The Steamboat Pilot & Today

Steamboat Springs — If you’ve ever looked up into spruce trees during a hike in the woods, you’ve probably noticed what looks like a large nest or the bristle end of an old-fashioned broom. While some birds — as well as squirrels and porcupines — may have made a home in these structures, the mass of twigs is most likely a deformity known as Witches broom.

Witches broom occurs on many types of trees, both deciduous and evergreen, and is caused by fungus, microorganisms or insects such as aphids or mites. The affliction causes the ends of the branches to grow very quickly to form a dense cluster of small twigs that can resemble a bird’s nest or the sweeping end of a broom. Hence the name “Witches broom,” which is derived from the German word “Hexenbesom” for bewitch (hex) and bundle of twigs (besom).

If the broom is quite large or there are many within the same tree, it can hurt the tree by stealing its nutrients. This weakens the tree and makes it susceptible to disease or harsh weather, which could lead to its death.

On our spruce trees, Witches broom can be caused in two ways. One is by an infection of dwarf mistletoe, which is spread by spores carried on the wind or by birds that deposit the seeds on the trees. (“Mistel” is an Anglo-Saxon word for dung, and “tan” is the word for twig, meaning that mistletoe translates to the not very romantic “dung on a twig.” Think about that this holiday season as you hang decorations!) Dwarf mistletoe is a parasitic plant whose roots actually grow into the limbs of the tree, stealing its nutrients.

The other way our spruces develop Witches broom is through a rust fungus also spread by spores. The rust disease usually infects another host plant first, such as kinnikinnick or Arctostaphylos uva-ursi, also known as bearberry. While the Witches broom in the spruce trees resembles a nest right now, in the summertime, the broom is a very noticeable orange or rust color. The color comes from pustules that produce thousands of spores that drop off with the spruce needles in the fall to infect more kinnikinnicks and then spread to more spruce trees.

There is no practical way to control Witches broom other than pruning and burning the brooms.

Witches brooms have some redeeming qualities. They provide food and habitat for a variety of animals, including grouse, the hairstreak butterfly (our state insect), squirrels, owls, goshawks and porcupines.

Additionally, some of the favorite plants of gardeners were propagated from Witches brooms. These ornamental cultivars are often classified as dwarf plants and provide slow-growing, compact plants with some striking characteristics.