The holiday hitchhiker: Why we kiss under a killer parasite

When we were stationed at Arnold Air Force Base, we went to the local Walmart and good old boy Tennessean was selling Mistletoe out front of the store.  

It is the “long-lost acquaintance” of the holiday season. We see it hanging in doorways, hear Bing Crosby croon about it, and use it as a legalistic excuse for a festive smooch. But beneath the leathery green leaves and white berries of the mistletoe lies a backstory far more “Grinch” than “Santa.”

While we embrace mistletoe as a symbol of home and hearth, biologists have a different name for it: a “charismatic plant pathogen.”

Mistletoe is a hemiparasite—a botanical bandit that grows high in the canopy of host trees, stealing water and nutrients to survive. 

“It’s a good lifestyle to steal your food rather than make it yourself,” Jim Westwood, a Virginia Tech plant biologist, told Popular Science.

Because it stays vibrant and green even in the dead of winter while its host tree appears skeletal, ancient cultures viewed it as supernatural. To the Greeks and Romans, it was a centerpiece of ritual; to the Norse, it was a plant of “liminal space,” existing between the divine and the human.

In one famous Norse myth, the god Baldr was slain by a mistletoe spear. Some historians believe our modern tradition of kissing under the sprig echoes his mother Frigg’s grief and eventual reconciliation with the plant—turning a weapon of death into a symbol of forgiveness.

The plant’s journey to our living rooms was paved by figures as diverse as 13th-century friars and “Sleepy Hollow” author Washington Irving. In the 1800s, mistletoe was so popular that newspapers tracked its seasonal arrival like a commodity.

Yet, for all its romantic associations, the plant carries a “deadly” reputation. It contains peptide toxins—particularly the European variety, which carries a toxin similar to ricin. While a few berries won’t likely be fatal to a human, the plant is certainly not for snacking.

Perhaps the most unromantic fact about mistletoe is its name. It literally translates to “dung on a twig.”

The plant spreads via bird droppings. Birds eat the berries, and the seeds—coated in a sticky substance called viscin—survive the digestive tract and glue themselves to new tree branches. There, they wait for spring to begin their parasitic hijack.

If you’re looking for a bit of “roadside botany” this December, look up. Once deciduous trees drop their leaves, the evergreen clumps of mistletoe become “almost impossible to ignore,” says Carolee Bull, a plant pathologist at Penn State.

So, the next time you find yourself standing under the mistletoe, remember: you aren’t just standing under a Victorian kissing prop. You are beneath an ancient, toxic, bird-delivered parasite that has successfully charmed humanity for two millennia. You’re welcome for this story about “poo on a stick”! Happy Collecting!