A winter woodland walk, an opportunity to admire the deciduous trees’ skeletal frames with their filigree of twigs against the lowering sky, to hear the creaking of the boughs in the wind. Something about a Silver Birch (Betula pendula) catches the eye, five dense, ball-like masses of stunted twigs hanging from its branches. The mind runs riot. Creations of an industrious mammal or home to an unusually large bird? The truth more prosaic; a woody deformity known as Witches’ broom.
While many woody plant species, whether deciduous or evergreen, are prone to developing Witches’ brooms, in Britain they are usually seen on birch. Some trees have one, others several, and they can form anywhere from the lower parts of the tree to the uppermost branches. They also vary in size, some barely detectible with the naked eye, others large and easily seen, even in the summer.
Witches’ brooms have an uncanny resemblance to a besom, a broom made from a bundle of twigs. As well as for sweeping floors, besoms, at least in popular imagination, were used by witches to fly around on, the first depiction of which appeared in marginalia of a 1451 edition of Martin Le Franc’s Le Champion des Dames. It was an easy leap for the mediaeval mind to believe that these masses of twiggy growths were deposited by witches in the first place, especially in the absence of a more rational explanation.
In mediaeval Germany they were called “Hexenbesen”, which, translated directly into English, gives us Witches’ broom as well as the verb “to hex”, to bewitch, and “besom”. Witches also used them as stopping places or nests (“Hexennester”), as did elves, hobgoblins, and mares. Mares were spirits whose particular trait was to sit on the chest of a sleeper and cause them to have bad dreams, from which we have derived the word nightmare. “Mahrnester” or mare’s nest is the alternative German word for a Witches’ broom.
Unlike Mistletoe, with which they are often confused, Witches’ brooms are not parasites stealing the water and nutrients of their unfortunate hosts but forms of abnormal growth in the tree’s cells. When growing normally, a tree or shrub will exhibit what botanists call apical dominance, the plant producing a hormone, auxin, which slows the growth of the lateral or side stems and allows the central or apical stem to grow taller and compete for light.
Organisms such as fungi, mites, aphids, ironically, Mistletoe, and in British birches the ascomycete fungus, Taphrina betulina, can upset this process by inducing the tree to create cytokinin, a form of phytohormone, which interferes with its ability to regulate bud growth in a certain area. Green buds first appear on the tree and can often remain as buds for several years until they grow into shortened branches or slender twigs. Each of these will then potentially produce more small buds which will either fall off or themselves sprout into yet more twigs. Over time the tree will have produced a bundle of tightly packed twigs in that area.
Some defect in the tree, often caused by scarring or clumsy pruning, offers the micro-organisms the opportunity to enter the tree and trigger the formation of brooms. They rarely harm the tree, just reducing flowering in the affected area of the tree. They also offer a haven for other organisms, although not to witches, several species of moth reliant upon certain types of Witches’ brooms for food and shelter for their larvae.