Is the Mistletoe That inspires Holiday Smooching A Menace to Trees?

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For for reasons that aren’t entirely clear, people started kissing under the mistletoe in 18^th century in England. Perhaps because mistletoe remains evergreen throughout the year, flourishing while trees are dormant, it was considered a symbol of fertility long before the smooching tradition began.

But paradoxically, mistletoe, which includes more than 100 species in the Viscaceae family, is a parasitic plant believed to kill host trees and produce poisonous berries. The sticky coating of its seeds allows it to adhere to branches, from where it draws food and water from the bark. However, a study accepted for publication in Northwest Science reports that Oregon mistletoe did not have negative effects on the health of oak trees.

Researchers from Oregon State University and the Oregon Department of Forestry used the urban forests of western Oregon cities as a natural laboratory to study the effects of western oak mistletoe (Phoradendron villosum) on exotic oaks. They collected data from 227 trees on mistletoe abundance and tree characteristics, including height, age and condition.

Read more: “Ancient wisdom stored in trees”

The results revealed no observable link between mistletoe infestation and tree health. “Of the 42 trees with more than 20 mistletoe plants in their crowns, none were in poor condition, one was moderate, and 41 were apparently in good condition,” study author Dave Shaw, OSU Extension Service forest health specialist, said in a statement.

So, at least for non-native trees in urban areas of Oregon, mistletoe does not appear to be a threat. Additionally, “western oak mistletoe is likely beneficial to wildlife in urban forests,” Shaw added. Its berries are toxic to humans, but popular with western birds, notably bluebirds, which disperse their seeds. In their article, the study authors cite other studies showing that mistletoe can promote greater diversity and abundance of birds.

Researchers also recognize that native white oaks in Oregon suffer negative health effects from heavy, long-term mistletoe infestations. A nuanced approach to mistletoe management is therefore warranted.

In the meantime, kisses!

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Main image: Lithiumphoto / Shutterstock

• Devin Reese

  Published on December 22, 2025

  Devin Reese is the editor-in-chief of Natural history and a science writer based in Alexandria, Virginia.