Pensacola News Journal

For: Sunday, March 4, 2001

By: Daniel E. Mullins

Extension Horticultural Agent

Santa Rosa County

Mistletoe, A True Parasite

Mistletoe, that mysterious plant that grows on tree branches, receives positive attention during the Christmas season. During February and March however, many homeowners voice concerns about its presence.

This is the time of year that most of our deciduous trees have dropped their leaves and mistletoe is most easily seen. A clump that seems to suddenly appear during the winter was there during the previous summer, but was hidden by green leaves during the growing season.

Mistletoe is a plant that is a true parasite on other plants. It prefers to infest certain species of trees. In the Gulf Coast environment, it is found mostly on laurel oak and water oak, but is sometimes found on other trees.

Mistletoe is spread from tree to tree primarily by seed. Anyone who has used branches of this plant for holiday decorating has seen the white berries that it produces. These berries are relatively sticky, and it is believed that they adhere to the feet of birds and are then moved to other trees.

A mistletoe plant or clump usually begins as a single seed. The berry, containing the seed, is deposited on a branch and lodges in a bark crack or crevice.

The seed germinates in much the same way that typical plants do, but the similarity ends there. Instead of a root, mistletoe develops a root-like structure known as a haustoria.

This structure grows under the bark and into the vascular system of the tree branch. It is so specialized that it actually grows into the phloem tissue. This tissue is made up of a series of small tubes that carry sugars that were synthesized by the leaves and were intended for other parts of the tree.

To call mistletoe a parasite is almost an understatement. Once it is established, the tree supplies all of the nutrients that it needs. All that it has to do is intercept its nourishment, grow and produce more berries.

Controlling mistletoe is very difficult. Various treatments have been attempted including sprays, and even injecting trees with herbicides. To my knowledge, none of these have been successful. Rates of herbicides that kill mistletoe generally severely weaken or kill the infested tree.

Physical removal of the mistletoe is practical in some cases. When found on smaller branches, the entire limb must be removed and the cut should be made at a point 2 to 3 feet back from the clump toward the tree trunk. This is to ensure that all of the haustoria is removed.

Simply pruning a clump of mistletoe away does little good. This stimulates new buds to break through the bark and continue to grow. It is possible to cause even more vigorous and widespread growth on an individual branch by periodic pruning.

Mistletoe removal on large tree limbs is sometimes done by removing a long, thick slice of bark and wood with each clump. In addition to the amount of physical damage done to the tree, such pruning provides no assurance that all haustoria will be removed and it could sprout again later.

Reducing Mistletoe Infestation

Locally, mistletoe infestations are found to be most severe on stressed trees and in areas where one species seems to dominate. With this in mind, following are some suggestions.

C Think diversity when planting trees. Avoid planting too many of the same species in the same general area.

C Protect established trees by limiting activities within the root zone. Filling, grading, ditching and compaction can severely stress trees and make them susceptible to parasites.

C Take care of the problem early. Remove the first clump of mistletoe seen, branch and all. This might break the cycle in your area and prevent problems later.

Note: Never “buckhorn” prune a large tree in order to remove mistletoe or for any other reason. Buck horning is the removal of all large limbs near the trunk. This kind of pruning results in a structurally unsound and unsafe tree.