Mistletoe: A Favorite Christmas (Hemi-) Parasite
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For some, the idea of a holiday parasite may conjure images of that in-law nobody likes, but for plant nerds, mistletoe is what comes to mind. Modern traditions of kissing under the mistletoe at Christmas (hopefully not with that in-law) have antecedents in working-class Victorian Britain. The ancient Celts and Norse had several of their own religious and mythological meanings for mistletoe, including the evergreen leaves on bare winter trees symbolizing the rebirth of the light at the winter solstice.
Rather than growing in the soil, mistletoe roots (called haustoria) are specialized for growing into other plant tissues. Mistletoe use their haustoria to extract water and nutrients, but not food, from host trees. Because they have green leaves and stems, mistletoe still use photosynthesis to make sugars from carbon dioxide and sunlight for food, and so is considered only a partial or hemi-parasite. True parasitic plants, like Orobanche minor, lack chlorophyll and depend entirely on host plants for food, nutrients, and water. Many orchid species are epiphytes, which grow on, but not into, other plants, and therefore are not considered parasites at all.
There are many species of mistletoe around the world. The European and North American species we’re most familiar with are in the sandalwood family (Santalaceae). Viscum album is the species associated with Celtic and Norse mythology, while Viscum cruciatum is more common on the Iberian Peninsula (Spain and Portugal). Species of the genus Arceuthobium parasitize junipers and pines in the northeast and Great Lakes, but the species common in the southeast is Phoradendron leucarpum, which favors parasitizing oaks, maples, hickories, and black gum trees.
Mistletoe is easy to recognize in winter. With their host trees bare of leaves, mistletoe’s small, roughly globular, shrub-like form and light-green stems and leaves are distinctive and easy to spot. Much less conspicuous are its tiny flowers, which usually occur as female and male flowers on separate plants. Blooming in late fall and early winter, native bees, honey bees, and some fly, ant, and beetle species are all important pollinators of mistletoe flowers. Once pollinated, the flowers develop into small, white, and sticky berries. The berries are important food sources of many species of songbirds. Birds disperse the seeds in mistletoe berries by eating and excreting them, or getting the berries stuck on their feet and beaks. Mistletoe leaves are also the host food for the caterpillars of great purple hairstreak butterflies (Atlides halesus).
Since mistletoe is native to our area and such an important food source for wildlife, removing mistletoe from trees is usually not recommended or necessary. Mistletoe grows slowly, and most host trees can tolerate moderate levels of parasitism. Heavy infestations can reduce tree vigor that, when combined with other stressors (drought, disease, root damage) may result in tree death. Generally, however, mistletoe will not substantially harm host trees.
If you decide to remove a few mistletoe for holiday decorations, be advised to keep them away from small children and pets; the leaves and berries are toxic, causing severe intestinal irritation and drops in blood pressure and pulse. For more information, visit the NC Extension Gardener Plant Toolbox or contact your local N.C. Cooperative Extension County office.
Matt Jones is Horticulture Agent for North Carolina Cooperative Extension in Chatham County.