Why Leaves Change Colour in Autumn

A world turning red and gold

In autumn, something beautiful happens. The green leaves of many trees slowly turn yellow, orange, red and brown. Soon they drift down to cover the ground. Have you ever wondered why leaves change colour, and why they fall at all? There is real science behind it.

Why leaves are green in summer

Leaves are green because they are full of a substance called chlorophyll. Chlorophyll is a green pigment — a natural colouring. It does a very important job: it catches sunlight so the leaf can make food for the tree. This food-making is called photosynthesis, and it is how the whole tree grows.

All summer long, the leaves are packed with chlorophyll. There is so much green that it hides every other colour.

The colours that were there all along

Here is the surprising part. Leaves also contain yellow and orange pigments — the same kinds of colours found in carrots and sweetcorn. These colours are in the leaf the whole time, even in summer. We just cannot see them, because the strong green chlorophyll covers them up.

So the yellows and oranges of autumn are not new. They were hiding under the green all year.

What happens in autumn

As autumn arrives, two things change: the days get shorter and the weather gets cooler. The tree senses this and gets ready for winter. It knows winter is too cold and dark to keep making food.

So the tree stops making chlorophyll. The green slowly breaks down and fades away. And once the green is gone — surprise! — the yellow and orange colours that were hiding underneath finally show through.

Some trees, like maples, also make a brand-new pigment in autumn that is red or purple. That is why a maple tree can blaze bright scarlet.

Why the leaves fall

If the colours are so pretty, why does the tree drop its leaves at all? Leaves are thin and full of water. In winter, that water could freeze and damage them, and leaves also lose water that the tree cannot easily replace from frozen ground.

So the tree does something clever. It grows a tiny wall of cells where each leaf joins the branch. This slowly cuts the leaf off. A gust of wind or a drop of rain, and the leaf falls. The tree seals up the spot, locks in its water, and rests through the cold months. In spring it will grow fresh new leaves.

Trees that do this are called deciduous trees. Trees that keep their leaves, like pine and fir, are called evergreen. You can read more about how a tree changes across the year in Trees Through the Year and how trees grow in the first place in How Trees Grow.

Real autumn colours to spot

• Maple — fiery red, orange and yellow
• Oak — deep reddish-brown
• Birch — bright golden yellow
• Beech — copper and orange

Observe it yourself: a leaf colour collection

You can see these hidden colours for yourself.

1. In autumn, take a walk and collect fallen leaves of different colours and shapes. Try to find leaves from at least three different trees.
2. Lay them out from greenest to reddest. Can you find one leaf that is half green and half coloured? That leaf was caught in the middle of changing!
3. Press your leaves flat inside a heavy book for a week, then tape them onto paper and label each tree.

Watch the same tree over a few weeks. Each week, note its colour. You are watching chlorophyll disappear, one leaf at a time.