Delve into all the research and you’ll find at least a dozen reasons why mistletoe is associated with the Yule season. Most of them have been clouded by time, myth and superstition. Nowadays most plant people know that mistletoe (Phoeradendron tomentosum) is a parasitic, native, mildly poisonous plant that we’d like never to see.
The soft, woody plant, with its dark green foliage and white (sometimes reddish) berries, lives off of other plants — usually weakened trees. In particular it latches onto oak trees of most species, but affects other species such as mesquite, hackberry and elm as well. It derives most of its nourishment from the unwilling host, but it carries on photosynthesis as well. Technically mistletoe is known as a “hemiparasite.” Mistletoe doesn’t kill directly since the death of the host means the death of the mistletoe itself. Instead the mistletoe slowly weakens and disfigures the tree by robbing it of nutrition and sunlight. Eventually the host, being weakened, will probably die of another cause.
Mistletoe sends out roots, which penetrate the host’s bark and enters the tissue of a tree. So simply breaking off the visible growth is only a temporary solution — it will result in a vigorous, well-rooted parasitic plant regrowing rapidly the next year. A typical mistletoe plant lives about ten years, but produces a large quantity of seeds (berries) in its lifetime. Birds such as cedar waxwings and bluebirds seem to relish these seeds, “depositing” them on the branches of nearby trees. Their seed-laden droppings fall on branches below. The seeds are very sticky and stick easily to branches The curious thing is that mistletoe berries aren’t very nutritious to birds and they prefer to munch on other things (other native berries, fresh seed, suet) when they are available.
In my particular area (around Denton), the birds prefer berries from beautyberry, yaupon holly (all hollies actually) agarita and mature junipers. Since monocultures (just one or two dominant species of vegetation) encourage mistletoe, planting other species augments an effective control program.
Some trees such as sweetgum, sycamore and conifers like pines and juniper do not seem to host mistletoe. There are many species of mistletoe however and conifers are parasitized by another type. The plant also has poisonous properties and should be kept out of the reach of small children and pets that may be tempted to eat the berries.
If there is a mistletoe-infected tree in close proximity to a healthy tree, the healthy tree will probably be invaded shortly. A healthy tree can fight off a few mistletoes plants, but if its weakened even slightly it’s all downhill from there.
The most successful method of control is to remove some of the host tree’s wood. If mistletoe is on a small limb, the entire limb should be removed back to at least one foot below the point where the mistletoe attaches.
Removing infected branches may temporarily disfigure the tree, but will make for a much healthier and more vigorous tree starting the very next growing season. Such pruning is best done in the winter, when the tree is dormant and the mistletoe is more easily seen.
Recently a plant growth regulator (ethephon) was found to be an environmentally safe method of control for mistletoe. When sprayed directly on mistletoe clusters it can cause complete removal of the visible parts, reducing the possibility of continued infestation. It does however need to be resprayed every few years, and within very narrow temperature parameters. The roots are untouched so this offers only a temporary setback. In addition the chemical may cause damage to other types of plants that have leaves at the time of spraying. Theis means of control has been generally unsatisfactory. There is no good research on its effect on birds, squirrels, nearby children or pets.
Mistletoe is usually symptomatic of a more generalized health problem with the host plant The host may be diseased in some way, there may be soil compaction, excessive root loss from construction, or any number of other stressors which have weakened it. The mistletoe, being a parasite, is taking advantage of a negative situation. So just treating the mistletoe by itself is often half of the battle.
Today, most people still think of the its attractive berries and evergreen foliage in its decorating context and are surprised to hear that it is harmful to trees. Overall the best method for preventing mistletoe infestation is to keep your landscape healthy and diverse.