Witch's broom growth may last several years and can be caused by many different types of organisms, such as fungi, insects, mistletoe, mites, nematodes, phytoplasmas and viruses. Human activity is sometimes the reason for the introduction of these organisms; for example, when a person prunes a tree improperly and leaves the tree susceptible to disease.
Witch's brooms occasionally result in desirable changes. Some cultivars of trees, such as Picea orientalis 'Tom Thumb Gold', were discovered as witch's brooms. Czech witch's broom collector Jan Slama image gallery of dwarf conifers derived from witches brooms.