Witches’ broom grows vertically on a limb. Submitted Photo
It’s that time of year when images of witches riding brooms are everywhere around us. However, witches’ brooms are more than transportation for our favorite characters around Halloween. There is also a connection between witches, brooms and botany.
The brooms of history known as “besoms” were simply large sticks with a bundle of thin twigs tied around them and used for sweeping. The plants used for making brooms were often members of the birch (Betulaceae) or pea (Fabaceae) families. Scotch broom (Cytisus scoparius) is an example. In folklore, witches traveled on brooms to gatherings of witches, and when they were tired, they rested in trees.
In the botanical world, a witches’ broom is the designation for a ball or mass resembling a nest observed in trees and shrubs. The formation became known as a witches’ broom when people perceived it as the location used by a resting witch on her broom.
In actuality, a witches’ broom forms when a proliferation of shoots grows close together. Often, the shoots are shorter, thicker and more vertical in growth habit than normal, characterizing it as a “broom.”
The causes of witches’ broom formations are varied and include fungi, powdery mildew, viruses, mites, genetic mutations, and adverse environmental conditions such as injury from herbicides, including glyphosate. Another cause is an infestation of one-celled organisms called phytoplasmas.
Phytoplasmas damage the phloem, affecting the ability of the phloem to move sap. Many species of trees and shrubs bear witches’ brooms, including lilacs (Syringa spp.), dogwoods (Benthamidia, formerly Cornus spp.), willows (Salix spp.), apple (Malus spp.), black locust (Robinia pseudoacacia), honey locust (Gleditsia triacanthos), peach (Prunus spp.), sassafras (Sassafras albidum), oaks (Quercus spp.), ninebarks (Physocarpus spp.), and many conifers.
Witches’ brooms caused by genetic mutations within a tree can be harvested, propagated and marketed as dwarf varieties. Collectors of bonsai and propagators of dwarf varieties look for witches’ brooms to purchase.
Many dwarf varieties originate from witches’ brooms on yews, junipers and pines, including one found in Pennsylvania. A witches’ broom on an eastern white pine (Pinus strobus) created a dwarf pine (Pinus strobus ‘Hershey’). Witches’ brooms can also be cultivated as bonsai of the parent species.
Unfortunately, witches’ brooms may indicate more serious diseases. The mite-borne virus that causes rose rosette disease forms witches’ broom growths in roses (Rosa spp.). A rust fungus can cause witch’s broom formations in firs and blueberries grown near each other. The fungus spends half its life cycle on fir and half on blueberries.
If you find a witches’ broom in your landscape, prune it out, removing the shoots to reach the original branch material. Additional growth may continue for several years, so be vigilant and aggressive in pruning until new growth appears normal. If the witches’ broom has formed because of a fungus, virus or phytoplasma, the entire plant may be affected, and pruning will not correct the problem.
Because exposure to glyphosate can harm trees and result in witches’ broom formations, Penn State Extension recommends safe use of this chemical. See more at extension.psu.edu/use-glyphosate-with-care-near-trees.
This fall, look for witches on their brooms and witches’ brooms in your garden. If you have questions about witches’ brooms, call the Penn State Extension Master Gardeners of Butler County Garden Hotline at 724-287 4761, ext. 7, or email the Master Gardeners at butlermg@psu.edu.
Mary Alice Koeneke is a Penn State Extension Butler County Master Gardener.
