(A) First, the myths and legends:
The common name of the plant is derived from the ancient belief that mistletoe was propagated from bird droppings. This belief was related to the then-accepted principle that life could spring spontaneously from dung. It was observed in ancient times that mistletoe would often appear on a branch or twig where birds had left droppings. “Mistel” is the Anglo-Saxon word for “dung,” and “tan” is the word for “twig”. So, mistletoe means “dung-on-a-twig”.
From the earliest times mistletoe has been one of the most magical, mysterious, and sacred plants of European folklore. It was considered a bestower of life and fertility; a protection against poison; and an aphrodisiac.
The mistletoe of the sacred oak was especially sacred to the ancient Celtic Druids. On the sixth night of the moon white-robed Druid priests would cut the oak mistletoe with a golden sickle. Two white bulls would be sacrificed amid prayers that the recipients of the mistletoe would prosper.
Later, the ritual of cutting the mistletoe from the oak came symbolize the emasculation of the old King by his successor. Mistletoe was long regarded as both a sexual symbol and the “soul” of the oak. It was gathered at both mid-summer and winter solstices, and the custom of using mistletoe to decorate houses at Christmas is a survival of the Druid and other pre-Christian traditions. (Mistletoe is still ceremonially plucked on mid-summer eve in some Celtic and Scandinavian countries.)
In the Middle Ages and later, branches of mistletoe were hung from ceilings to ward off evil spirits. In Europe they were placed over house and stable doors to prevent the entrance of witches. It was also believed that the oak mistletoe could extinguish fire. This was associated with an earlier belief that the mistletoe itself could come to the tree during a flash of lightning.
In parts of England and Wales farmers would give the Christmas bunch of mistletoe to the first cow that calved in the New Year. This was thought to bring good luck to the entire herd.
Kissing under the mistletoe is first found associated with the Greek festival of Saturnalia and later with primitive marriage rites. Mistletoe was believed to have the power of bestowing fertility, and the dung from which the mistletoe was thought to arise was also said to have “life-giving” power.
In Scandinavia, mistletoe was considered a plant of peace, under which enemies could declare a truce or warring spouses kiss and make-up.
In some parts of England the Christmas mistletoe is burned on the twelfth night lest all the boys and girls who have kissed under it never marry.
And for those who wish to observe the correct etiquette: a man should pluck a berry when he kisses a woman under the mistletoe, and when the last berry is gone, there should be no more kissing!
(B) Now the Scientific Facts: Warning - do not read this section if you are a romantic!
The mistletoe of literature (Viscum album) is distributed throughout the warmer parts of the Old World. It is most abundant on apple trees, poplars, willows, lindens, and hawthorns. Its North American counterpart ( Phoradendron serotinum) is parasitic on many species of deciduous trees of eastern North America. Another eastern North American species, the dwarf mistletoe ( Arceuthobium pusillum), is parasitic primarily on coniferous trees. The scientific name of the mistletoe family is Loranthaceae, which consists of about 20 genera.
Some people have sentimental whims about the myths of mistletoe, but this evergreen, parasitic plant is a common pest that reduces the vigor and even threatens the survival of many shade trees. Hosts of broadleaf mistletoe (Phoradendron macrophyllum ) include ash, alder, birch, box elder, cottonwood, locust, silver maple, and walnut. Another species of broadleaf mistletoe in California, P. villosum, infests only oaks. Dwarf mistletoes (Arceuthobium spp.) infest pines, firs, and other conifers in forests, but are rarely a problem in landscape plantings.
IDENTIFICATION:
Leafy mistletoes have green stems with thick leaves that are nearly oval in shape. Plants often develop a roundish form up to 2 feet or more in diameter. The small, sticky, whitish berries are produced from October to December. Evergreen clumps of mistletoe are readily observed on decidous trees in winter when leaves are off the trees.
LIFE CYCLE AND BIOLOGY:
Mistletoe plants are either female (produce berries) or male (produce only pollen). The berries of the female plant are small, sticky, and whitish; they are very attractive to robins and other birds. The birds feed on and digest the pulp of the berries, excreting the living seeds that stick tightly to any branches on which they land. In most cases, the initial infestation occurs on larger or older trees because birds prefer to perch in the tops of taller trees. A heavy buildup of mistletoe often occurs within an infested tree because birds are attracted to the berries, and may spend a good deal of time feeding on them. In addition, seeds may fall from mistletoe plants in the upper part of the tree, creating new infestations on the lower branches. The rapidity with which mistletoe spreads is directly related to the proximity and severity of established infestations, and newly planted trees can be quickly infested if they are growing near old, heavily infested trees.
After the mistletoe seed germinates, it grows through the bark and into the tree’s water-conducting tissues, where rootlike structures called haustoria develop. The haustoria gradually extend up and down within the branch as the mistletoe grows. Initially, the parasitic plant grows slowly; it may take years before the plant blooms and produces seed. Old, mature mistletoe plants may be several feet in diameter, and on some host species, large swollen areas develop on the infected branches where the mistletoe penetrates. If the visible portion of the mistletoe is removed, new plants often resprout from the haustoria.
DAMAGE:
Broadleaf mistletoe absorbs both water and mineral nutrients from its host trees. Healthy trees can tolerate a few mistletoe branch infections, but individual branches may be weakened or sometimes killed. Heavily infested trees may be reduced in vigor, stunted, or even killed, especially if they are stressed by other problems such as drought or disease.
MANAGEMENT:
In newly developed areas or in older established areas where trees are being replaced, the ideal method of controlling or preventing mistletoe is to plant trees believed to be resistant or moderately resistant to mistletoe. Avoid trees like Modesto ash, known to be especially susceptible to mistletoe infestation. Where many new trees are being planted, control mistletoe in any surrounding infected trees to reduce the infection of new trees.
For treatment of existing trees it is important to remove mistletoe before it produces seed and spreads to other limbs or trees. Mechanical control through pruning is the most effective method for removal. Growth regulators provide a degree of temporary control but repeated applications are required. Severely infested trees should be removed and replaced with less susceptible species to protect surrounding trees.
Mechanical Control:
The most effective way to control mistletoe and prevent its spread is to prune out infected branches, if possible, as soon as the parasite appears. Using thinning-type pruning cuts, remove infected branches at their point of origin or back to large lateral branches. Infected branches need to be cut at least one foot below the point of mistletoe attachment in order to completely remove embedded haustoria. Done properly, limb removal for mistletoe control can maintain or even improve tree structure. Severe heading (topping) is often used to remove heavy tree infestations; however, such pruning weakens a tree’s structure, and destroys its natural form. In some cases it is best to remove severely infested trees entirely because they are usually a source of mistletoe seed.
Mistletoes infecting a major branch or the trunk where it cannot be pruned may be controlled by cutting off the mistletoe flush with the limb or trunk. Then wrap the area with a few layers of wide, black polyethylene to exclude light. Use twine or tape to secure the plastic to the limb, but do not wrap it too tightly or the branch may be damaged. Broadleaf mistletoe requires light and will die within a couple of years without it. It may be necessary to repeat this treatment, especially if the wrapping becomes detached or if the mistletoe does not die. In some tree species callus tissue will form under the plastic, further weakening the limb. Additional problems with wrapping are the difficulty of wrapping to exclude light, heat damage to the limb, and providing a refuge for insects such as earwigs.
Simply cutting the mistletoe out of an infested tree each winter, even without wrapping, is better than doing nothing at all. Even though the parasite will grow back, its spread is reduced because broadleaf mistletoe must be several years old before it can bloom and produce seed.
Chemical Control:
The plant growth regulator ethephon (Monterey Florel brand) may be used as directed by the label to control mistletoe in dormant host trees. To be effective, the spray must thoroughly wet the mistletoe foliage. By treating when trees are dormant, the tree foliage will not get in the way of the treatment and the mistletoe clumps are more visible than when leaves are on the tree. Spraying provides only temporary control, especially on well-established infestations, by causing some of the mistletoe plant to fall off. The mistletoe will soon regrow at the same point, requiring retreatment.
Resistant Species:
Some tree species appear resistant to broadleaf mistletoe. Bradford flowering pear, Chinese pistache, crape myrtle, eucalyptus, ginkgo, golden rain tree, liquidambar, persimmon, sycamore, and conifers such as redwood and cedar are rarely infested. These or other resistant species should be considered when planting in infested areas, or when replacing infested trees.
The process of phytoremediation - a process by which plants absorb toxins and lock them up in their cells, thus reducing pollution - has recently been given a more attractive image. Formerly, plants used for such purposes looked uninteresting, or plain ugly, drab relatives of the cabbage, or that most uninspiring of houseplants, Chlorophytum. But it has now been discovered that water lilies are masters of phytoremediation. In Israel, they have been shown to absorb metal pollution to a maximum of 16% of their dry weight, trapping it in plant proteins called phytochelatins. The researchers who have identified the phytochelatin-producing gene propose to modify the genes of other plants with it, thus widening the scope for cleaning up contaminated sites. Could this be a politically attractive application of GM technology?
For several years, David has spent his holidays resting and relaxing in the tropical paradise of Hawaii. This year we are lucky enough to join David and the Canadian Gardener crew on location in Hawaii for two glorious shows of tropical flowers and sunshine to air on December 24/26 and December 31/Jan 02. Check your local CBC listings or visit the