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What makes a copper beech tree purple?

It's still unknown exactly why the colour variation occurs in beech trees (Photo: WTPL / Juliana Ditsche)
It's still unknown exactly why the colour variation occurs in beech trees (Photo: WTPL / Juliana Ditsche)

We’re predominantly interested in native trees; trees that are naturally distributed in the UK rather than trees that have been moved about by humans.  However, sometimes science can’t actually decide if a species is native and many tree species fall into this category.

Common and copper beech trees

One tree of particular interest is the copper beech, known by the scientific name of Fagus sylvatica f. purpurea. 'Fagus sylvatica'  indicates that it is a beech tree and 'f. purpurea'  indicates that it is a purple variety of the common beech. The difficulty with the copper beech is that the purple colour is a naturally occurring mutation, but has also been selectively bred by horticulturists for ornamental garden trees.  Furthermore, the common beech (and the naturally occurring purple variety) is native to the South East of the UK, where it can naturally occur in woodland, but its spread northwards has been facilitated by people.

What does 'naturally occurring' mean?

When we describe something as 'naturally occurring', we mean that something can occur naturally in the environment without human intervention.  In the case of the copper beech, we use the term to indicate that the copper colour has occurred spontaneously in a tree without human interference.  Purple beech leaves occur in the natural environment in one of two ways: either a mature, green common beech puts out purple leaves, usually restricted to one limb, or a new sapling germinates that is predominantly purple rather than green.  There are a few beech woods in the UK where you may come across a copper beech growing in amongst green beeches.

The mystery of the new copper beech

A supporter got in touch with us recently to discuss a beech tree in an urban setting that had spontaneously changed colour.  The photo below shows the beech tree in 2014, with a purple tint, but quite clearly still a green tree. The same tree completely changed to purple this year.

Whilst it’s not unheard of for a common beech to change colour, or to produce offspring that are purple, it is very unusual for a mature specimen like this tree to completely change colour in one season.

A green beech tree that turned completely purple in one season (Photos: Google Maps (left); David/Woodland Trust Supporter (right) )
A green beech tree that turned completely purple in one season (Photos: Google Maps (left); David/Woodland Trust Supporter (right) )

Despite copper beeches being known to humans for many centuries, we still don’t understand why this colour variation occurs.  All beech trees seem to have the genetic potential to be purple, but the gene is usually ‘switched off’ and so each year the tree produces green leaves.  In this tree, it would appear that the purple gene has been ‘switched on’ this year, changing the crown from green to purple.

Why did this happen?

What has actually caused this change is a mystery.  There are still many things we don’t know about plant genetics.  We assume that the tree has responded to a threat or stress, and that switching to purple leaves confers some sort of advantage over the green leaves.  The tree is showing no other signs of stress so it’s difficult to identify what the stressor might be, and it’s likely we will never know why the tree needed to change colour.

As mentioned, the copper beeches that you find in gardens are often cultivated for their purple colour, i.e. they have been specifically bred to have this trait, it isn't naturally occurring.  As their genetics are different to the native beech, you shouldn’t expect your copper beech tree to suddenly change colour!

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