You’ve seen Witch’s Broom in your walks, but most people have no idea what it is, or why it is there, and they often just think it a bird or squirrel nest – and it could be. But where does it come from?
A Witch’s Broom is a mass of live tree growth, and several things can cause it. The most common cause around these parts is “broom rust”, but brooms can also result from fungi, moulds, insects, mites, viruses or even some plants. All of these things can intentionally or unintentionally introduce cytokinin, a plant hormone that interferes with the other hormones that regulate plant growth. Once the cytokinin gets inside the tree, a small bit of the tree grows almost uncontrollably in a mass of sticks, needles and twigs all coming from a single point on the tree. This creates what looks like a nest, or the switch from an old stick broom, hence the name.
Witch’s Brooms look dead in the winter, then come alive in May or June in a burst of colour (usually yellow or orange) that are spores full of cytokinins. The small one to the left (with a glove for scale) is just starting to colour up. These spores blow around in the wind to potentially infect other trees (and probably make people sneeze, too). Spruce trees are the most commonly affected in Kananaskis, but firs – especially sub-alpine – can be hit as well, as can most any other woody tree. The brooms don’t appear to adversely affect the tree, and trees can have more than one broom.
Brooms range in size from 1 foot across to much, much bigger. Because of the way they propagate, brooms often come in clusters, with up to 50 broom-containing trees in a small area. Some brooms can be upward of 2 meters in size. The tree to the right has 11 brooms in it, all highlighted in red in the photo.
Brooms themselves are actually ecologically important here. Some insects such as moths or spiders uses them as nests, and some critters nest in them as well; northern flying squirrels often hollow out ones high up in trees to use as nests. Some plant fanatics around the world have used Witch’s Brooms to deliberately infect trees to make dwarf cultivars. Brooms are commercially less desirable in tropical climes in cacao trees, where their presence reduces the yield of the beans to make chocolate.
Read about some of the other fascinating plants of Kananaskis Country here!