“You don’t want to buy that lot—the trees have mistletoe!” Our realtor pointed at a shrubby mass growing among the branches high in the Ponderosa pine.

It didn’t look anything like the mistletoe I was familiar with, coming from California. There, the live oaks often support huge masses of mistletoe. And neither plant resembled the old plastic “mistletoe sprig” I inherited from my parents, that we hung in our doorway at Christmastime to encourage kissing. Curious, I did some research. It turns out that there are hundreds, if not thousands of barely-related species of parasitic plants called mistletoe.

Mistletoes share some common traits. They are all flowering plants that live on or in the branches and trunks of other plants. Most have green leaves and stems filled with chlorophyll, thus providing their own food via photosynthesis. They depend on the host mainly for water, minerals (such as N, P, & K), and support.

There are exceptions, however. For example, some species (in the genera Cactaceae and Euphorbiaceae) live almost completely inside their host, with only their flowers and seeds protruding from the branch. Others (e.g., Viscum capense) have vestigial leaves, significantly reducing their ability to photosynthesize. These mistletoes are much more dependent on their hosts.

Sometimes mistletoe overwhelms and kills its host tree by blocking needed sunlight and consuming too much food and water. As a result, it’s considered a pest species, even though some kinds of birds feed on the berries. (The Phainopepla is one North American bird species that dines on mistletoe berries.)

Our plastic holiday decoration probably looks most like the British species, Viscum album, native to Europe. This is the plant the druids believed to have mystical properties. (Its association with Christmas probably has something to do with ancient winter solstice celebrations.) This mistletoe parasitizes around 200 species of plant, including the druids’ sacred oaks.

Several species of birds (e.g., Mistle Thrushes and Blackcaps) eat the poisonous white berries. The seeds inside pass through the digestive system unscathed and are deposited onto a branch in the droppings. When these seeds sprout, the embryos extend their hypocotyls into the bark, where they eventually reach the branch’s food and water supply. Since this can take a year or more, the mistletoe relies on its own chlorophyll for photosynthesis in the meantime. Technically, that makes it a hemi- (partial-) parasite, rather than a full parasite.

V. album has been imported to North America where it is cultivated for sale during the holidays, but it’s not naturally found here. Instead, we have plenty of native mistletoes that fill the same ecological niche.

For example, the mistletoe I was most familiar with in California is Phoradendron villosum, which infests only oaks. When the oaks drop their leaves, the extent of the mistletoe infestation becomes clear, as you can see in this photo.

Then there’s the mistletoe our real estate agent was pointing out to me. Southwest Dwarf Mistletoe (Arceuthobium vaginatum) weakens and kills primarily Ponderosa pines. (Other species in the same genus affect conifers.) Infected trees die slowly; how slowly depends on the initial health of the tree and how much mistletoe is present, and varies from 7 to 60 years.

Since there is no way to kill mistletoe in an infected pine, control focuses on prevention. The seeds ripen in late summer, when the fruit in which they have been growing explodes, flinging its contents outwards at speeds up to 60 MPH! Because these seeds are sticky, they adhere wherever they land. In addition, birds and other animals spread the seeds even further. Clearly, it’s best to eliminate the potential for seed production by remove severely infected trees, as well as infected branches elsewhere. For more information, read Colorado State University’s excellent factsheet.

Now that you’ve read this far, I have some important advice. If you happen to encounter a sprig of mistletoe this holiday season, please put these fascinating facts aside and just enjoy the kiss. You’re welcome.
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Photo of dwarf mistletoe on Ponderosa courtesy of the University of Minnesota.