Why do leaves change colour in autumn?

Beautiful October days are a pleasure we all enjoy as those autumn leaves turn from green to bright yellow, orange and red. These colour changes are of great importance to many plants – read on to find out why.

The transformation of green leaves into beautiful shades of orange, brown and red only occurs a few weeks each year. We all know this happens, but have you ever wondered why? And why do only certain plants undergo this colour change? We will give you the answers.

Why do leaves change colour in autumn?

The colour change we see in leaves come autumn is what happens when trees and plants start preparing for winter. Most of the trees and plants that change colour in autumn are deciduous. Unlike evergreens, they go dormant to preserve their energy for the next year. Instead of forming frost-resistant leaves that can survive the cold, they shed their leaves. Evergreens work differently; they store the energy from new shoots, enabling them to keep their foliage.

As the days get shorter and the temperatures get colder, deciduous plants start recycling and storing as many nutrients from their leaves as possible. The chlorophyll that gives leaves their green colour is converted back into valuable nutrients such as iron and nitrogen. During this process, chromoplasts and chloroplasts come to light. Due to their carotenoid pigments they appear in shades of yellow and orange. The dark red and purple colour we see on some plants comes from the pigment anthocyanin. This pigment works as sun and pest protection during the winter months.

Leaves are sensitive to sunburn in autumn, as photosynthesis is inhibited by the low temperatures. The red and purple colours, therefore, are not just nice to look at, they perform an important function: insects are more likely to stay away from red-coloured leaves because they usually contain a lot of plant defences.

Tip: the colour changes that occur throughout a so-called Indian summer are particularly radiant. The combination of cold temperatures and a lot of sunshine can cause leaf damage and sunburn. To prevent that from happening, the pigment anthocyanin is released, causing the leaves to turn bright red or purple. In grey weather, however, the colour changes are less vibrant.

Which plants change colour in autumn?

It is mostly the leaves of deciduous plants that change colour in autumn. Gingko (Ginkgo biloba), red oak (Quercus rubra), sweet gum tree (Liquidambar styraciflua), staghorn sumach tree (Rhus typhina) and birch (Betula spec.) trees all display particularly stunning autumn colours.

There are some deciduous plants that only have a brief colour change with very pale colours. Ash (Fraxinus spec.), alder (Alnus spec.) and elderberry (Sambuccus spec.) only appear in a pale yellow and brown for a very short period of time before the leaves are shed.

There are some evergreen species such as the Mahonia (Mahonia spec.) and Cotoneaster species (Cotoneaster spec.) that also change colour in autumn. They mostly turn red instead of yellow as the chlorophyll breakdown is only minimal. Instead, they form the red and purple pigment anthocyanin.

Tip: conifers also shed their leaves but this happens continuously, needle by needle, so that the foliage always stays green.

Why do leaves fall in autumn?

Deciduous plants shed their leaves in autumn as a means of protecting themselves. There are many factors that make it difficult for plants to retain their foliage and to photosynthesise in winter. One of these is soil frost, which essentially prevents plants from absorbing enough water into the roots. Along with the leaf evaporation that would occur if trees were to keep their leaves in winter, this would result in so-called frost-drought. This is common, for example, with evergreen rhododendrons.

By becoming dormant in winter, deciduous plants save energy. If the plant were to retain its foliage, the soft leaf tissue would have to be made frost-hardy before winter, which would require a great deal of energy. With the reduced amount of nutrients in the soil and the lack of photosynthesis that takes place in winter, plants do not have the energy required to protect themselves from frost. Consequently, they go dormant and shed their leaves.

A bare tree is also much less likely to suffer from branches or twigs breaking because of heavy snow. With little foliage weighing down the branches, a build-up of snow cannot occur.

How does leaf shedding work?

The mechanism behind leaf shedding is simple and effective. A thin layer of cork forms at the base of each leaf. This serves as protection because it closes the conduits and protects from frost and germs. When the leaves first start changing colour and the cork layer has not yet fully formed, the connection to the plant is still quite strong and leaves are not easily dislodged. A week or two later when the cork has fully formed, the leaves lose their connection to the branch at the slightest pressure – a flutter of wings or a gust of wind will cause the leaves to fall.

How do evergreen plants manage to keep their foliage?

Evergreen plants have foliage that is better protected against frost. It takes a lot of effort to build up such stable foliage, so they work hard to keep it on the tree. The genetic make-up of evergreens means that they have a lower capacity for photosynthesis, but, if conditions permit, they can use their leaves in winter for short periods of time.

Would you like to enjoy the wonderful bright colours of autumn in your garden? Find a list of the most vibrant shrubs and trees with stunning autumn colours here.