What scientists are still discovering about mistletoe
New species of the plants are still being discovered, and their parasitic biology is being probed for potential cancer treatments.
Kissing under mistletoe is a practice that hearkens back to pagan fertility traditions—a joining together of random strangers, longtime lovers or the newly infatuated.
But most winter holiday revelers may not know that the biological reality of mistletoe plants is perhaps even more interesting and romantic than whatever goes on underneath them.
For example, most mistletoe species would literally die without the host they are attached to. That’s because mistletoe is a widespread category of parasitic plants that grows on other plants, leeching water and other resources by tapping directly into the host’s root system.
While this relationship can occasionally be deadly to the host plant, a growing body of research has revealed mistletoes are more often complementary to the ecosystem around them—including the host plant and near-threatened Andean marsupials. The impact of these often small, hanging plants is so outsized that researchers who study them characterize mistletoes as keystone species. They may also have healing properties, used in both traditional and Western medical settings.
“They are so weird and they haven’t had the attention they should,” says Isabel Carmona Gallego, a PhD student in biology at National University of Comahue in Argentina.
(How mistletoe became everyone's favorite parasite)
A diverse world of mistletoe
European and North American mistletoes are most commonly associated with the Christmas kissing tradition, but mistletoes are found widely across the world and belong to the Santalales order of flowering plants. Only Santalales that act as parasites to the branches or stems of other plants are considered mistletoes, excluding some Santalales that grow off roots on the ground. In other words, if you can’t hang, you’re not a mistletoe.
There are plenty that can hang, though. The Loranthaceae family alone has about 77 genera and 1,000 species, which parasitize trees, bushes and even cacti.
Mistletoes produce nutritious berries as part of their reproduction strategy. These berries contain seeds surrounded by a glue-like substance called viscin, which can survive the digestive system of birds and mammals. When the birds and small mammals that consume the fruit poop it back out, the seeds will stick to branches or trunks and begin to penetrate through the bark and into the host’s root system to steal resources.
They may also parasitize each other. Recent research published in Oikos by Francisco Fontúrbel, a botanist at the Catholic University of Valparaiso in Chile and his colleagues has revealed that one species found in the Chilean Andes, Tristerix corymbosus, doesn’t usually reach past the elevations of its preferred host trees.
These mistletoes can also parasitize another mistletoe species, Desmaria mutabilis. Since Desmaria mutabilis’ hosts are found at elevations of 4,000 feet and above, Tristerix corymbosus manages to extend its reach up the mountain by growing on top of the mistletoes that can parasite upper elevation trees, “one parasite of another parasite,” Fontúrbel says.
Mistletoes can bring the kiss of death to their host plants in the right conditions. In South American cities, for example, some people view them as pests, since mistletoes can kill trees outright by draining their fluids. But Fontúrbel says that mistletoes usually don’t derive a benefit from killing their hosts, as this spells their own end as well. Most often, parasites are only deadly for their hosts in times of drought, when they take too much water, or for introduced species often found in cities, which haven’t evolved alongside them. “They rarely kill native trees,” he says.
Mistletoe benefits for animals, plants and people
In most cases, parasitic mistletoes benefit the ecosystem around them. Fontúrbel first got interested in mistletoes while studying monitos del monte—diminutive marsupials from the southern Andes with tails that help them climb and grasp branches. Small enough to fit in the palm of your hand, their Spanish name translates to “little monkeys from the mountain.” He says these tiny creatures rely extensively on mistletoe berries, especially as the elevation increases. These marsupials likely disperse the seeds of the lower-elevation Tristerix corymbosus mistletoe onto its Desmaria mutabilis cousin higher up on the mountain, for example.
While these creatures are the chief beneficiaries of mistletoes’ fruit and pollen, the plants also give back to their hosts. Fontúrbel and Carmona Gallego say that mistletoe flower nectar has a spillover effect: Host trees with mistletoes on their branches often get pollinated more, increasing their reproductive output by up to four times that of trees without mistletoes, Fontúrbel says.
“You have these rewards by having a parasite on you,” Carmona Gallego says.
Many of these species are still not well understood by botanists, Carmona Gallego says. She hasn’t yet finished her PhD and has already identified several new species of mistletoe.
Or at least mistletoes aren’t as well understood by western science. In South America, many Indigenous cultures harvest mistletoe for medicinal uses. In Chile, for example, Fontúrbel says that the same mistletoe species that might parasitize one tree will be used for treating flu or stomachache, while those of another are used for treating wounds.
In Colombia, where Carmona Gallego is from, she says people place mistletoe leaves on the skin to help mend broken bones, which supposedly helps to join them once more. “We don’t kiss under the mistletoe, but communities use mistletoes in a different way,” she says.
Western medicine is catching up, though. While in the U.S. mistletoe extracts have not been approved by the Food and Drug Administration, in Europe they are commonly prescribed for cancer patients, and research into its various uses in oncology has been ongoing for decades. A 2025 study, for example, revealed that components of mistletoe can kill certain types of cells in a way that kick-starts the body’s anti-tumor response.
Fontúrbel speculates that the plants’ penchant for healing has something to do with how these plants draw secondary compounds from their host species along with the water they leech, which then remain in the mistletoes in concentrated form. These substances may be what provide medicinal properties.
“Indigenous people have plenty of knowledge of that,” he says.
