Witches Brooms on Hornbeam

Dear Readers, as Halloween approaches, it seems like just the right moment to talk about something that often goes unnoticed, or is mistaken for the work of a particularly industrious squirrel – witches’ brooms. The tree in the photo is a hornbeam, but unlike the trees around it, it’s covered in dozens of masses of tangled short twigs – the Royal Horticultural Society describes them as ‘disorganised’, which is putting it mildly. The lateral buds of the tree, which make the side stems, suddenly go berserk, creating this ‘birds nest’.

Although they look impressive, witches brooms are thought not to harm the tree – although the leaves in the ‘brooms’ are often misshapen, and so don’t photosynthesise very well, there will be other areas of the tree that are perfectly normal. But what causes them in the first place? One culprit, particularly in birch trees, is the fungus Taphrina betulina, and in Hornbeams it’s a close relative, Taphrina carpini. Technically, witches’ brooms are galls – the plant itself produces the twigs as a result of the fungal infection, and the fungus probably benefits as its spores are spread by the wind, so being higher up probably provides an advantage.

However, fungi are not the only reason that trees and other plants might develop this particular growth pattern: viruses, other pathogens and even genetic mutations can also cause this particular kind of chaotic growth. In fact, in some spruce trees the genetic mutation is stable, allowing the development of cultivars with a particular growth pattern. One such is the Little Gem cultivar of the Norway Spruce, which grows low to the ground and is often seen in rock gardens.

Norway spruce (Picea abies var Little Gem) Photo By Krzysztof Ziarnek, Kenraiz – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=118095457

Now that I’ve seen these witches’ brooms, I’m sure I’ll be noticing them everywhere – it’s unusual to see a tree with such an impressive crop, but there are lots of trees with just one or two, which are very easily missed. I’m always intrigued by the way that a pathogen can ‘persuade’ a plant to provide a shelter, or a way of reproducing, by hijacking the plant’s own biology. Now, that is really quite a trick.