Why Do Leaves Change Colour? The Red Was Not There in Summer

Why do leaves change colour in autumn? Walk down a street with birch and rowan trees at the end of September. The birch is yellow. The rowan glows red. It is easy to think both leaves did the same thing. They did not. One leaf has lost something. The other has made something new.

What happens when leaves change colour?

Autumn colours come from pigments inside the leaf. Green is chlorophyll, the pigment that captures sunlight so the tree can make sugar. In summer there is so much of it that it hides everything else. In autumn the tree breaks it down and takes the nutrients back before the leaf falls.

Yellow was there all along. The yellow and orange pigments sat in the leaf all summer, masked by the green. That half of the story is in our autumn laboratory guide.

Here is the surprise: the red was not there in summer. Many trees make a brand-new pigment in autumn, called anthocyanin. Rowan makes it. Birch makes almost none, so it turns yellow. Why spend energy on a colour just before the leaf falls? Scientists still debate it. One common idea is that the red works a little like sunscreen while the tree pulls out the last nutrients.

Why it matters for children

Two things that look alike can have different causes. Norway's curriculum asks pupils to experience nature through the seasons, to test hypotheses, and by Year 7 to distinguish between observations and conclusions. The strip is the observation. "The red is new" is the conclusion. This is what STEAM learning means.

Try it at home: a colour hunt on a coffee-filter strip

You need a green, a yellow and a red leaf, a white coffee filter, a coin, nail polish remover with acetone, a small glass, a pencil and tape. Suitable for ages 6–12.

Before you start, ask: if the red had been hiding all summer like the yellow, what should a green rowan leaf show on the strip?

1. Cut three strips of coffee filter, about two centimetres wide. Draw a pencil line two centimetres from the bottom of each.
2. Lay the green leaf over the line and roll the coin's edge hard until the line is dark. Repeat with the other leaves on their own strips.
3. An adult pours one centimetre of nail polish remover into the glass.
4. Tape the strips to the pencil and rest it across the glass, with the line above the liquid.
5. Wait until the liquid almost reaches the top.
6. Lay the strips side by side. Which colours do you find on each?

⚠️ An adult handles the nail polish remover: it is flammable, so air the room and keep it away from flames.

Glue the strips next to a pressed leaf in a herbarium, or try leaf printing to see the veins instead of the colours.

What happens if you put pieces of the red leaf in plain water instead?

Questions to wonder about

• Why do some leaves just turn brown instead of yellow or red?
• If two rowan trees stand on the same street, why can one be much redder than the other?
• If red works like sunscreen, how could you test that on a single tree?

One street, two stories

Next time you pass the birch and the rowan, you will see two stories: one tree letting go of its green, and one making something new.

Every child is made of good atoms. Explore free content at goodatoms.com.