Mistletoe is actually a tree-killing parasite — here's how it became a Christmas icon

That holiday mistletoe? Not as romantic as you would think.

Mistletoe is actually a parasitic plant. It crowds trees in densely packed balls. Weighing up to 50 lbs, it absorbs water and nutrients. Its berries are toxic to humans and pets and will cause vomiting and stomach pain if ingested.

So, why kiss under it?

Mistletoe's uses go back thousands of years. The Celtic druids associated it with fertility because it would bloom in winter. A Norse myth tells of mistletoe being used to kill, and then resurrect, the god Baldur. And his mother vowed to kiss all who walked underneath it. But the real tradition of kissing under mistletoe began during the Greek festival of Saturnalia, Dec 17th-23rd. And would later appear during marriage ceremonies.

Its incorporation into Christmas festivities may have developed in the middle ages. As Christmas adopted traditions from other cultures' winter holidays. By the 18th century, it had become common for men to steal a kiss from a woman who wandered under the mistletoe. Victorian English custom reportedly denied marriage proposals to any woman who refused a kiss.

The proper etiquette for mistletoe kissing:

"The gentleman should pluck one white berry while kissing the lady on the cheek. One kiss is allowed for each berry. When the last berry is gone, there should be no further kissing."* – Linda Allen, Decking the Halls.

(*Rules may not be up to date for modern holiday celebrations.)

This video was originally published on Dec. 12, 2016.

Join the conversation about this story »