Witches' brooms: what they look like and why they form

It’s reasonably common to see weird growths on trees when you’re out and about, and one growth in particular can easily catch your eye. Looking like a dense tangled bird’s nest, witches’ brooms are a growth distortion that results in bunches of stem, leaf or root material growing from one point on a tree. 

What is witches' broom?

The term ‘witches’ broom’ relates to the deformity in trees. They can be small or large, and a single tree may have several or only one. In herbaceous plants this deformity is known by the scientific names of phyllanthies, phyllodies or chloranthies. Collectively, these are known as virescences. A virescence is a growth consisting of distorted and multiplied parts of a plant, either green and leafy or woody and twiggy. It comes from the Latin virescens which means ‘becoming green’, and is not the same root as the word ‘virus’, although some viruses do cause virescences!

What causes witches’ brooms to form?

Witches’ brooms are caused by microorganisms, and are therefore technically a type of gall. It’s thought that witches’ brooms are caused by fungal, viral or bacterial activity, and occasionally insect activity. This might sound like plant pathologists are hedging their bets, but really there are multiple causes of witches’ brooms. Witches’ brooms seen on birch trees, for example, are likely caused by a fungus called Taphrina betulina, and create some lovely examples of witches’ broom.

In a witches’ broom, the growth of a lateral bud – the buds that make twigs and side shoots – loses control and causes multiple stems to form in a tangled, disorganised manner. Multiple years of growth is required to create big brooms.

Do witches’ brooms harm the tree?

By themselves, witches’ brooms don’t tend to have an impact on the long-term health of a tree. The growth in the deformed area doesn’t usually function as it should. Witches’ brooms are a woody deformity, and whilst leaves sometimes grow on the broom, photosynthesis is usually limited. The tree as a whole continues to grow as normal though, and if witches’ broom occurs on fruit trees it doesn’t usually affect the volume or quality of crop.

Should I leave witches’ broom on the tree?

Whether to leave a witches’ broom on the tree is a personal choice. Some people don’t like the appearance of trees that have brooms, in which case judicious pruning of the damaged limbs by a professional should solve the problem. For trees where their aesthetic qualities are not so important, we think you should leave the brooms in place. They’re a natural occurrence and are an interesting feature in their own right!

Where can I find a witches’ broom?

Witches’ brooms are reasonably common, so you should not have too much trouble finding one. Trees develop them in both woodland settings and urban areas, and if you start looking at your local trees when you’re out and about you may spot one. They’re usually easier to find in winter when deciduous trees are bare, but they’re permanent features and can be seen in green tree crowns too.

At Snidley Moor in Cheshire, several trees are developing a lovely spread of witches’ brooms. You’ll likely find a tree or two similarly affected in your local wood.