MISSOULA — Witches’ broom sounds like something for Halloween, but for trees, it is not part of a costume. The broom, a dense mass of branches, is the result of dwarf mistletoe, a parasitic plant impacting Montana forests.
“A lot of times infected trees look really green and healthy, but it's actually just the mistletoe, the parasitic plant,” said Amy Gannon, who runs the Forest Pest Management Program for the Montana Department of Natural Resources & Conservation (DNRC).
Every species of Montana conifer, except Ponderosa pine, has been targeted by its own specific mistletoe. The infection starts small, then grows for years, or even decades, slowly choking the tree beneath it.
“They have to have a living host and they draw the nutrients from the host tree,” Gannon said. “That's robbing the tree of all the nutrients, and the rest of the tree is dying. So, the top of that tree is dying.”
Watch to learn more about how witches' broom is impacting trees in Montana:
As the infection progresses, the parasite becomes more and more visible. As hormonal changes occur in the tree, witches’ brooms grow large, protruding out in heavy, dense clusters. The infection spreads, attacking more than one tree in a stand.
“It's important to recognize that these plants are raining seeds down on the understory,” Gannon said. “They shoot seeds, and they can go 20, 30 feet, and then they'll stick on the understory.”
The parasites are very prolific. They have spread across much of the country and the state, infecting a patchwork of forests. Gannon said this causes concerns from both a management and ecological standpoint.
While the brooms have some benefits, like providing habitat for some insects and rodents, they also affect the whole structure of a stand. Heavy brooms can fall off, posing a threat to whatever is underneath.
Dry brooms can act like a ladder, spreading flames from the ground into the crown of the trees.
“The concern is the heavy foliage and the dense brooms that are in the mistletoe-infected trees can bring the fire up to the tops of the trees and make it a much more severe fire,” Gannon said.
Mistletoe-infected stands are actually a sign of an area that has not been burned in a while. Fire is one of the most effective ways to get rid of the parasite. But, eradicating the infection is not an easy task.
“Just because you prune off a broom, the parasite is still in the tree and can grow and so they'll have new brooms,” Gannon said. “You have to be assertive, because you have to remove all of the mistletoe and the susceptible understory, any of the regenerating trees underneath, and it requires at least a 20-foot buffer around it.”
It is difficult to both physically root out the parasite and to conceptualize the problem, because it takes so long to kill the host trees. The DNRC Forest Pest Management Program offers consultations and assistance to help manage mistletoe.
“We all remember the mountain pine beetle outbreaks of about 2009 to 2015. They swiftly kill the trees. Mistletoe does not. It's a slow decline,” Gannon said. “I think a lot of managers and homeowners and landowners are able to ignore it for a little too long, until it becomes quite a problem.”
