Birch trees often carry clusters of twigs called Witches' Broom
Nature on our doorsteps: Tangles of twigs
Rosaleen Dwyer is the County Heritage Officer at South Dublin County Council – every week she gives us an insight into the natural heritage around us and the beautiful biodiversity of the plants and creatures.
In winter, when trees are bare, nests that were built last springtime by crows and wood pigeons can sometimes be seen remaining amongst the branches.
Some trees, however, appear to have many nests scattered throughout the branches.
A closer look at these trees shows that these nest-like structures are not birds’ nests.
They are, in fact, dense tangles of small twigs that appear to be growing from single points along the tree’s branches.
These unusual growth masses are known as witches’ brooms.
They occur when the tree experiences some form of stress, caused perhaps by an infection caused by bacteria, a fungus, or a virus.
They can also be caused by aphids, mites, or some other type of insect damage.
Normal hormone activity in trees controls the regular growth of buds along the branches.
When a tree is under attack these normal hormonal processes are disturbed and the buds respond differently.
The dense tangles of twigs offer places for insects to hide away in wintertime
One result is the rapid and exaggerated growth of buds and twigs in locations along an otherwise normal branch.
Birch trees appear especially vulnerable to this stress response.
These trees can carry many tangles of twigs, ranging in size from quite small clusters to bundles that are so heavy that they cause light branches to visibly droop.
Witches’ brooms tend not to affect the overall health and vigour of the tree.
They can also act as natural mini ‘insect hotels’, where the dense tangle of twigs offers a place for insects and their larvae to rest, feed and develop.
These in turn can attract birds who will search through the mass of twigs in search of something tasty.
Old fashioned sweeping brooms were once formed using bundles of twigs tied to the end of a long handle made from a light branch of a tree.
Also, in the past, as witches were presumed to be the cause of anything unusual, the strange tangles of twigs that appeared like magic in trees became known as witches’ brooms.
