Witches broom tree

If you find something that looks like a messy bird’s nest in a birch tree, or even better, lots of messy bird nests, then you have found a very unusual and very large gall. It’s called a witches broom and isn’t caused by a gall-making insect but by a fungal infection which makes the tree grow compact bunches of small, thin twigs. They can be as small as oranges or as large as beachballs. The peculiar name comes from an old superstition that witches made their magical flying brooms out of these strange growths. Witches brooms are very common, especially on older downy birch where almost all trees have a witch’s broom or two.

Birch trees don’t seem to be very negatively affected by the witches brooms, and because they are composed of birch branches, they can stay in the trees for years.

If you can get hold of a fallen or low-hanging witches broom in the winter, then shake it over a white tray if you want to see something interesting. Hundreds of small insects use many tiny cavities in the witches brooms for overwintering.