Mistletoe: parasite, seldom a killer

Partial parasite has a reputation as a tree killer, but proper management leads to much less dire results
2011-12-18T00:00:00Z 2014-07-02T08:26:04Z Mistletoe: parasite, seldom a killerAlex Dalenberg Special To The Arizona Daily Star Arizona Daily Star

Some mistletoe begs for a sweet kiss under its leaves. And then there's the mistletoe that takes root in your trees, seemingly ready to devour them.

Desert mistletoe, a partial parasite that feeds off host plants such as ironwood and mesquite trees, is a fact of life living in the Sonoran Desert, and its distinctive green clumps can be seen infesting yards throughout the Old Pueblo.

"It's a year-round plant," said Nancy McCue of the Arizona Cooperative Extension Master Gardeners Program. "We get a lot of calls on it. Mistletoe is very common."

For many homeowners, it's an unwelcome sight. Due to its parasitic nature, mistletoe has earned a reputation as a tree killer. The reality, however, is less dire. With proper management, mistletoe doesn't have to mean the kiss of death for your plants.

What to do about it, however, may not be common knowledge. So, for the holidays and beyond, here is your guide to anything and everything mistletoe.

What is mistletoe?

Desert mistletoe is one of about 1,000 mistletoe species worldwide, according to the Arizona-Sonora Desert Museum website. Research Associate Mark Dimmitt writes that the plant is native to the deserts of southern Nevada and California, south to central Baja California and southern Sonora in Mexico.

Desert mistletoe is essentially leafless and grows in dense clusters of brittle stems. Like other plants, mistletoe contains chlorophyll and is capable of photosynthesis, but takes water and nutrients from its host, most commonly mesquite trees.

Mistletoe, which is popular among the avian set, is spread when its extremely sticky seeds are deposited on other host plants. Birds wipe their bills on branches or deposit droppings that come from eating the plant's berries, Dimmitt writes.

Diagnosing the problem

A mistletoe infestation is subtle, until it isn't anymore.

"It will spend several years setting down roots in the tree system, then it sends out vegetative growth," McCue said. "You'll know it when you see it when you have the vegetative growth. It's obvious because you have this green growth on the tree that's unlike the rest of the tree."

Visible infestations are likely advanced and are more likely to spread to other trees, McCue said.

"If you see clumps it's been in there a while. Once you've got vegetative growth it produces more seeds, takes it to another branch and takes it to neighboring trees. That's how it spreads," she said.

What's the prognosis

Here's the good news: If mistletoe is a killer, it's a slow killer. Managed properly, trees infested by mistletoe can live a long time.

"It will eventually kill them, it's a parasite, but it's possible (the infestation) will go on 15 to 20 years," McCue said.

Beth Hargrove, one of the owners of Rillito Nursery and Garden Center, 6303 N. La Cholla Blvd., takes a similar view. Mistletoe seldom kills; it's not in the parasite's best interest to kill its host.

"Generally speaking, parasites won't kill the host tree, because it would kill them as well," she said.

Dimmitt writes: "A heavy infestation of mistletoe can damage or kill the host plant, but this is uncommon."

What to do

Here's the bad news: With the infestation typically settling down inside the tree, mistletoe infestations can be devilishly difficult, if not impossible, to remove.

With that in mind, it's important to do preventive maintenance to stop mistletoe from becoming a problem, said Hargrove.

"If they're drought-stressed or prone to bugs and you don't take care of those issues when they first come up, as with people, unhealthy trees are prone to more problems," she said. "Generally speaking, if you keep them happy healthy and pest and disease free that's certainly the best start."

Healthy trees are less likely to become infested and more likely to weather an infestation, she said.

Once a tree has mistletoe, however, the problem becomes one of managing the condition. Pruning is one option, Hargrove said, but it should be done with care.

"It depends on how bad it is and where it is in the tree. If you spot it fairly soon before it gets ahold in the tree you can generally prune it out," said Hargrove.

However, cutting on a tree can weaken it if not done properly, leaving the plant more susceptible to disease and other ailments. Also, with mistletoe, it's impossible to tell how far into the tree the infestation has gone.

"Cutting out the branches has been a common practice but it is not foolproof," McCue said. "In many cases if you start whacking at a tree it's going to do more harm than good."

Perhaps the best thing homeowners can do is to keep the mistletoe from growing further or spreading to other trees, McCue said. This can be done by constantly knocking down the green foliage with a stick or a long pole.

"You have to keep an eye on it. If is has flowers and fruit it will not only spread to other trees on your property, but within the original tree and neighboring trees," she said. "You can kind of starve it out. A plant like mistletoe will never completely starve, but it's not going to flourish."

Alex Dalenberg is a Tucson-based freelance writer. Email: alex.dalenberg@gmail.com

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