As we approach the holiday season it may be a good time to review the good and the bad of the mistletoe plant. Mistletoe at this time of year can often be found hanging above a doorway. This can be a good thing if used with your sweetheart.
Now mistletoe hanging in your trees is another story, it is just bad. Mistletoe is a parasitic plant meaning that it needs another host plant to survive. Mistletoe needs live tissue to survive and it has a root-like system that it imbeds into its host to extract food and water. We have two types of mistletoe in our area, the "true" mistletoe which infects the juniper and the dwarf mistletoe which attacks all the other conifers here. The mistletoes are also host specific which means that the mistletoe plant that has infected a Ponderosa pine tree will not infect a Douglas-fir tree, the mistletoe plant on one species will only infect trees of that species.
The mistletoe plant on a tree affects it in several ways; since it draws water and nutrients from its host, if the tree is heavily infected the mistletoe can severely reduce the growth of the tree limiting the amount of fiber that the tree or your stand will produce. Many times the limb or limbs on a tree that are infected will appear to be the greenest and healthiest as they are taking away the water and nutrients from the rest of the tree. The mistletoe will also cause swelling and a deformity where it attaches itself to the tree; if this is on the bole or the trunk of the tree it can affect the grain of the wood lowering the grade of the tree. When the mistletoe attaches to a limb many times the deformity will form a mass of very compact limbs which is referred to as a "broom." If these brooms are close to the ground they can pose a threat in the case of a wildfire because they contain so much fine, compact, dead fuel they can cause a fire to intensify and spread.
Another way that mistletoe can affect your tree or your stand is that during times of drought or if your trees are growing on a poor site the mistletoe can take so much away from the tree that it can make it susceptible to other insects or diseases that can kill the tree. East of the Cascades different bark beetles will attack trees that have been weakened from the affects of mistletoe and drought. Usually the mistletoe will not kill trees by itself but, the trees are weakened by the mistletoe and are not able to fend off the bark beetle attack and will eventually lead to the trees' demise.
Since mistletoe is a plant it will produce seed and this is how it propagates when it lands upon an appropriate host. The seeds are spread by a couple different mechanisms. Birds will feed upon the berries produced by the mistletoe and as it passes through the birds system the seeds come out in the droppings of the bird. When those droppings are deposited on tree the seeds will take root and start to grow. The seeds are also sticky so when they stick to a bird's beak, feet or another small mammal they can be spread throughout a stand. The seeds are also spread by another device, the berries when ripe will actually explode dispersing the seeds outward, then gravity takes over and since the seeds are sticky when they land and stick on an appropriate host a new plant can start. After a new plant starts it will usually take around five years for the mistletoe to produce seed so the spread can be slow. The spread will be the most effective below an infected limb or tree, then to the sides and slowest upward in the tree.
So with all this working against your trees it can be very difficult to eliminate mistletoe from a stand, but a forest landowner can do several things to their stand to get the mistletoe to a manageable state. Forest landowners should strive to only have the mistletoe in the lower one third of the crowns in their stand. That way the mistletoe is closer to the ground and can be easier to deal with and the tree can also grow "past" what the mistletoe is extracting from the tree and remain healthy. Since the mistletoe lives off of another living limb or tree; by pruning a limb you will kill that mistletoe plant. If a tree has mistletoe throughout the whole crown it may be necessary to remove the infected tree to kill all the mistletoe. So any time a landowner starts removing trees they should have a well thought out plan in place. Is there enough young tree reproduction to take the place of the removed trees? Will trees have to be replanted? Will the same or a different species or a mix be planted? How will I control the mistletoe in the future? If you need help answering these questions you may want to contact your local OSU Extension Forester or Stewardship Forester with the Oregon Department of Forestry.
So if you have mistletoe in your trees prune as much out of it as you can to make a healthier forest, and if its hanging over a doorway always exercise caution during this time of year.