Extension Communications 3614 Administrative Services Building Ames, Iowa 50011-3614 (515) 294-9915

10/20/03

Contacts:
Paula Flynn, Plant Pathology, (515) 294-0581, pflynn@iastate.edu
Jean McGuire, Continuing Education and Communication Services, (515) 294-7033, jmcguire@iastate.edu

Yard and Garden Column for the Week Beginning October 24, 2003

Witches' Broom Sightings in Trees

By Paula Flynn
Extension Plant Pathology
Iowa State University

At this time of year it's not uncommon to spot a black-cloaked character with pointed black shoes and hat perched on a broom. This broom is often humorously attached to a telephone pole or the side of the house. But have you ever spotted a witches' broom in a tree?

Witches' brooms in trees appear as dense clusters of twigs. The term witches' broom comes from the German word Hexenbesen, which means to bewitch (hex) a bundle of twigs (besom). In medieval times, brooms were made of bundles of twigs, and mysterious occurrences were often blamed on witchcraft.

Witches' brooms occur on many different woody plant species, including deciduous trees such as hackberry and maple, and coniferous trees such as pine and spruce. Brooms can form just about anywhere on a tree, from the lower branches to the uppermost part of the tree. There may be only one broom in a tree, or they may be many scattered throughout the tree. In some cases, they are quite large in size and are easily spotted. In others, brooms are so small and well-hidden that only keenly observant broom hunters can spot them. The quest for witches' brooms in trees is a real passion for some and can be likened to morel hunting by mushroom lovers.

Fortunately, investigation into the causes of witches' brooms has revealed that we do not have bewitched trees in our midst. It turns out that a number of stresses, both pathological and environmental, can lead to the formation of brooms. Organisms such as fungi, mites, aphids and mistletoe plants can cause abnormal growth when they attack a host tree. Environmental stresses that injure the growing points of branches can also trigger the formation of brooms.

In Iowa, witches' brooms caused by living organisms are common on hackberry trees and honeysuckle bushes. Brooms on hackberry are often so common and numerous that they are sometimes used to identify the tree. Hackberry brooms are thought to be caused by the activity of a tiny mite and a powdery mildew fungus. Pruning out these brooms is usually not practical. The trees typically tolerate the brooms without much problem. Dense clusters of twigs also are fairly common on the ends of honeysuckle branches. Here, the brooms are caused by the activity of aphids. In many cases, brooms caused by an infectious organism are numerous on the host tree.

Some brooms appear to be caused by genetic mutations in the buds of the branches. Unlike brooms caused by living organisms, there is usually just one broom per tree when the cause is a genetic mutation. On occasion, a large broom in a tree can lead to the decline of a tree, even though it is not associated with a disease problem. Big brooms seem to divert energy away from the rest of the tree, causing it to suffer from poor vigor over time.

On the positive side, these abnormal growths have created great opportunities for diversifying the plant world. Witches' brooms have led to the development of new varieties with unique characteristics. Plant enthusiasts with skillful hands have taken cuttings from witches' brooms and propagated them. Seeds collected from cones in witches' brooms also have been grown to produce trees with eye-catching characteristics. Plants propagated from witches' brooms are commonly classified as dwarf plants. Like the "mother" broom, these dwarf plants grow very slowly and have a compact form.

Hundreds of rare and unusual dwarf conifer species that are now commercially available began as an observant sighting of a witches' broom in a tree. Dwarf conifers vary in size, shape, color and texture, and are especially desirable when yard space is limited. Dwarf plants typically grow only 3 to 6 inches per year. Some plants grown from witches' brooms are classified as miniature and grow less than 3 inches every year. You don't need a yard at all to grow these trees, only a container or trough.

One focus of broom hunters tends to be conifer trees, commonly known as "evergreens." A popular hunting spot for witches' brooms on conifers is in a cemetery. Practical people say that brooms tend to be more common in cemeteries because the trees there tend to be older and the open space of a cemetery makes witches' brooms easier to spot. But others may believe that there is a more supernatural explanation for these witches' brooms in old graveyards. After all, it is the season to be a bit superstitious.

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ml: isugarden

Editors: Three color photos, suitable for publication, are available at right. Click on each thumbnail photo to go to the fullsized photo. The top picture's fullsize photo is 168K; the middle picture's fullsize photo is 140K; and the bottom picture's fullsize photo is 120K. Caption: Witches' brooms on a hackberry tree
Caption: White spruce (Picea glauca) mother tree and broom. Photo credit: Chub Harper
Caption: White spruce graft from mother broom (9 yrs. old) Photo credit: Chub Harper

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