumn with sunny days and cold nights, one after another, brings out the brightest reds.

NaruBut if a severe frost kills the leaves, no red gets made. They turn brown and drop early.

NaruYellow is already in the leaf, so it's fairly steady from year to year. But how bright the red gets changes a lot with each year's weather.

3:58Outer leaves and inner leaves

NaruNow, back to that fullmoon maple from the start.

NaruThe outer leaves, soaked in sunlight, could make plenty of red.

NaruThe inner leaves, which get less light, make only a little red. So the yellow underneath shows through, and they look yellowish.

4:19Where was the red all summer?

NaruSo where was the maple's red all summer?

NaruFor the most part, it wasn't anywhere yet.

NaruThe leaf made it fresh in autumn, from sunlight and the sugar trapped inside.

NaruYellow is the color that was hiding. Red is the color made in autumn. Naruhodo!

4:38What is the red for?

NaruBut wait a moment.

ChiefPainting leaves you're about to throw away!? What a waste!

NaruExactly. What the leaf makes the red for is actually still not settled.

NaruThere are two leading ideas. To protect the leaf. Or to keep insects away.

4:57The parasol idea

NaruIdea one: the parasol.

NaruWhen strong light hits a weakened leaf that's losing its green, the leaf's cells can be damaged.

NaruThe idea is that the red softens the light like a parasol, and protects the leaf.

NaruBefore dropping a leaf, the tree pulls precious nutrients like nitrogen back into its branches and trunk. While the leaf is protected, it has time to finish that move.

NaruIn an American experiment, researchers put mutant plants that can't make red side by side with normal ones, under harsh conditions: strong light and cold.

NaruThe leaves of the plants that couldn't make red were damaged and dropped while still green. They also pulled back less nitrogen.

NaruBut other studies haven't clearly confirmed that red helps pull nutrients back.

5:50The signal idea

NaruIdea two: a signal to insects.

NaruAphids come to trees in autumn to lay their eggs. The idea is that red tells them: this tree is tough, there's nothing good for you here.

NaruAnd here, apples show up again.

NaruIn one study, aphids avoided apple trees whose leaves turn red in autumn. In a separate spring test, aphids placed on trees with red autumn leaves did worse. The trees really were tough.

NaruBut there's pushback. Avoiding red leaves doesn't prove red is a signal. Red leaves may simply be hard for insects to notice.

6:32Still not settled

NaruParasol or signal? Each has studies for it, and doubts against it. Why leaves turn red is still being worked out.

Chief...Naruhodo. Parasol or signal... When it's settled, let me know right away.

NaruThe Director gave me his maple-leaf stamp of approval!

6:52When you see the leaves this year

NaruWhen you see the autumn leaves this year, compare the leaves on the sunny side and the shady side of the same tree.

NaruWhere are the reddest autumn leaves near you? Tell us in the comments.

NaruThis has been Marusuke's Naruhodo Lab!

NaruIf you went 'naruhodo!', please hit like and subscribe!

Explained in pictures

A green summer maple leaf and a magnified circle of its inside: round green dots fill it, and yellow diamonds behind them barely show — Inside a summer leaf (the number of dots is just an illustration). The circle on the right magnifies part of the leaf on the left. The round green dots are the green pigment chlorophyll, which catches sunlight and makes food. The yellow diamonds behind them are the yellow pigments, carotenoids. They are in the summer leaf too, but there's far more green, so they stay hidden. As it gets colder in autumn, the green is broken down and the yellow shows through.

Magnified circles of a green summer leaf and an autumn leaf compared: in summer, many green dots and only a few red stars; in autumn, red stars everywhere and only a few green dots — The same magnified view of a summer leaf (left) and an autumn leaf (right); the number of shapes is just an illustration. The red stars are the red pigments, anthocyanins. There's a little in the green summer leaf, but most of it is made brand new in autumn. Peel off the green wallpaper and there's almost no red wall underneath. The red wasn't hiding: the leaf goes out of its way to paint itself red in autumn.

White squares of sugar piled up in a leaf at night, and a magnified circle of the vein inside the branch, narrowing from both sides so that sugar has trouble passing — A leaf and branch on an autumn night. The white squares on the leaf are sugar it made in sunlight during the day. Normally the sugar flows out through the veins, down into the branches and trunk. But in autumn, the veins carrying sap in and out of the leaf gradually narrow (see the orange squares at the leaf's base and the magnified circle). So sugar made on a sunny day can't easily flow out on a cold night, and it piles up inside the leaf.

A red leaf in sunlight, with bars for the amounts of green, sugar and red: almost no green, and plenty of sugar and red — What happens in a single leaf in autumn (the bar lengths are just an illustration). The bars on the right show, from the top, the amounts of green, sugar and red. The green fades and sugar builds up in the leaf. When sunlight hits it, the leaf uses the stored sugar to make red. So autumns with sunny days and cold nights bring the brightest reds, while a severe frost kills the leaves: no red gets made, and they turn brown and drop early.

A fullmoon maple tree: the outer leaves in the sun are bright red, and the inner leaves are a yellowish color with little red — This picture shows what has been observed on a Japanese maple, the fullmoon maple. The outer leaves in the sun turn bright red, while the inner leaves end up a yellowish color with little red. The outer leaves, soaked in sunlight, can make plenty of red. The inner leaves, which get less light, make only a little, so the yellow underneath shows through. This may not hold for every tree, so try comparing the leaves yourself this autumn.

More details

What are the green and yellow in a leaf?

The green is a pigment called chlorophyll. It catches sunlight and makes food. It's the star of the leaf.

The yellow comes from pigments called carotenoids. They are in the leaf all year, but in summer there's far more green, so they stay hidden.

As it gets colder in autumn, chlorophyll is broken down and the yellow underneath shows through. It's like peeling off green wallpaper and finding a yellow wall underneath.

Is there really no red pigment in the summer leaf?

The red pigments are anthocyanins. It turns out there's a little in the green summer leaf, but most of it is made brand new by the leaf in autumn.

Different colors on the same tree can't tell you on their own whether the red was there in summer. You have to check the pigment.

Plenty of sunlight helps the leaf make this red, and it is made from sugar the leaf produced in sunlight.

Why does the red get brighter in some years than in others?

An autumn with sunny days and cold nights, one after another, brings out the brightest reds. But if a severe frost kills the leaves, no red gets made. They turn brown and drop early.

Yellow is already in the leaf, so it's fairly steady from year to year. But how bright the red gets changes a lot with each year's weather.

Why do apple farmers turn the fruit?

The red skin of an apple gets its color from anthocyanins too, the same kind of pigment as the maple's red.

In autumn, farmers in Japan turn each apple so the side that was in the shade gets sunlight too. It helps the color spread evenly all the way around.

Why make red in leaves that are about to drop?

That is actually still not settled. There are two leading ideas: the parasol idea, to protect the leaf, and the signal idea, to keep insects away.

Each has studies for it and doubts against it, and why leaves turn red is still being worked out (as of 2026).

What is the parasol idea?

When strong light hits a weakened leaf that's losing its green, the leaf's cells can be damaged. The idea is that the red softens the light like a parasol and protects the leaf.

Before dropping a leaf, the tree pulls precious nutrients like nitrogen back into its branches and trunk. While the leaf is protected, it has time to finish that move.

In an American experiment, researchers put mutant plants that can't make red alongside normal ones under harsh conditions: strong light and cold. The leaves of the plants that couldn't make red were damaged and dropped while still green, and they pulled back less nitrogen. But other studies haven't clearly confirmed that red helps pull nutrients back.

What is the signal idea?

Aphids come to trees in autumn to lay their eggs. The idea is that red tells them: this tree is tough, there's nothing good for you here.

In an apple study, aphids avoided trees whose leaves turn red in autumn. In a separate spring test, aphids placed on trees with red autumn leaves did worse.

But there's pushback. Avoiding red leaves doesn't prove red is a signal. Red leaves may simply be hard for insects to notice.

Takeaway

Yellow is the color that was hiding. Red is the color made in autumn. This year, compare the leaves on the sunny side and the shady side of the same tree.

Sources and how we checked

AI wrote this page from the video script and the sources above. A different AI from the one that wrote the script checked it against the text of those sources (November 2, 2026). A person with a master’s degree in science and a Japanese high-school science teaching license also read and checked it before publishing. Numbers are as of that date. If anything was hard to follow, you spot a mistake, or there’s a question you’d like us to look into, please write to [email protected].

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