I love all things spooky and Halloween, so when I first heard the term “witches’-broom,” I was super excited. What is this magical thing, what causes it to happen?
I later learned that witches’-broom can certainly be a sign of a serious problem – but in some cases they can be no big deal at all. Perhaps sometimes they’re downright magical after all.
Wildlife love them, humans have used them as literal brooms, and they can make for some pretty awesome growth, too.
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Plus, these deformed growths are responsible for some of the coolest plant cultivars that we have available on the market.
Let’s talk about the ins, the outs, the good, the bad, and the downright ugly. Here’s what’s on the agenda:
What You’ll Learn
What Are Witches’-Brooms?
At its most basic, a witches’-broom is a deformity in the wood of a tree or shrub.
It usually looks like a big cluster of shoots growing from a central point, resembling a bundle of twigs, a broom, or even a nest.
The deformity can also contain color variations from the main plant.
To be clear, most of the time these growths appear on woody plants, but they can occur on grasses, garlic, and other herbaceous plants as well.
But they don’t look the same as woody growth types, so we usually exclude these from the conversation.
While they’re sometimes listed as “plant diseases,” that’s not technically true.
Nor are they parasites like mistletoe or epiphytes. They’re actual growth emerging from the tree or shrub.
These growths can be long-lived and though they develop slowly, they can sometimes reach up to six feet or more tall and wide.
This unusual growth happens when the hormones in the emerging bud of the plant are interrupted.
At the tip of each woody branch are auxins, the hormones that influence growth.
Auxins are responsible for regulating the growth of new tissue in such a way that the developing side shoots won’t grow so quickly or numerous that they overwhelm the lead shoot.
If the auxins are disrupted for some reason, chaos ensues.
The result is the loss of apical dominance.
What’s that? Apical dominance is when the central growing shoot is stronger than the side shoots. When a shoot loses apical dominance, those side shoots take over the lead.
Often the dominant leader dies and the side shoots take over.
We’re going to talk about what can cause this disruption in growth next.
Causes
A witches’-broom can be caused by anything that disturbs the growth in the young bud.
That could be insects, pathogens like viruses, phytoplasms, and bacteria, fungi and oomycetes, nematodes, eriophyd mites, and disruptive phytohormones like cytokinin.
Parasitic plant growth can also cause hormone disruption.
It can also be a result of genetic mutations or even environmental conditions. Solar radiation is thought to be the reason that conifers at higher elevations have more witches’-brooms, for example.
Places where herbicides are used tend to have more odd deformities, too.
It’s common to see them on oak trees and ninebark, usually caused by powdery mildew, and on incense cedars and firs, caused by rust.
Dwarf mistletoe infestations commonly cause them on conifers like Douglas firs, junipers, and pines. In walnuts, it’s often phytoplasms that cause bunching disease that results in witches’-brooms.
Horticultural Significance
Some growers and experts keep an eye out when walking in the woods or through parks and nurseries for witches’-brooms. That’s because these growths can be propagated to make new cultivars.
In fact, some of the most popular dwarf conifers were grown out of witches’-brooms. Collecting them has become increasingly common and profitable in recent years.
In the housing boom after World War II, more people found themselves with backyards, most of them somewhat petite, and gardeners were looking for compact versions of the plants they love.
That trend continues today. So breeders started looking for witches’-brooms because they produce plants that stay small.
Plant breeders look for the growths that are caused by genetic mutations rather than disease or damage because they’re more likely to be stable and reproducible.
Once they find a good specimen, they’ll grab a ladder or a tree saw and will snag the growth. Believe it or not, on taller conifers, people are known to use a rifle and shoot them out of trees!
Then, they’ll graft or root the growths to develop a new plant. Some of the most common evergreens available are from witches’-brooms. Dwarf mugo pines are one common example.
The perennial favorite eastern white pine ‘Sea Urchin’ was a growth discovered by horticulturalist Dr. Sidney Waxman.
North Carolina State University has even asked people to help them hunt for witches’-brooms on loblolly pines for their Tree Improvement Program.
The growths also provide a home to critters like squirrels. They’re like a pre-made nest!
The bottom line is that, in general, these mutations are nothing to worry about. Unless it’s a serious disease causing the growths, it’s mostly just Mother Nature doing her thing.
Associated Problems
For the most part, witches’-brooms are no big deal. They don’t harm the tree, but they can be ugly.
When the auxins are disrupted, it usually causes the lead shoot to stop growing. This can lead to unequal and stunted growth that is unattractive.
Though they aren’t harmful themselves, their presence can indicate that your tree is sick. If the growth is the result of a genetic mutation, it’s nothing to worry about.
But if it’s caused by a fungal infection, that could be an indication that your tree or shrub is very sick.
The cacao industry in Brazil was decimated when Witches’ Broom Disease (WBD), caused by the fungus Moniliophthora perniciosa, decimated cacao trees (Theobroma cacao) in the 1990s.
The bacteria Rhodococcus fascians threatens many different species, and the mite Phyllocoptes fructiphilus spreads rose rosette disease (RRD).
Without the help of professionals, it’s hard to tell what the cause of a mutation is.
You’ll have to look for other signs of pests or diseases to figure out if something is plaguing your tree and whether you need to take action.
Management
You don’t have to do anything if your plant has witches’-brooms. But if you hate the look of them, just prune out the deformities.
Cut the entire shoot off to the main branch with a sharp, clean pair of clippers or a saw.
Don’t be surprised if the weird growth comes back. It might, but if you keep pruning for several years, it usually stops eventually.
If the witches’-broom is caused by a fungal or viral infection, pruning probably won’t help.
The problem is systemic and will just return even if you cut off the symptomatic parts. You’ll need to deal with the disease before the strange growth will stop happening.
You’ll also need to examine the plant closely to make sure what you’re seeing truly is a witches’-broom and not mistletoe, which can look similar.
But mistletoe is a parasite and not a part of the plant itself. Learn more about mistletoe here.
Join the Witches’-Broom Coven
I know the stomach drop sensation of seeing something unusual happening with your trees.
Those frightening moments when you look up and see something that definitely seems out of place. In the case of the bushy growth of a witches’-broom, it might not be a problem at all.
In fact, if you’re a plant breeder, they can be downright magical!
Have you seen witches’-brooms on your trees or shrubs? Let us know in the comments section below and feel free to share a picture!
And for more information about issues with your landscape trees, check out these guides next:
