As a youth I was introduced to mistletoe during the Christmas season; however I never thought much about where the plant grew. In our house a sprig of mistletoe was attached over a doorway and anyone caught under it was kissed. Now there is mistletoe etiquette – when a man kisses a woman he must remove a berry from the sprig. When all the berries are gone, there’s no more kissing permitted.
Mistletoe is a partial parasitic flowering plant that attaches to a host plant and relies on the host for water and minerals. However, it produces its own food through photosynthesis. The seeds from mistletoe are spread by birds or the wind. Birds are attracted to the berries and may spend a good deal of time feeding on them. They either wipe the sticky seeds from their beaks onto neighboring branches or excrete them after eating the berries.
A mistletoe seed germinates in as little as six weeks. It begins growing through the bark and into the tree’s water-conducting tissues where fine root-like threads, called haustoria, develop. The haustoria gradually extend up and down within the branch as the mistletoe grows. The plant will flower in four to five years. Mistletoe plants are either female (producing berries) or male (producing only pollen). If the visible portion of the mistletoe is removed (i.e. the part of the plant on the outside of the branch), new plants often sprout from the haustoria lying beneath the branch surface.
There are several species of mistletoe. In our area, the broadleaf mistletoe (P. villosum) only infests oak trees. I was very surprised last year when mistletoe appeared on a lower branch of a maple tree in my yard. This must have been another variety of mistletoe which is also found in California, Phoradendron macrophyllum. This variety infests alder, Aristocrat flowering pear, ash, birch, box elder, cottonwood, locust, silver maple, and walnut.
Mistletoe is poisonous to people and pets, although deer eat it as a source of protein. Mistletoe can weaken and damage trees. Therefore, it is smart to manage the spread of mistletoe by replacing susceptible cultivated trees with resistant species such as Chinese Pistache, ginkgo, sycamore and conifers such as cedar. P. Infected branches need to be cut at least a foot below the point of mistletoe attachment to completely remove embedded haustoria.
Although mistletoe can compromise a tree’s health, it is also a source of food and nesting sites for birds and animals worldwide. Areas with mistletoe support a greater variety and higher densities of animal populations. Like most of Mother Nature, the mistletoe has a positive and negative side. At Christmas time, it is definitely a positive plant which can engender a spark of love between the people standing under it.
This article adapted from Pest Notes: Mistletoe, UC ANR Publication 7437. Authors: E. J. Perry, UC Cooperative Extension, Stanislaus County, C. L. Elmore, CE Weed Science, UC Davis Emeritus Produced by IPM Education and Publications, University of California Statewide IPM Program.
Jim Gormely is a University of California Cooperative Extension Master Gardener of Tuolumne County.