The traditional use of mistletoes during holiday seasons, their involvement in folklore and legend, their consumption by domestic and wild animals, and their
use for medicinal purposes make mistletoes of widespread interest to the public. The fact that these plants are parasites that injure and can eventually kill both
conifers and hardwoods is not well known. Mistletoe, interestingly, is both a holiday tradition and a health threat to trees.
Our environment is chock full of plants threatening our landscape, but if the truth be told, there are largely forgotten uses for mistletoe varying from culture to
culture. For instance, misteltoe was considered an affective treatment for seizures. Mistletoe has also been investigated as a treatment for cancer. Who knows
what benefits mistletoe may provide? So, should it be eliminated from our environment? Probably not, but it would help to remove it from trees in urban areas.
True Mistletoe Facts & Why It Needs To Be Controlled
There are many types of mistletoe, and all are evergreen, flowering plants that require a living host. Some are rather
specific and grow on only a single type of tree; while others occur on a wide range of hardwood species. Even though they
are completely parasitic, they do manufacture much of their own food materials by photosynthesis and in general require
only water and mineral elements from the host plant. They gain these elements by establishing root-like structures within
the vascular system of the host tree. This root can grow up to two feet long within the host branch, robbing vital elements
and presenting a health threat.
Two general kinds of mistletoes grow in the United States: the "dwarf mistletoes" (genus Arceuthobium),and the "true
mistletoes" (genus Phoradendron). Trees in the San Bernatrdino Mountains suceptible to true mistletoe are the Califfornia
black oak, canyon live oak, and interior live oak. Dwarf mistletoe grows in our conifers, such as white fir, ponderosa pine,
lodgepole pine, and Jeffery pine, among others. This page addresses true mistletoe. To learn about dwarf mistletoe,
please click the link above.
True mistletoe grows in clumps up to five feet in diameter, as shown in the photo on the right. The root system of true
mistletoe is called "haustoria," and can grow up to 2 feet inside the host branch, often necessitating complete branch
removal to eradicate it.
Trunk swelling on California
Large California black oak (Quercus kelloggi)
bearing massive clumps of mistletoe. Such
far-gone trees should be removed before
birds spread the mistletoe to other trees.
Unopened male mistletoe flowers.
Mature female mistletoe flowers.
If you have an infected oak, maple or other hardwood or a juniper or
cypress, you are dealing with a "true" mistletoe species. Birds feed on
the berry-like fruits of these mistletoes and can widely disperse the
seeds. Small trees are rarely infected with misteltoe since birds tend to
be attracted to the upper limbs of large trees. Birds feeding on the
berries can spread the mistletoe infections over a large area. The
berries are round, white to pink in color, occur in spikes and are about
one-quarter inch in diameter. A berry usually holds a single seed
surrounded by a sticky pulp. Birds digest the pulp of the berry and
excrete the living seed. Berries can also fall into lower parts of the host
tree, and begin growing wherever they stick to the bark. The berries can
also stick to squirrels and be transferred.
Young or small trees are seldom infected by mistletoe. In nearly all
initial infection occurs on larger or older trees because birds prefer to
perch in the tops of taller trees. Severe buildup and spread of mistletoe
can kill trees, and often occurs when trees are neglected. Timely pruning
can eliminate or greatly slow the advance of this parasitic plant.
Branch and trunk swellings frequently result from mistletoe infection.
Swellings weaken the tree and provide an entrance point for decay fungi.
How do I get rid of mistletoe?
For treatment of existing trees it is important to remove mistletoe before it produces seed and spreads to other limbs or trees. Severely infested trees should be
removed and replaced with less susceptible species to protect surrounding trees.
The good news is that mistletoes are obligate parasites, which mean they survive only in living tissue. The most effective treatment is to prune out an infected
branch, thereby killing the mistletoe on that branch. The bad news is that you cannot eliminate mistletoe from a tree unless all infected limbs are removed. If a tree
were completely infected, you'd kill the tree if you removed all infected limbs. Often times there are less drastic measures you can take in order to enjoy your trees
and tolerate mistletoe because mistletoes spread slowly and it takes many infections and years to kill a tree.
Using thinning-type pruning cuts, remove infected branches at their point of origin or back to large lateral branches. Infected branches need to be cut at least one
foot below the point of mistletoe attachment in order to completely remove embedded haustoria. Done properly, limb removal for mistletoe control can maintain
or even improve tree structure. Severe heading (topping) is often used to remove heavy tree infestations; however, such pruning weakens a tree's structure, and
destroys its natural form. In some cases it is best to remove severely infested trees entirely because they are usually a source of mistletoe seed.
Mistletoe infecting a major branch or the trunk where it cannot be pruned may be controlled by cutting off the mistletoe flush with the limb or trunk. Then wrap the
area with a few layers of wide, black polyethylene to exclude light. Use twine or tape to secure the plastic to the limb, but do not wrap it too tightly or the branch
may be damaged. In some tree species callus tissue will form under the plastic, further weakening the limb. Broadleaf (true) mistletoe requires light and will die
within a couple of years without it. It may be necessary to repeat this treatment, especially if the wrapping becomes detached or if the mistletoe does not die.
Simply cutting the mistletoe out of an infested tree each winter, even without wrapping, is better than doing nothing at all. Even though the parasite will grow back,
spread is reduced because broadleaf mistletoe must be several years old before it can bloom and produce seed.
Click here to learn about dwarf mistletoe