Why Do Leaves Change Colour in Autumn? The Science, Simply Explained
Why do leaves turn red, orange and yellow in autumn, then fall? The science behind autumn colour, explained in plain English.
Every autumn, trees across the UK turn gold, orange and red. It's one of nature's best shows, and the science behind it is simpler than you might think.
Why leaves are green
Leaves are green because they're full of a chemical called chlorophyll. Chlorophyll captures sunlight, which the tree uses to make food from water and carbon dioxide. This process is called photosynthesis. There's so much chlorophyll in a summer leaf that its green colour hides all the other colours.
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What happens in autumn
As the days get shorter and colder, trees get ready for winter. They stop making new chlorophyll, and the old chlorophyll breaks down. As the green fades, the colours that were there all along start to show.
- Yellow and orange come from chemicals called carotenoids, the same family of pigments that make carrots orange. They were in the leaf all summer, hidden by the green.
- Red and purple come from anthocyanins. Unlike yellows, many trees make these new in autumn, using sugars trapped in the leaf.
- Brown is what's left when the other colours have broken down.
Why do leaves fall?
Big, thin leaves would lose too much water and could be damaged by frost and snow in winter. So the tree grows a special layer of cells where the leaf stalk joins the branch. This layer slowly cuts the leaf off, and eventually it falls. The tree then rests through winter and grows new leaves in spring.
What makes a great autumn colour year?
The brightest reds usually come after a run of sunny days and cool, but not freezing, nights. Sunshine helps leaves make sugars, and cool nights keep those sugars in the leaf, where they help produce red pigments. Strong winds and heavy rain can strip leaves early and cut the show short.
Common questions
Why do some trees keep their leaves in winter?
Evergreen trees, such as pines and holly, have tough, waxy leaves or needles that cope well with cold and dry weather, so they don't need to drop them.
When is the best time to see autumn colours in the UK?
Usually from mid-October to early November, though it varies by year, tree and location.
Why are some leaves red and others yellow?
It depends on the type of tree. Some trees make lots of red anthocyanins in autumn, while others mainly show the yellow carotenoids already in the leaf.
Is this a good science topic for kids?
Yes. Collecting leaves of different colours and talking about why they change is a great, free autumn activity.
Related guides
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