A friend, who is putting together decorations for a Christmas season event, contacted me with a request for some mistletoe. She considers me the go-to guy for anything tree-related. Of course, I said I would see what I could do, even though I do not have a ready supply I can reach and cut with my pole saw.

Scouting my neighborhood, I found the street trees and the trees in the park remarkably free of mistletoe. Wouldn’t you know it? It appears all over the place when you are not even looking for it.

The most notorious area for this parasitic plant is in the Modesto ash trees in the Bel Aire neighborhood. It is a good example of urban planning with unforeseen results. We now know that tree variety is susceptible to infestation. Who knew back then? They were planted as a monoculture throughout the neighborhood. Once the infestation got started and went untreated, it spread like a plague.

Not a “disease” as such, mistletoe is a seed-producing green plant. Think of it as a weed that cannot grow in soil, only on tree branches. It has root-like structures called haustoria that embed in the water-and-mineral carrying xylem in tree branches.

The scientific name for the common mistletoe we see locally is Phoradendron, meaning “tree thief." There are several species, and two other genera of mistletoe, but here we are speaking in general terms about broadleaf mistletoe. It produces its own food by photosynthesis, but it robs water and minerals from its host. When tree branches dry up and die, so does the mistletoe. Conversely, when trees absorb abundant water, so does the mistletoe.

It often goes unnoticed in the mix of tree foliage, but in winter on deciduous trees its evergreen leaves and branching clumps stand out in sharp contrast to bare tree branches.

Around Napa Valley, we also see it on black walnut, flowering pear, silver maple, birches, native oaks and various other tree species. In contrast, some of our common street trees, like Chinese Pistache and crape myrtle are resistant.

Once established in a tree, mistletoe can survive for a long time. One reference reported a case of it living 419 years, documented by tree ring examination. Though some trees can live with it for a long time, whole trees can be overgrown and killed by mistletoe. In some trees, like some of the old black walnuts along Trancas Street near Silverado Trail, long-term infestation can cause hypertrophy, an abnormal swelling in branches.

Because of its seed-bearing ability, one tree can become a major infestation center for a whole neighborhood. A dioecious plant, mistletoe produces seed on female plants, pollinated by male plants. The seed is enclosed in extremely sticky whitish berries.

Birds serve as a vector for new infestations. They feed on the berries and excrete viable seed as well as carrying sticky berries inadvertently on their feet.

One tree with both male and female plants in it can be the start of something big. Control is difficult and it may take a concerted effort in a neighborhood.

The only chemical control I know of is Florel, by Monterey Lawn and Garden Products Inc. It has specific requirements for timing and temperature. It must be used during the dormant season and it is not effective if the nights are cold directly after the spray is applied.

In any case, if your trees have it, they’re not going to get rid of it by themselves. And as it is with other “weeds,” letting it go to seed compounds the problem.

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Bill Pramuk is a registered consulting arborist. Visit his website billpramuk.com, email questions to info@billpramuk.com or call him at 707-363-0114.