You won’t see them being ridden on Halloween, they’re no good for sweeping, and they aren’t caused by witches. Instead, witches’ broom is the term for a dense cluster of leaves or twigs growing haphazardly among the branches of a plant, usually a shrub or tree. At times, the appearance of the broom is strikingly different from normal growth, as in the example shown at left.
Any plant can grow a witches’ broom, from deciduous trees such as willow or cherry, to conifers including pines, firs and spruces. Hackberry is especially susceptible. Aster Yellows is an example of a damaging broom that affects non-woody plants.
Many factors can cause broom formation, including viruses and other microorganisms, fungi, mites and insects, nematodes, mistletoes, and even random genetic mutations. An environmental stress that affects the growth point of a branch will also result in a witches’ broom. Sometimes, two of more factors are involved. Often, the cause cannot be determined. In most cases, the best way to control a witches’ broom is to simply prune it out.
Witches’ brooms have a severe economic impact on several industries. Crinipellis perniciosa is a fungus that causes the formation of brooms on cacao trees in Latin America. Because the brooms do not produce seeds, cocoa yields are reduced by 90% in infected trees. Evergreen trees grown for timber become unusable if enough brooms form in them. In this case, mistletoes are the most common culprit.
Not all witches’ brooms are unwelcome. Many of the dwarf conifer cultivars sold in garden centers for fancy prices originally started as a witches’ broom in a full-sized tree. Some enterprising grower took a cutting of the broom and propagated it for the nursery trade. The result is a extremely slow-growing, bushy tree that keeps its compact shape without pruning. Depending on the original cause of the broom, these cultivars may even come true from seed. The dwarf spruce Picea pungens ‘Montgomery’ [left photo] is one example. Bird’s Nest Spruce (Picea abies ‘Nidiformis’) [shown on the right] is another.