Despite the fact that mistletoe remains a popular Christmas decoration, with its bright green leaves and white berries, it is a parasite of trees. And, as a result, it can harm trees. It gets water, essential elements and some food from its host tree. Mistletoe also has poisonous properties and should be kept out of the reach of children and pets who may be tempted to eat the berries.
Mistletoe harms trees in several ways. Chemical signals from the mistletoe confuse tree growth and transport systems. Local food supplies are continually used by the parasite. Eventually as more and more clumps of mistletoe grow, the tree begins to decline and die. Old, slow growing trees are the most susceptible to attack. Mistletoe seldom infests young, vigorous trees.
Every clump of mistletoe is male or female, not both. Only female clumps produce the white- to pink-colored fruit, which has been traditionally used for hanging to perhaps kiss the suspecting or unsuspecting person who stood beneath it.
Mistletoe is spread mainly by birds. Birds eat the fruit, and then seeds pass through the birds and stick to branches. Falling fruit from clumps of mistletoe can also stick to limbs below. Once the seed is stuck or lodged on the branch, growth of a new plant can begin. Once the seed is on a branch, it germinates. The seed first grows a root-like stub that pushes against the branch. This modified root works its way into bark openings to the cambium layers. Soon the root of the parasite is well connected to food supplies. Several growing seasons may pass between seed germination and leaf development. During this time the root stub continues to grow and is fed by the tree.
Mistletoe is difficult and in many cases impractical to control. This is partly because of the way it is connected to the tree. In addition, many times the mistletoe is growing very high up in the tree and is difficult to reach. Just breaking away the mistletoe is ineffective because it may redevelop at the point of removal. Systemic herbicides can move through the mistletoe and into the tree. This can injure your tree. You may have heard of a chemical growth regulator (ethephon) used as a method to control mistletoe, but its practicality is questionable.
The most successful method of eliminating this parasite from a tree is through proper branch removal. The entire limb should be pruned at least one foot below the point where the mistletoe is attached. Once mistletoe has been removed, there is always the chance that additional mistletoe will again find its way to your tree via birds.