Why do leaves change colour in autumn?

Chlorophyll is the pigment responsible for the green colouring we see on leaves throughout spring and summer. It helps trees capture sunlight and convert it into energy through photosynthesis.

As temperatures begin to cool, and daylight hours shorten, deciduous trees start preparing for winter by slowing down energy production. This means they gradually stop producing chlorophyll. As the green pigment breaks down, other hues begin to emerge.

The orange and yellow shades we associate with autumn come from compounds called carotenoids — the same things that make carrots orange! These pigments are present in leaves for much of the year, but they're usually hidden beneath the stronger green colouring of chlorophyll.

Some trees also produce red pigments called anthocyanins, which give autumn leaves their distinctive crimson and purple shades.

When do leaves start changing colour in the UK?

In the UK, leaves typically begin changing colour between September and November, though timing can vary depending on weather, location, and tree species.

Wind, drought, rainfall, and early frosts can all influence the timing and intensity of autumn displays — that's why every autumn looks a little different, even for the same tree!

Through our work planting and caring for urban trees, we've seen how local conditions within towns and cities can affect timing too. Trees growing in sheltered parks or warmer urban environments often keep their leaves for longer than those in more exposed locations.

Why do trees lose their leaves in autumn?

Trees lose their leaves in autumn to conserve water and energy through the winter months. As days become shorter and temperatures drop, there's no longer enough sunlight for efficient photosynthesis, so keeping leaves becomes more costly than beneficial.

By shedding their leaves, deciduous trees reduce water loss, conserve valuable resources, and protect themselves from winter weather. This helps them survive the colder months and prepare for new growth in spring.

Why might leaves fall before autumn?

Sometimes, trees begin shedding leaves earlier than expected. This can be a sign of stress rather than part of the normal seasonal cycle.

Prolonged drought and heatwaves (such as the conditions experienced during summer 2026), pests, disease, and root damage can all cause trees to shed their leaves prematurely in an attempt to conserve resources. This is sometimes referred to as 'false autumn'.

Although some early leaf drop is normal during particularly hot or dry years, significant leaf loss during summer may indicate that a tree is under stress.

Why do we still see green trees in winter?

Not all trees lose their leaves in autumn. Trees that retain their foliage throughout the year are known as evergreens, and those that shed their leaves each autumn are deciduous trees.

Evergreens continue to photosynthesise during winter thanks to adaptations such as waxy coatings, smaller leaf surfaces, and needle-like leaves that help reduce water loss in colder conditions.

Should fallen leaves be cleared away?

Where it's safe to do so, letting fallen leaves lie can be of real benefit to nature. Fallen leaves (as well as fallen trees) help support biodiversity, insulate tree roots from the winter cold, nourish the soil as they break down, provide food for worms and other decomposers, and keep carbon locked up and out of the air.