Take a walk in the woods or look into the crowns of birch trees in winter and you’ll see not only a beautiful tracery of branches, but some unusual clumps of twigs that you might mistake for nests. In most cases, these will be witches’ brooms and there’ll usually be more than one on an individual tree. And generally if one birch tree has them, you’ll often find others exhibiting the same distinctive clusters close by.
Look closely and you’ll see that each witches’ broom comprises a dense cluster of short, closely packed branches coming from a single point on an otherwise ordinary branch. In winter, with the stubby twigs bare, you’ll should spot closely spaced buds along their length; in summer the clumps will be closely covered with leaves.
These witches’ brooms haven’t been constructed by birds or squirrels, but are usually a result of damage caused by something far smaller.
What causes witches’ brooms
There are a whole host of things that can cause dense clusters of branches or witches’ brooms to form. In fact, even physical damage by the wind or a pair of rubbing branches may cause an abnormal growth habit of close packed branchlets to form. But much more frequently, witches’ brooms are produced in response to a pest or pathogen. This can be viral, bacterial or fungal disease, or alternatively an infestation by insects, nematodes or mites. Any of these can cause the cells in the branch to divide rapidly and produce a proliferation of growth in an attempt to try and ‘shrug off’ the problem.
In the case of the birch tree shown in the picture taken in my local wood, the witches’ brooms are most often caused by Taphrina betulina, a widespread fungus which infests silver birch, downy birch and dwarf birch.
Can witches’ brooms form in any tree
Witches’ brooms can form in a wide range of woody plants – both trees and shrubs. Most susceptiable are trees such as alder, Amelanchier, birch, cherry (and other prunus relatives), eucalyptus, gleditisia, mulberry and oak, as well as conifers like pine, juniper spruce and fir.
You may also see witches’ brooms on roses, heathers and rhododendrons, in addition to some other shrubby plants. Perhaps surprisingly, similar structures can form on non-woody species such as strawberry and potato, although these are temporary and die away on herbaceous stems. Once infected by the pest or disease, however, the witches’ brooms reappear on the following season’s growth.
Will witches’ brooms kill a tree
In most cases witches’ brooms will not cause damage or affect the overall health of vigorous trees. However, the specific pathogen which caused them to form will be either living in the tissues of the plant or, in the case of insects, mites or nematodes, on the surface of the plant. This will divert some of the plant’s energy in the form of sugar to the pathogen which is living in or on it. And the plant is also likely to expend more energy in rapid cells division in the witches’ broom, in order to combat the infection.
In some cases, a severe case of the pathogen may weaken the host plant to such an extent that its health suffers and it may even die. Less vigorous woody plants, as well as a few herbaceous species, can be more vulnerable to such health effects.
What is the treatment for witches’ brooms
There is no effective treatment for stopping witches’ brooms forming on infected plants. Insect and mite infestations can, in some cases, be combatted by hand picking, biological control or organic pesticides specific to the pest. Fungal, bacterial or viral causes (as well as less common mycoplasmas) are not generally treatable once the host plant has been infected.
Prevention measures are also limited on woody plants. Avoiding high nitrogen fertilisers will reduce the chances of soft, sappy growth being produced which is most at risk of infection by pests and diseases. By contrast, applying high potash fertiliser can help to toughen up tissues and make them less prone. More generally, keeping the soil in good condition by regular applications of compost or organic matter will help keep plants healthy and more able to shrug off infection.
And lastly, limiting pruning to spring and summer for vulnerable plants, will allow any cuts to heal more quickly which may reduce the chance of infection by a witches’ broom-causing pathogen. Meantime, where the visual impact of the witches’ broom is unacceptable, the structures can be pruned out. However, in most cases, this is not a permanent solution as new brooms are likely to appear elsewhere on the plant.