Why Leaves Change Colour

In spring and summer, trees look green from top to bottom. But in autumn, many leaves turn brilliant shades of yellow, orange and red before they fall. Where do all those colours come from?

The answer is that the colours were hiding there all along! Leaves are full of tiny green packages called chlorophyll, which capture sunlight and use its energy to turn water and air into food for the tree. Because chlorophyll is so strong and plentiful, its green colour covers up everything else — like loud music drowning out quiet voices.

When days get shorter and cooler in autumn, trees know winter is coming. They stop making chlorophyll and slowly pull the useful parts out of each leaf to store them in the trunk and roots for next year.

As the green fades away, other colours that were there the whole time are finally revealed. Yellow and orange pigments show up first. Some trees, like maples, also make brand-new red pigments in autumn — the brightest leaves often belong to trees that get lots of sunshine.

Eventually a special layer of cells at the end of each leaf stalk seals it off like a little cork. When the wind blows, off comes the leaf, leaving behind a neat scar on the twig. The tree tucks itself in for winter, ready to grow fresh green leaves in spring.