Saturday, December 25, 2010

Mistletoe Control


Kissing beneath a sprig of mistletoe hung in the house is a popular Yuletide tradition. 

Not far behind: muttering obscenities under your breath, while looking up into a backyard tree filled with this energy-sapping pest that clings - and sucks the life from - infected branches.

Although it may look harmless hanging in your hallway this time of year, mistletoe in trees is slowly killing your source of summertime shade. Healthy trees can tolerate a few mistletoe branch infections, but individual branches may be weakened or sometimes killed. It may take years to kill a tree; in the meantime, mistletoe adds another burden to a tree suffering from drought or disease, hastening its demise.

Susceptible trees in California to this evergreen, parasitic plant include alders, Aristocrat flowering pear, ash, birch, box elder, cottonwood, locust, silver maple, walnut, and zelkova.  

 Mistletoe does its damage by sprouting root-like structures (called haustoria) which enters the growing tissues of limbs, robbing the tree of water and nutrients. It is spread mainly by your local neighborhood birds who carry the sticky mistletoe seeds to trees in newer residential areas from old trees along stream banks and from established neighborhoods and parks.
The small, sticky, whitish berries are produced from October to December. In addition, seeds may fall from mistletoe plants in the upper part of the tree, creating new infestations on the lower branches. The rapidity with which mistletoe spreads is directly related to the proximity and severity of established infestations, and newly planted trees can be quickly infested if they are growing near old, heavily infested trees.

Removing and controlling mistletoe this time of year is not an easy task. Usually its located high up in trees, reachable only by birds, squirrels and junior high schoolers.  But instead of arming your fearless 12 year-old with a pruning saw, it's safer to hire a reputable tree service to do the branch removal. "Reputable": an arborist who is certified, bonded, insured.
Prune out infected branches, if possible, as soon as the mistletoe appears. Using thinning-type pruning cuts, remove infected branches at their point of origin or back to large, lateral branches. If you're doing your own pruning, try to remove more than just the mistletoe; cut off the suffering limb one foot or more below the point of mistletoe attachment. Done properly, limb removal for mistletoe control can maintain or even improve tree structure.

 However, even if all you can reach is the mistletoe itself, it is better to remove that than to do nothing at all. It will take several years for the mistletoe to get back to a size where it will produce seeds.
 After removing the mistletoe, try wrapping that area of the tree branch with black plastic. That will help stop any resprouting. But you may have to replace the black plastic on a regular basis; it's the lack of light that keeps the mistletoe from regrowing.
 Chemical controls that contain the plant growth regulator ethephon, such as Monterey's Florel, may provide some mistletoe containment. The University of California's Integrated Pest Management website says that for the Florel to be effective, the spray must thoroughly wet the mistletoe foliage. The ideal time to treat is in spring as temperatures begin to warm, but before the tree begins to grow new leaves. Daytime temperatures must be above 65 degrees (F) for good results. Spray only the individual mistletoe plants, not the entire tree. By treating when trees are dormant, the tree foliage will not get in the way of the treatment and the mistletoe is more visible than when leaves are on the tree. 

However, this spray may only provide temporary control and mistletoe may soon regrow at the same point.
 The most effective means of mistletoe control? Chop down that infected tree and plant a mistletoe-resistant tree, such as the Chinese pistache, crapemyrtle, ginkgo, golden rain tree, London plane tree, sycamore or a conifer such as a redwood or deodar cedar.

In the meantime, get creative.

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