Episodes of “Monsters Inside Me” on Animal Planet demonstrate how internal parasites can slowly drag a human being down over many years’ time. It got me motivated to finally cut the mistletoe out of my big mesquite tree. This weird plant the ancient Celts called “a different twig” is forgotten outside the Christmas season, but in times of drought, it should be a top concern for everyone with trees in the yard. Just as with people, stress can exacerbate the negative effects of parasites, increasing their ability to damage the plant host.
Mistletoe is a plant that actually lives on other plants, sending its root-like haustoria into branches, where it sucks the life out of a tree. If you study where they attach you’ll discover the limb is swollen at that point to make room for the parasite’s anchorage.
When we brought a big clump down by cutting off its support limb, it fell with a thud. This surprising weight demonstrates how much moisture a mistletoe ball can deny the tree. That moisture would otherwise support new growth and foliage that contribute to its overall health. This demonstrates how a single ball of the stuff on any tree can be a significant water waster.
Mistletoe produces small, round pearly fruits filled with a sticky substance that surrounds the seed. When the skin of the fruit breaks, this “glue” helps the seed stick to the nearest branch, where it will germinate, and then haustoria penetrates the bark and water-carrying tissues. Seeds falling from fruiting mistletoe high in a tree rains down seed on the lower branches, causing rapid infestation. Many species of birds eat the fruit, then excrete the seed onto branches of trees nearby, furthering the spread of the parasite.
When there’s a lack of rainfall, large trees dry out internally, reducing their hydrostatic pressure. In the wild, the driest trees often experience the greatest mistletoe infestations because it’s easier for the parasite to enter due to lack of pressure. When drought strikes, even healthy trees lose hydrostatic pressure, making them more vulnerable to mistletoe. Under these stressful conditions it’s important to be vigilant to destroy mistletoe startups as soon as they appear.
When mistletoe becomes established on a branch, chemicals can’t kill it without killing the tree since their vascular systems have merged. The only real solution is to cut off the infected branch at least a foot above the mistletoe where tissue is healthy. This makes sure all the remnants of the internal haustoria are removed. Merely cutting off the mistletoe stem may slow it down and limit immediate seed production, but this parasite always grows back.
It takes a sharp eye to distinguish the little startups from the foliage in a leafed-out tree, which is why it’s better to remove them when trees are deciduous. The “different twigs” are green and stand out like sore thumbs in winter. However, when the parasite appears in any season, or on evergreens during drought, it’s important to eradicate it immediately before it fruits and spreads.
Because birds like to perch atop trees, infestation often begins well beyond the range of pruning equipment. Attempting to remove them yourself can be risky at these heights. For serious infestations of one or more trees, consult a certified arborist to assess the problem and recommend a control plan. A professional tree trimmer with a cherry picker or a good tree climber may be required to get the job done properly.
With such limited rainfall in the West this year, more trees are going to be vulnerable to startups. Don’t become the genesis of a communitywide infestation by failing to control it in your own trees. It’s best to work with the entire neighborhood to keep all startups minimized before they spread. Otherwise this year of drought may evolve into an epidemic of monsters inside trees that will undoubtedly kill the hosts.
Maureen Gilmer is an author, horticulturist and landscape designer. Learn more at MoPlants.com. Contact her at firstname.lastname@example.org or P.O. Box 891, Morongo Valley, CA 92256.
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