What is holly, mistletoe?

"It's time for mistletoe and holly."

Lots of people are familiar with Frank Sinatra's classic Christmas carol, "Mistletoe and Holly," and many of us decorate our homes with these familiar and attractive plants this time of year. However, my guess is that few people know much about the natural history of mistletoe and holly.

When I was a boy in Virginia, I remember walking through the woods on a winter day and spotting a tree with dark green leaves, bright red berries and light gray bark. It looked almost magical, set against the bare brown branches and fallen leaves of the winter forest.

The tree was an American holly (Ilex opaca), the same tree that is commonly used to make Christmas wreaths. American holly occurs in the wild from coastal Massachusetts to northern Florida and as far west as eastern Texas. It grows up to 50 feet tall, and it is especially abundant in the maritime forests (forests which grow near the ocean) of the mid-Atlantic states.

I observed this first hand last summer while assisting biologists from Drexel University in Pennsylvania with a research project in Barnegat Bay, N.J., where holly seemed to be the most common tree in the maritime forest adjacent to their research station. I also discovered that holly leaves have sharp, pointy tips on them. Ouch!

There are about 400 species worldwide in the holly family, several of which are native to the United States, including winterberry, possumhaw, yaupon and dahoon. Most of these plants are native to the Southeast, but the bright-red berries of common winterberry (Ilex verticillata) are common in the wetlands throughout much of the Northeast during the fall and winter. Unlike American holly, winterberry loses its leaves in the fall.

American holly is an important tree for native wildlife. The thick, evergreen foliage of hollies provides shelter and nesting sites for birds and other small animals. They produce small, white flowers in the spring, which are pollinated by bees, wasps and other insects. Their berries provide food for animals, such as deer, squirrels and wild turkeys, as well as songbirds like robins, cedar waxwings and mockingbirds.

The practice of using holly in rituals and festivals can be traced back to the early Celts, Druids and Romans in Europe. Holly was eventually co-opted from winter solstice festivals into Christmas celebrations, where it is still used today. Many states banned the sale of wild holly branches in the early 1900s because they were being over-harvested for Christmas decorations; but many nurseries still sell wreathes made from cultivated varieties of holly. The thick, evergreen leaves of the holly make for an excellent hedge. The trees can also be planted in yards as ornamentals.

Mistletoe, another traditional favorite for holiday decorations, is usually hung over doorways where it's used to solicit a kiss from a passersby. Few holiday revelers realize that this evergreen plant with thick, rounded leaves and white berries is actually a parasite, a hemi-parasite to be specific.

The term ‘hemi-parasite’ means part-parasite. The mistletoe grows on the branches of trees and shrubs where it taps into its host plant with a specialized root that produces structures called haustoria, which obtain water and nutrients from the host plant. The mistletoe's green leaves also conduct some photosynthesis, the process by which plants use sunlight to make sugars and other organic (carbon-based) molecules from water and carbon dioxide. The scientific name of the American mistletoe, Phoradendron, actually means "thief of the tree" in Greek, an appropriate name considering the mistletoe's lifestyle.

There are about 1,300 species of mistletoe, occurring on every continent except Antarctica. In the United States, we have the American mistletoe (Phoradendron flavescens), which can be found from New Jersey to Florida and as far west as Texas, as well as several species of dwarf mistletoe (mainly Arceuthobium species) native to the western U.S. and Canada.

Like holly, mistletoe is important to wildlife. Several species of birds, including grouse, mourning doves, bluebirds, robins and pigeons, feed on mistletoe berries. The dwarf mistletoe of the western United States is also used as a nesting site by many bird species, including spotted owls, goshawks, house wrens and chickadees. Many insects, including several species of flies, ants and beetles, are responsible for pollinating mistletoe.

Birds spread the seeds of many mistletoe species in their droppings, and by eating the soft pulp of the berries without swallowing the entire berry, and then wiping the sticky seeds off their beaks onto a tree branch where seeds can later sprout. The dwarf mistletoe also has another trick for dispersing its seeds –– the ripe berries can explode, propelling the seeds up to 50 feet in the air where some of them will land on the branches of prospective host trees and shrubs. While the American mistletoe parasitizes a wide variety of trees and shrubs, the dwarf mistletoe mainly grows on coniferous trees, such as pines and junipers.

As you might have guessed, mistletoes can damage trees and shrubs, reducing the growth rates of their hosts. While many foresters regard mistletoe as a pest, some biologists say this view needs to be balanced with the role they play as a source of food and nesting sites for a variety of animals. And, of course, there's the role mistletoe plays in sparking holiday romance...

Information for this article was obtained from Wikepedia, the U.S. Geological Survey, the USDA Forest Service, the Smithsonian National Zoological Park, and V.H. Heywood's Flowering Plants of the World. Don Lyman is an adjunct instructor in the Biology Department at Merrimack College in North Andover, Mass. He can be contacted at Donald.Lyman@merrimack.edu.