A witch’s broom (witches’ broom) is a deformity mostly found in woody plants, typically trees and conifers. The natural structure of the plant changes and a dense mass of shoots begins to grow from a single point, resulting in a structure that resembles a broom or a bird’s nest.
Witch’s brooms can be caused by host of different organisms: bacteria, dwarf mistletoes, fungi, insects, mites, phytoplasmas, viruses and others. Sometimes they are the result of a combination of two organisms: witch’s broom on hackberry (Celtis occidentalis), for example, is caused by a mildew fungus carried by a mite.
Typically a witch’s broom lasts many years, often for the life of the host plant, slowly growing. You’ll often see huge witch’s brooms on conifers that can be decades old. Some though are annual structures. For example honeysuckle witch’s broom, caused by an aphid, Hyadaphis tataricae, rarely survives the winter and new ones are produced each spring.
Source of Dwarf Plants
The organism provoking the witch’s broom often modifies the genetic structure of the broom. That means if a cutting is taken from a witch’s broom and if it can be grafted or rooted, the resulting plant will retain the dense, dwarf habit of the witch’s broom. Most of the dwarf and miniature conifers so commonly grown in gardens were originally taken from witch’s brooms.
Controlling Witch’s Broom
Witch’s brooms tend to show up here and there and don’t really harm the host to any degree. You can simply let them grow as a curiosity if you want. If they aren’t to your liking, prune them off at the point of origin (in which case though they might grow back in future years) or remove the branch that bears them.
There are a few that are highly contagious. Honeysuckle witch’s broom is one and the most obvious treatment is to avoid planting honeysuckles that are infested and to destroy any you are growing. Before buying any shrubby honeysuckle (Lonicera spp.) always ask if it is resistant to witch’s broom.