Why Do Leaves Change Color in the Fall?
The yellow and orange were hiding inside the leaf all summer. Here is what really happens when trees turn gold and red, explained for curious kids and the grown-ups who get asked.
Every autumn someone asks me this question, and every autumn I love the answer a little more. Because the truth is a small surprise: most of those golden colors were inside the leaf the whole summer. You just couldn't see them.
Green is a disguise
A leaf is a tiny food factory. To make food from sunlight, water and air, it uses a green pigment called chlorophyll. In spring and summer a leaf is packed with so much chlorophyll that the green covers everything else, like a thick coat of paint.
But chlorophyll doesn't last forever. It breaks down in sunlight, and all summer the tree keeps rebuilding it. As long as it keeps making more, the leaf stays green.
The tree reads the calendar
In late summer the nights get longer and cooler. Trees notice this. Shorter days are the signal that winter is coming, and winter is a hard time for a leaf. The air is dry, the ground can freeze, and a big flat leaf would lose more water than the tree can spare.
So the tree slows down and then stops making chlorophyll. Without new green, the old green fades away. And underneath it, the hidden colors appear.
Where the yellow and orange come from
Leaves also contain pigments called carotenoids. They are the same family of pigments that make carrots orange and corn yellow. They help the leaf catch light during the summer, but the green chlorophyll hides them.
When the green disappears, the carotenoids finally get their moment. That is why birches, aspens and many other trees turn bright yellow almost overnight. Nothing new was painted on. The cover just came off.
Where the red comes from
Red is different. Red and purple come from pigments called anthocyanins, and in most trees they are made fresh in the fall.
Here is how it works. As autumn starts, the tree begins sealing off the leaf at its stem. Sugars that the leaf made during the day get trapped inside. Some trees, like many maples, turn those trapped sugars into red pigment. Scientists think the red may work a bit like sunscreen, protecting the leaf while the tree pulls useful nutrients back out of it before it drops.
That is why some years are redder than others. The brightest reds usually come after sunny days and cool, but not freezing, nights. Sunny days make lots of sugar. Cool nights keep the sugar in the leaf.
And the brown?
Oaks often turn brown or rusty. Brown comes from tannins, bitter substances left behind in the leaf when the other pigments are gone. It's the last color in the show.
Why do the leaves fall?
While the colors change, the tree is building a special layer of cells where the leaf stem meets the branch. Little by little this layer cuts the leaf loose and seals the spot like a bandage. One windy afternoon, the leaf lets go.
Evergreen trees, like pines and spruces, keep their needles because the needles are narrow and covered in wax, so they lose much less water in winter.
Try it at home
Want to see the hidden colors yourself? Tear two or three green leaves into small pieces and put them in a glass jar. Ask a grown-up to cover them with a little rubbing alcohol and set the jar in a bowl of warm water for about an hour. Then dip a strip of coffee filter into the green liquid and wait. As the liquid climbs the paper, it separates into bands. Look closely for a yellow band. That's the color that was waiting all summer.
The short answer
Leaves change color because the tree stops making green chlorophyll before winter. The green fades and reveals the yellow and orange that were always there, and some trees make brand new reds. Then the tree lets the leaves go to save water until spring.
Next time you walk through fallen leaves, pick up a yellow one and remember: you're looking at a color that was hiding in plain sight all year.
Quick answers
Why do leaves change color in the fall?
As days get shorter and cooler, trees stop making chlorophyll, the green pigment. When the green fades, yellow and orange pigments that were in the leaf all along become visible, and some trees also make new red pigments.
Why are some fall leaves red and others yellow?
Yellow and orange come from carotenoids, which are always in the leaf. Red and purple come from anthocyanins, which some trees, like many maples, make only in autumn from sugars trapped in the leaf.
Why do leaves fall off trees?
Before winter, the tree grows a layer of cells where the leaf stem meets the branch. This layer slowly cuts the leaf off, seals the wound and lets the leaf drop, which helps the tree save water through the cold months.
