Witch’s Broom on Maple

2021 October 31

I’ve been seeing this growth at the top of one of the maples west of our house for some time, and now that the leaves have fallen off of the trees I was finally able to get clear pictures of it on October 31, 2021 (Halloween).


It was about 20 to 25 feet off of the ground, so it was hard to get a closeup image.



While I couldn’t get to the big one, there was another smaller, less-dense one that was closer to the ground.


This one I could reach with our pruning loppers, so I cut it off and brought it in for Sam to hold, to give a size reference.


The branches and leaves on these growths look so different from the rest of the tree that they grow on, that they could be mistaken for a different plant altogether. They aren’t, though. It is in fact all one tree.


The leaves not only come in deformed, they also don’t fall off properly in the autumn, and so these growths really stand out on the otherwise leafless trees in late fall and winter.


So, this is not some sort of parasitic plant, it is more like the tree equivalent of a wart or tumor. This type of growth is called a Witch’s Broom. It is caused by a misfunction in the hormones that normally cause trees to grow with a characteristic shape. There can be a number of causes, including cell mutations; various types of fungal, bacterial, or virus infections; or certain insects, mites, and nematodes. These can all cause the auxins and cytokinins (plant hormones) to be produced inappropriately. This results in the tree producing new buds and branches at much too high of a density, producing a dense, tangled mass. It does not appear to harm the tree particularly, but the growth does generally persist for the life of the tree.

The effect is not limited to maples like the ones we have here. Witch’s Brooms occur in most types of trees. These dense growths are actually helpful for wildlife. Northern Flying Squirrels like to nest in them, as do a lot of different kinds of small birds.

One Response
  1. Anne Bingham permalink
    November 3, 2021

    Fascinating. I’m realizing there probably are a lot of those in our treeful neighborhood that I’ve been mistaking for squirrel nests.

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