Witches' broom is a deformity in a woody plant, typically a tree, where the natural structure of the plant is changed.

witches broom tree deform
Witches' Broom - Photo by Pjt56 on Wikimedia Commons.

A dense mass of shoots grows from a single point, with the resulting structure resembling a broom or a bird's nest.

They occur on a number of conifers and deciduous tree species. The brooms are caused by a number of factors including infection by fungi, infestation of mites, genetic mutations, or adverse environmental conditions that kill the terminal bud of shoots. Those caused by genetic mutation may be stable allowing for them to be propagated vegetatively as dwarf of these witches’-brooms in confers has been the source of many dwarf cultivars. Soft foliaged conifers like Chamaecyparis and Thuja will rapidly overgrow any mutation and they will be lost if not spotted quickly. Many of these are color variations such as yellow or variegated white on a green plant.