Witch's Broom: Plant Oddity, Garden Variety
What possible reason would a witch have to leave her broom in a tree? No reason, really, but the witch's brooms occasionally found on woody plants, particularly conifers, resemble the traditional besom, a broom fashioned by attaching a bundle of gnarly twigs to the end of a handle, used by witches to fly and, incidentally, by players to brush the ice in the sport of curling. The witch's broom in the tree seems almost magical, one example of the unusual structures that can result from the disruption of a plant's normal growth processes.
What is a Witch's Broom?
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, often resembling a besom's bundle of twigs or a more ball-shaped bird's nest. Witch's brooms can be found on both 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. A witch's broom may grow for many years, sometimes appearing as a mini tree within the larger tree. Although it may look unsightly to some observers, it rarely causes serious damage to the tree.
What Causes a Witch's Broom?
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 angiospermous (flowering) woody plants as well as on coniferous trees, results from one or more external factors: extreme environmental conditions or infection by a wide range of different 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. On roses, for example, infection by a virus that is transmitted by a tiny eriophyid mite causes rose rosette disease, which appears as a witch's broom of congested, pliable reddish stems, small, deformed leaves, and sometimes excessive prickles. 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, and these pathogenic witch's brooms may need to be controlled in the home landscape.
On many conifers, dwarf mistletoe (Arceuthobium species in the mistletoe family, Viscaceae) is a cause of witch's brooms. Dwarf mistletoe is a flowering parasitic plant whose main host in North America is conifer trees in the genus Pinus. It lives on conifer branches, inserting a haustorium, a root-like structure, into the branch to obtain nutrients and carbohydrates from its host. The response of the pine to this parasitic infection is often to form a witch's broom; the proliferation of shoots and foliage provides more sites for more mistletoe plants, and infected trees may be overwhelmed with mistletoe-induced witch’s brooms.
The other type of witch's broom is the result of a 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.
Witch's Brooms in Horticulture
Dwarf cultivated varieties of conifers are often started from mutant witch's brooms found on full-size conifers 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. How do they get the brooms down? One famous broom hunter, Dr. Sidney Waxman, a botanist and horticulturist at the University of Connecticut, would roam New England forests and cemeteries, looking for enticing brooms. Returning in October, when cones on the brooms were ready to harvest, he occasionally would use a rifle to shoot the cones down, but he also employed professional tree climbers (not at the same time, of course).
Seedlings grown from the collected seeds would then be evaluated for unique and desirable features, leading to the development of many new conifer cultivars that retain the dwarfed size of the original witch's broom. Practical and beautiful in the smaller home landscapes of today, these dwarf conifers may have been one good reason for the witch to leave her broom in the tree.
