Witch’s brooms are the result of odd disruptions in a plant’s normal growth, but also the source of many cultivated varieties of dwarf conifers for the home garden.

What reason would a witch have to leave her broom in a tree?

No reason, but the witch’s brooms occasionally found on woody plants, particularly conifers, resemble a broom fashioned by attaching a bundle of gnarly twigs to the end of a handle, used by witches to fly.

The witch’s broom is one example of the unusual structures that can result from the disruption of a plant’s normal growth processes.

A witch’s broom is a very dense, twiggy mass of small shoots and dwarfed foliage growing from a single bud or shoot on a woody plant.

Witch’s brooms can be found on deciduous trees and conifers; a tree may have only one, but occasionally there is a proliferation of witch’s brooms on branches throughout the tree.

They may grow for many years, sometimes appearing as a mini-tree within the larger tree.

Although it may look unsightly, it rarely causes serious damage to the tree.

In normal tree growth, the leading shoot produces auxins, plant hormones that inhibit the growth of lateral or side buds below the apical bud.

When that growth pattern is disrupted by an external factor, side shoots can grow unchecked, sometimes resulting in structures such as witch’s brooms.

There are two main causes of witch’s brooms, depending on what has caused the disruption in the normal development of the bud or shoot.

The more common type of witch’s broom, which can be found on flowering woody plants as well as coniferous trees, results from one or more external factors: extreme environmental conditions or infection by a range of pathogens, insects or parasites, often host-specific.

Some pathogens that can infect woody plants and cause witch’s brooms include fungi, phytoplasmas, viruses and nematodes.

Infection by insects or mites, sometimes in concert with pathogens, can lead to bizarre plant structures, technically known as galls.

A multitude of tiny biotic agents infecting a plant can result in an outbreak of witch’s brooms appearing on one plant as well as on nearby plants of the same species. These pathogenic witch’s brooms may need to be controlled in the home landscape.

The other type of witch’s broom is the result of spontaneous genetic mutation that happens within a bud meristem cell, a copying error occurs in the DNA when the cell divides, with no apparent external factor causing it.

If this happens, usually only one or a few witch’s brooms appear on a tree, and they may be longer-lived and more genetically stable than those caused by a biotic agent.

Some conifer species seem to be more prone to these spontaneous mutations than other kinds of woody plants; their “mutant brooms” have become a source of new horticultural varieties of dwarf conifers.

Dwarf cultivated varieties of conifers are often started from mutant witch’s brooms found on full-size confers growing in the wild.

There are “broom hunters” who search conifer stands for interesting witch’s brooms growing on trees, often high up in the canopy.

Practical and beautiful in our smaller home landscapes of today, these dwarf conifers may have been one good reason for the witch to leave her broom in the tree.

“Growing Green” is contributed by Lehigh County Extension Office Staff and Master Gardeners. Information: Lehigh County Extension Office, 610-391-9840; Northampton County Extension Office, 610-813-6613