By Paula White
In my frequent trips to the Sacramento area, I have observed big clumps of mistletoe growing on large deciduous trees, especially oaks. I had heard that these trees were stressed out from the parasitic mistletoe and suffering a slow death. I even trimmed off some of the mistletoe that I could reach, thinking that I was helping the tree (and I also wanted to hang some up as a Christmas decoration). But last February while hiking in Mt. Diablo State Park, I read a fascinating article in the Mount Diablo Review by Jenn Roe that gave me a whole new perspective on mistletoe. Instead of harming the tree, the Pacific Mistletoe (Phoradendron villosum) commonly found in California oak forests appears to contribute to a healthy oak ecosystem.
Why then does mistletoe get such a bad rap? It’s complicated–there are over 1300 mistletoe species worldwide, and some are more harmful than others. In the United States dwarf mistletoe species (Archeuthobium spp.) cause a lot of damage to high elevation conifer forests in the Pacific Northwest and northern California. Dwarf mistletoe is a full on parasite, meaning that it robs the tree of carbohydrates, while species in the “True mistletoe” genus (Phoradendron) are hemi–parasites. The literal translation of Phoradendron villosum is hairy tree thief, a name that acknowledges mistletoe’s appropriation of its host tree’s water and minerals. However, unlike Dwarf mistletoe which has no real leaves, Pacific mistletoe has green fleshy leaves that photosynthesize, and small flowers that bloom July through September. This later blooming time provides important food for pollinators in drier months which then become abundant clusters of yellowish white berries that are eaten by animals, especially birds, during the winter months when other food is scarce.
Birds often build nests in trees that contain mistletoe in order to conveniently access the berries. Their droppings, which contain mistletoe seeds, fertilize the soil under the tree and also enable new mistletoe plants to spread to other trees or parts of the same tree. In fact, the word mistletoe was coined by Anglo-Saxon speaking druids who noticed that mistletoe grew on trees that were covered in bird poop. “Mistel” = “dung,” and “tan” is the word for “twig.” So, mistletoe literally means “dung-on-a-twig.”
In the long run, any harm done by Pacific mistletoe’s theft of the tree’s water appears to be outweighed by the benefits provided by the plant. Research conducted on oak forests throughout California for nearly forty years concluded that oak trees that had Pacific mistletoe growing in them were just as likely if not more likely to be healthy than oak trees that did not contain mistletoe. Of the 667 trees surveyed, 93.6% of those that survived did not contain mistletoe, while 95% of survivors did. Furthermore, oak trees that had more mistletoe clumps also had higher acorn productivity. Other research has noted the importance of mistletoe as a food source for western bluebirds (Sialia mexicana) and as a host plant for Great Purple Hairstreak caterpillars, and several studies have noted that mistletoe litter has a higher carbon to nitrogen ratio than other plant litter and breaks down more quickly, improving soil nutrients. Other species that browse on mistletoe leaves include deer and rabbits, whose droppings further enrich soil nutrients. Finally, research on medicinal properties of plants in Mexico found that Pacific mistletoe (Phoradendron villosum) had high potassium and nitrogen content, both of which promote protein metabolism and other enzymatic activity beneficial for the control of diabetes. A word of caution– both the berries and leaves of mistletoe are toxic to humans if eaten straight off the plant, so consult with an expert before consuming mistletoe!
Shrubs in the Treetops: A Portrait of Mistletoe. Jenn Roe. Mt. Diablo Review, Fall/Winter 2022.
Effects of mistletoe (Phoradendron villosum) on California oaks. 2018. Biology Letters, 14:20180240. http://dx.doi.org/10.1098/rsbl.2018.0240
Oak mistletoe (Phoradendron villosum) is linked to microhabitat availability and avian diversity in Oregon white oak (Quercus garryana) woodlands. 2017. Botany. 95:3 https://doi.org/10.1139/cjb-2016-0249
Physiological Aspects of Parasitism in Mistletoes (Arceuthobium and Phoradendron).1964. Plant Physicology. 996-1007 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC550208/pdf/plntphys00415-0128.pdf
Macro and micro-nutrient contents of 18 medicinal plants used traditionally to alleviate diabetes in Nuevo León, Northeast of México. 2015. Pakistan Journal of Botany, 48 (1), 271-276 http://eprints.uanl.mx/24847/
Dwarf Mistletoe. Forest Pathology. https://forestpathology.org/parasitic-plants/dwarf-mistletoe/
Not Just for Kissing : Mistletoe and Birds, Bees, and other Beasts. 2016. USGS Communications and Publishing. https://www.usgs.gov/news/featured-story/not-just-kissing-mistletoe-and-birds-bees-and-other-beasts-0