Extension News

Mistletoe: Friend or Foe?

Note to media editors: This is the Garden Column for the week of Dec. 22, 2006.

12/18/2006

By Paula Flynn
Plant Pathologist
Iowa State University
Extension

Mistletoe has a pretty good reputation around Christmas time. It's commonly associated with romance and peace. Walk under it, accidentally or intentionally, and you'll likely find yourself getting a kiss.

Many different kinds of mistletoe plants can be found around the world, but the one used as a holiday ornament is known as a true or leafy mistletoe. The leaves are evergreen, meaning they stay green year-round. White berries, attractive but poisonous, can be found nestled in the scale-like leaves.

Rich in symbolism of harmony, mistletoe at a glance seems to be a plant of virtue. Unfortunately, an investigation of its home before the household doorway or ceiling will uncover a tainted past. Mistletoe is a parasite of trees. In fact, the scientific name of the holiday mistletoe,  Phoradendron, means "thief of the tree" in Greek.

You might be doubtful, recalling that the green leaves of mistletoe mean that it can make its own food. Mistletoe often is found in the upper branches of trees where there's usually plenty of sunlight for photosynthesis. Mistletoe plants do manufacture much of their own food, but if you take a close look at the plant, you'll find something is missing -- the roots. There are no roots that reach into the ground to absorb water and minerals.

Instead of roots, mistletoe plants produce finger-like projections, called haustoria,  that grow through the tree bark and into the tree branches. Making a connection with the water-conducting vessels of the tree, mistletoe absorbs the water and minerals it needs to grow and reproduce. Some mistletoe species grow only a few inches in diameter while others grow into impressive masses measuring several feet across.

A selfish plant, mistletoe can divert water from the tree to itself when conditions are dry. This siphoning ability gave rise to another common name, “vampire plant.” In addition, mistletoe is wasteful of water. It continues to transpire (lose water) when water is short. Trees, on the other hand, strive to limit loss of water through leaves when drought conditions occur.

Under stress from the theft of a portion of its water and minerals, trees infected with mistletoe often become weak and decline over a period of years. The area where mistletoe connects into the tree becomes swollen and distorted. Over time, these weakened branch areas often die and break.

Mistletoe can grow on more than 100 different shade trees and evergreens, including ash, birch, cherry, elm, maple, oak, sycamore, walnut, willow, cypress and juniper. Mistletoe occurs in walnuts in southeast Iowa, but elsewhere in Iowa the climate is too cold for this warmth-loving plant.

Since mistletoe is green year-round, it can make a deciduous tree (one that loses its leaves) look like an evergreen during the winter months. Tufts of green foliage appear along branches that appear to be dying.

Birds such as robins, thrushes and bluebirds help spread mistletoe from tree to tree. They eat the seed-containing berries and later disperse the unharmed seeds on the tree branches where they perch. The seeds are sticky, helping to glue them to the surfaces of the tree.

Will an increased demand for this parasitic plant help free host trees from its leeching ways? Not likely -- simply harvesting the leafy portion of the plant does not remove the network of woody strands embedded in the branches or stems of the host tree. Another mistletoe clump will develop where the stems and leaves were.

The best way to control mistletoe is by cutting out infected branches below the point where it's attached to the branch. Herbicide or growth regulator sprays can be used to control mistletoe and are applied when the trees are in the dormant stage.

Though an unwelcome visitor in trees, mistletoe is likely always to be welcomed into homes as a holiday tradition and promoter of romance. Watch where you walk.

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Contacts :

Paula Flynn, Plant Pathology, (515) 294-3494, pflynn@iastate.edu

Jean McGuire, Continuing Education and Communication Services, (515) 294-7033, jmcguire@iastate.edu