Mistletoe: Holiday Kiss or Tree Menace?Saturday, December 08, 2007 - 09:00 AM
Submitted by: Rebecca Miller-Cripps is the program coordinator for the U.C. Master Gardeners of Tuolumne County.
Mistletoe is what is known as a hemiparasite (or semiparasite). That means it draws some of what it needs—water and mineral nutrients—from its host, but it also performs photosynthesis like other plants, creating its own sugars and other carbohydrates. It's an evergreen plant and there are seven native species of American mistletoe in California. Most landscape trees are parasitized by Phoradendron macrophyllum, know as broadleaf or “leafy” mistletoe, but our native oaks act as hosts to Phoradendron villosum, oak or “hairy” mistletoe, only. And the Christmas mistletoe used for decoration and kissing is an entirely different species, Phoradendron serotinum, which is cultivated for Christmas use.
Mistletoe plants come in two genders—male and female. The female plant bears the flowers and the small, white, sticky berries that are so attractive to birds. For our birding friends, trees with mistletoe attract robins, cedar waxwings, and phainopepla. According to the California Department of Food and Agriculture (www.cdfa.ca.gov) “Mistletoes provide food and nesting habitat for many animal species and may be important contributors to the health of natural communities.”
Also, Wikipedia made reference to the increased biodiversity present in trees that also contain mistletoe. Writing in a 2002 issue of Science News, Susan Milius, in an article entitled “Mistletoe, of All Things, Helps Juniper Trees” related her study that more juniper berries sprouted around juniper trees already infested with mistletoe. It seems the mistletoe attracted birds that also ate juniper berries. Being digested and eliminated by birds actually enhanced the juniper berry's sprouting ability.
Another study by David M. Watson in a 2001 edition of Annual Review of Ecology and Systematics, reported that “mistletoe has recently become recognized as an ecological keystone species, an organism that has a disproportionately pervasive influence over its community.”
Although many animals use mistletoe as a food source and nesting site, and the ancient Druids regarded it as an antidote to poison, all parts of some of the mistletoe species are poisonous to humans. However, in Europe some medicines derived from mistletoe are prescribed as a treatment for cancer and, although not available in the U.S., research is ongoing.
Of course, historically, traditionally and through scientific study, some of mistletoes' influence is recognized as not being beneficial. Many trees, especially some of the ashes, are much more susceptible to weakening by mistletoe. The antidote is to plant landscape trees that are resistant to mistletoe. According to CDFA, most trees can withstand minor occurrences of mistletoe, but heavy infestations can lead to tree weakening and even branch die-back.
However, here again, the jury isn't in agreement. Dr. Robert Raabe, Professor Emeritus in plant pathology at U.C. Berkeley, has lectured for 30 years that leafy mistletoe does not kill deciduous trees. In fact, if you think about it biologically, it doesn't make sense for a parasite to kill its host. By doing so, it only hastens its own demise.
And in a recent lecture to Tuolumne County Master Gardener trainees, Ken Churches, Calaveras County Farm Advisor advised that the dead oak trees in which you see many mistletoe clumps were not killed by the mistletoe. It is simply much more visible in a tree that has died.
Once again, California Department of Food and Agriculture maintains that “trees weakened by mistletoe infections are more susceptible to attack by insects and fungi, which may lead to increased mortality rates.” And the Hastings Biological Field Station of the University of California website (www.hastingsreserve.org) contends that to prune out a large mistletoe infestation can be threatening to the health of an old tree. They say that “mistletoe is probably just another one of the things on an oak that takes 300 years to kill a big tree.”
If you would like information on ways to control mistletoe, call the Master Gardener office at 533-5696 or log on to www.ipm.ucdavis.edu and look up the pest note on “mistletoe.”
In the meantime, whether you believe that mistletoe traditions began with the Romans, the Druids, or the English….whether you believe that mistletoe is a menace or a treat for the birds, hang a piece of it as part of your winter holiday decorations, kiss your favorite person and reflect on the complexity that this plant represents.
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