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Knobbly growths on tree trunks: burrs and how they form

Trees can often have strange knobbly growths, and people wonder what this growth is and why it formed.

The conversation will then usually turn into whether this is harmful for the tree. Perhaps you have seen these strange growths on a tree yourself - they're easy to spot once you start looking!

A knobbly burr (Photo: WTML/Emilie Bonnevay)

In the photo above we have a mature oak tree. It's fairly common for its type with its cracked bark, green lobed leaves and... wait, what is that weird bulbous bit on the trunk?

This abnormal growth is called a burr (in American English called a 'burl', so you may find it referred to as both online).

It's a mass of shoot and bud tissue that grows manically and forms a distinctively knobbly growth on a tree. At its most basic, it's not unlike a benign tumour in an animal.

Some burrs seem to cover tree wounds (Photo: WTML;/ Emilie Bonnevay)

Why do burrs form?

It can be difficult to work out why a burr has formed on a tree. Burrs are thought to be a response to stress, but the stressor can be difficult to identify.

The burr usually forms over a wound, which may have been caused by anything; fungi, bacteria, virus, insect activity, animal activity, weather or human damage.

Although to some people burrs can look unsightly, they don’t cause any harm to the tree. Whatever wound originally caused the burr is usually healed over during the period of irregular growth, protecting the tree from any further damage at that site.

By themselves they’re not a cause for alarm, and are simply a unique feature of the tree in question! The late great British nature writer Roger Deakin described them as “like pearls in oysters”.

Burrs can have economic value, as the erratic growth of the plant tissue results in unusually-patterned wood that can be beautiful when worked carefully.

The car manufacturer Jaguar uses walnut burrs to create the veneer that lines their car interiors, and uncut walnut burrs have been known to sell for £10,000. At the lower end of the scale, woodturners can make unique products from burrs, though the wood is difficult to work as it has no grain and a lack of uniformity. It’s not a wood for beginners!

A burr plate needs experienced craftsmanship (Photo: WTML/ Andy Coates)

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