Witches' Broom on Trees

High in the branches of trees, you may sometimes spot what looks like an abandoned bird’s nest—a dense, tangled mass of twigs clustered together in a tight ball. But look closer and you may find that no bird built it at all. The strange growth is known as a witches’ broom.

A witches’ broom forms when a tree’s normal growth pattern is disrupted, causing a dense cluster of shoots to sprout from a single point. Instead of growing outward in an orderly fashion, dozens or even hundreds of small branches crowd together, creating a broom-like shape. From a distance, these clumps often resemble large nests wedged in the crook of a tree.

A birch tree infected with Taphrina betulina causing witches' broom. Credit: Wikimedia Commons

The phenomenon can be triggered by several different agents. Fungi are among the most common causes, infecting the tree and altering its growth hormones. For example, Taphrina betulina, a fungus, creates spectacular witches’ broom on birch trees. Viruses, bacteria, mites, and even parasitic plants like dwarf mistletoe can also interfere with normal development. In some cases, genetic mutations are responsible.

While witches’ brooms may look dramatic, they do not always pose a serious threat to the tree. Some remain small and harmless for years. Others, particularly those caused by parasites, can weaken branches or stunt overall growth.

Some witches' broom have proven useful. Horticulturists sometimes propagate cuttings from these dense clusters to produce dwarf ornamental varieties of trees.

So the next time you notice what seems to be a bird’s nest high in the canopy, consider that it might not shelter eggs or chicks at all. It may instead be one of nature’s botanical curiosities.

Witches' broom in a birch tree. Credit: Wikimedia Commons

Hornbeam (Carpinus betulus) with witch's broom. Credit: Wikimedia Commons

Witch's broom on a cherry tree. Credit: Wikimedia Commons

A European larch with witches' broom. Credit: Wikimedia Commons

Credit: Wikimedia Commons

Credit: Vertigogen

Credit: Wikimedia Commons

Credit: Wikimedia Commons

Scots Pine with witch's broom. Credit: Wikimedia Commons

References:
# Witches' Broom on Trees. College of Agriculture and Natural Resources
# Witches' brooms: what they look like and why they form. Woodland Trust