To me, they look like a coven of witches gathered in the treetops, and then disappeared, leaving their broomsticks behind. But even though they’re called “witches brooms," those clusters aren’t supernatural. They’ve interested foresters for centuries.
In most trees, the leading, or terminal, shoot produces a hormone called auxin, which slows and directs the growth of the lateral, or sideways, branches. This lets the whole tree maintain a shape that maximizes sunlight capture and growth.
But that form can be altered by agents that create another hormone, called cytokinin, which makes the terminal shoot lose control over its lateral branches. As a result, lots of lateral branches grow into a disorganized cluster.
What causes the cytokinin to materialize? Well, just as there are different triggers that make you sneeze — a virus, dust particles, putting too much pepper on your eggs — there are multiple causes that stimulate the production of cytokinin: fungal infections, insect invasion, genetic mutations or wind gusts that remove the terminal bud of the parent shoot.
And different types of trees are susceptible to different factors. In oak, the culprit is powdery mildew. In hackberry trees, it's a mite — a spider relative — that makes for blobs of tangled twigs.
So, although witches are not something you want to meet up with on a dark night — especially around Hallowe'en — the witches’ brooms of trees illustrate the marvelous complexity of our arboreal world.
