New Research Shows Holiday Mistletoe Isn’t Harming the Trees It Grows On
If you’ve ever looked up at a clump of mistletoe hanging from a bare winter tree and wondered whether that festive plant is secretly hurting its host, new research from Oregon suggests you can set that worry aside. A recent scientific study led by an Oregon State University researcher has found no clear evidence that western oak mistletoe harms the health of the trees it grows on, at least in urban environments and at the infestation levels observed.
The findings are especially timely, arriving right in the middle of the holiday season, when mistletoe is most visible and most culturally celebrated. Beyond the seasonal charm, the research offers valuable insights for urban forest managers, ecologists, and anyone curious about how this often-misunderstood plant actually interacts with trees.
What the Study Looked At
The research focused on western oak mistletoe, a flowering plant that grows attached to the branches of trees and shrubs. While mistletoe is commonly labeled a parasite because it draws water and nutrients from its host, the study aimed to determine whether this relationship truly leads to declining tree health.
To answer that question, scientists surveyed urban forests in seven western Oregon cities: Salem, Corvallis, Eugene, Medford, Central Point, Rogue River, and Ashland. The research team included experts from Oregon State University, the U.S. Forest Service, and the Oregon Department of Forestry. The lead researcher was Dave Shaw, a professor emeritus in OSU’s College of Forestry and a longtime forest health specialist.
Rather than focusing on wild forests, the researchers chose urban settings, where trees are planted close together and often include many non-native species. These environments make it easier to observe mistletoe spread and host preferences across a wide range of tree types.
How the Survey Was Conducted
The researchers used a hands-on approach, walking, biking, and driving through city streets, parks, university campuses, and arboreta. Their goal was to visually inspect the crowns of non-native trees for the presence of western oak mistletoe.
In total, they identified 227 non-native trees infected with mistletoe. What stood out immediately was how selective the mistletoe appeared to be. Even though non-oak species were far more common in the urban landscapes surveyed, about 97% of the infested trees were oaks.
More specifically, 85% of the infected trees were either pin oak or northern red oak, while another 12% were other oak species. Only 3% of the infested trees were not oaks at all, despite the abundance of other tree types nearby.
Tree Health Results That Surprised Researchers
One of the most important parts of the study involved evaluating the overall condition of infected trees. The researchers categorized tree health as good, moderate, or poor based on visible signs such as crown condition, branch dieback, and general vigor.
The results were striking:
- Only one of the 227 mistletoe-infested trees was rated as being in poor condition.
- Fourteen trees were classified as being in moderate condition.
- The remaining 212 trees appeared to be in good condition.
Even among trees with heavier infestations, the findings remained consistent. Of the 42 trees that had more than 20 mistletoe plants in their crowns, none were in poor condition. One was rated moderate, and 41 were still considered healthy.
These observations suggest that, at least in these urban environments, mistletoe presence does not automatically translate into declining tree health.
Size, Age, and Infestation Patterns
The study also looked at the size and age of infested trees. Western oak mistletoe was found across trees of many sizes, but it appeared far more frequently on larger, more mature trees.
Only six infested trees had trunk diameters under 10 centimeters, and just nine were shorter than 10 meters tall. This pattern likely reflects how mistletoe spreads. Birds, which play a key role in dispersing mistletoe seeds, tend to perch on larger trees, making them more likely targets for infestation.
Understanding Western Oak Mistletoe
Western oak mistletoe is one of more than 1,400 species of mistletoe worldwide. It is commonly found from Baja California north to Oregon’s Willamette Valley. Unlike dwarf mistletoes, which can cause severe damage to conifer forests, western oak mistletoe is a hemiparasite. That means it draws water and minerals from its host but still produces its own food through photosynthesis.
The plant produces berries only on female plants. These berries ripen in late fall or early winter and are toxic to humans, but they are eaten by birds such as western bluebirds. The seeds are coated in a sticky substance, allowing them to adhere to branches where they can germinate and grow.
A Benefit for Urban Wildlife
While mistletoe often gets a bad reputation, the researchers pointed out that it may actually offer ecological benefits, particularly in urban forests. Mistletoe provides food for birds and can create dense branch structures that serve as shelter or nesting sites.
In this context, western oak mistletoe may be more of a contributor to urban biodiversity than a threat, especially when infestations remain moderate.
Why Urban Forests Matter
Urban forests are unique ecosystems. Trees are planted for shade, aesthetics, and adaptability, often bringing together species that would never naturally grow side by side. This diversity creates opportunities to observe how plants like mistletoe interact with a wide range of potential hosts.
For urban forest managers, the study offers practical reassurance. While mistletoe management may still be necessary in specific cases, its presence alone should not be seen as an automatic sign of tree decline.
Cultural Roots of a Misunderstood Plant
Mistletoe has been linked to health, fertility, love, and vitality across multiple cultures for centuries. The tradition of kissing under mistletoe dates back to 18th-century England, and despite its parasitic label, the plant has long been viewed as something special rather than sinister.
The researchers noted that the word “parasite” often carries negative assumptions, but in the case of western oak mistletoe, those assumptions may be overstated.
The Bigger Picture
This study does not suggest that mistletoe can never harm trees. Heavy infestations or stressed trees may still face challenges. However, the findings clearly show that western oak mistletoe is not inherently destructive and, in many cases, coexists with healthy trees.
For anyone spotting mistletoe this winter, the takeaway is simple: that green clump overhead is likely not a silent tree killer, but rather a complex plant with ecological value, cultural significance, and a far gentler impact than once believed.
Research paper:
https://northwestscience.org/Volume_98/98_4/Shaw_et_al_2025.html
