I will confess right now to not being a huge fan of the holidays. However, I am very fond of both natural history and kissing, and that has led me to investigate the botanical life history of mistletoe.
Mistletoe is not one species of plant, but hundreds. They are not all related, rather their common traits evolved independently a number of times across the globe. Most mistletoes are not terrestrial plants, but evergreen hemi-parasites. They perform photosynthesis, but send their roots into the living wood of a tree to get water and nutrients. In most cases, this pilfering doesn’t kill the tree – which would be a bad adaptive move for the mistletoe – but can cause stunted growth and deformities in the host.
Worldwide, 66 families of birds consume mistletoe fruits, which are produced only by female plants. Forty-three bird families have species which nest in mistletoe clumps, and hundreds of insects are associated with the flowers, which have abundant nectar.
The common mistletoe in Europe is Viscum album, one of 30 species of Viscum. This is the species that was probably the source of the custom of kissing under the mistletoe. In the U.S., there are two primary genera: the true mistletoes, Phoradendron, and the dwarf mistletoes, Arceuthobium.
Phoradendron species are widespread and their large clumps (above) are found on many hardwood trees and some conifers. The berries (top) are eagerly eaten by birds, although all parts are toxic to humans. The seeds are extraordinarily sticky, so they cling to branches when a bird wipes them off its bill or defecates. The bird probably most closely associated with mistletoe in the U.S. is the Phainopepla (female at right). In winter, these birds defend trees laden with mistletoe, and have a specialized gut that can rapidly process the fruit – they are able to consume over 1000 berries a day. Phainopeplas frequently nest in mistletoes, as do Cactus Wrens and Abert’s Towhees, among others. Great Purple Hairstreak larvae feed only on mistletoes in this genus.
The dwarf mistletoes are radically different. Rather than relying entirely on birds for dispersal, dwarf mistletoe berries can eject their seeds, which travel up to 40 feet. Most dwarf mistletoes are leafless, and do little of their own photosynthesis. They grow on conifers in the American west, which respond by producing a lot of twigs and branches at the site of the mistletoe implantation, resulting in a structure called a witches broom. The damage they can cause to valuable timber species makes them unpopular with commercial forestry managers, but dwarf mistletoes, in fact, have considerable wildlife value. Witches brooms provide nest sites for Red Squirrels, wood rats, goshawks, and Long-eared Owls and other birds. Dwarf mistletoes are the only host plants of the Thicket Hairstreak and Johnson’s Hairstreak, as well as a number of other insect species.
Mistletoes are an incredibly fascinating group of plants, with ecological value far exceeding their utility as kissing prompts. You can read more here:
- Mistletoes on hardwoods in the U.S. — Forest Service page with range maps and characteristics of common Phoradendrons in the U.S.
- Not Just for Kissing: Mistletoe and Birds, Bees, and Other Beasts — Great article by the USGS.
- More on the mistletoe from the eastern U.S. from Hilton Pond Center (SC), including lots of close-up photos.