A demonstration on how to remove the tree parasite mistletoe will be offered April 29, Arbor Day, in La Entrada Park, outside the Corrales Library. The free event has been arranged by the Corrales Tree Committee, which has warned that “Corrales is facing the loss of its cottonwood forest in the Bosque Preserve and a major portion of the Corrales tree canopy to drought and mistletoe.”

The demonstration in La Entrada Park, by certified arborist Harrison O’Connor of Legacy Tree Company, begins at 9 a.m. 

The committee appointed by Village government explained that “The continuing drought conditions in the Rio Grande Valley and infestations of mistletoe are gradually killing off the beloved cottonwood tree canopy that is integral to the culture and charm of Corrales. We may be able to offset some effects of these problems by acting early.”

Mistletoe is an evergreen parasitic plant that grows on a number of tree species.  It has a green stem and thick green, oval leaves. It can grow into a rounded form up to two feet or more in diameter. Its small, sticky, whitish berries are produced from October to December, but the  clumps are readily observed on deciduous trees in winter when tree leaves have fallen.

The committee submitted the following information to Corrales Comment to alert villagers to the growing problem. “The number of trees in the Corrales tree canopy is over 10,000 mature trees. The tree canopy in the 662 acres of the Corrales Bosque Preserve is 50 percent, with most of the trees being cottonwoods. There are an estimated 5000 mature cottonwoods in the Bosque. The greenbelt has a 14 percent tree canopy that includes over 2,000 cottonwoods, twice as many Siberian elms. Mature cottonwoods in the Bosque and greenbelt may be as old as 80-100 years.

“The sandhills have a scattering of younger and smaller cottonwoods  planted in residential landscapes that are also vulnerable to mistletoe.

“The north end of Corrales appears to have the worst infestation with hot spots where 30-50 percent of cottonwoods might be affected. The 12-mile narrow strip that borders the greenbelt and Clear Ditch are heavily infested. The south end of Corrales is the least infested with up to 10 percent of trees affected. Sandhills cottonwoods have high levels of infestation up to 50 percent in areas.

“Overall, of the 7,000 plus cottonwoods, there could be 1,500-2,000 trees infected with mistletoe. The cost for an arborist to remove mistletoe from a single large tree can be as much as $2,000. Treatment of the total Corrales canopy could cost in the millions.

“What causes the infestation? The berries of the female plant are attractive to birds who feed on the berries and excrete the seeds that stick tightly to branches on which they land. The seeds germinate on the branch and grow through the bark into the tree’s water conducting tissues.

“Infestations initially occur in older, taller trees because birds prefer to perch in the tops of those trees. Berries can fall from the top branches to cause further infestations on lower branches. The rapidity of spread depends on the proximity and severity of established infestations. That is why you notice that there are areas in Corrales with high levels of infestation and others with little or no infestation.

“What happens to an infected tree? Healthy trees can tolerate a few mistletoe branch infections, but individual branches may be weakened or killed. Heavily-infested trees may be reduced in vigor, stunted or killed. It may take many years for the damage to kill a tree.

“Trees that are stressed by drought are more susceptible to mistletoe damage and damage from other pests such as insects or fungi.

“The most effective way to control mistletoe and prevent its spread is to prune out infected branches, if possible, as soon as the mistletoe appears. In heavily-infested cottonwood trees, severe pruning weakens a tree’s structure and destroys its natural form, especially if pruning larger branches (greater than 2-3 inches in diameter). In some cases, severely infested trees should be removed entirely because they provide a continuous source of seed. Removing mistletoe clumps flush with the branch each winter is better than nothing, even if the mistletoe will continue to return. Trees that have had mistletoe removed can be reinfected by birds or by residual mistletoe rooting in branches.

“Mechanical management usually requires the services of a certified arborist and special equipment to reach upper branches of large trees. The cost of arborist services, especially if needed for multiple trees and repeated every few years may be prohibitively expensive.

“Chemical treatments have a low probability of success. Homeowners with a large, old, well-structured cottonwood tree should consider the cost of mistletoe management against the added property value provided by the tree canopy (possibly 10 percent increase over value of property with no trees).

“Managing mistletoe infestations in the Corrales Bosque where hundreds of trees are infected and multiple government organizations have interests would require a large budget that currently does not exist.

“What are pragmatic solutions? Unfortunately, mistletoe infestations are probably not going away, especially with increased vulnerability of trees from climate change. Municipal budgets are not going to increase dramatically to sustain or enhance the tree canopy, even with scientific evidence about the importance and value of the tree canopy. What budget we do have to work with should be focused on the saving the high-value cottonwood trees that give Corrales its unique character.

“This should be a joint effort of Corrales residents, Middle Rio Grande Conservancy District, and the governing body of Corrales.

“Our second priority should be a tree planting program that emphasizes use of tree species that are resistant to drought and not susceptible to mistletoe.

Recent research reported by the Nature Conservancy on climate ready trees provides guidance on tree species that are known to be drought tolerant and suitable for the changing climate in this area.

“Mistletoe research has also shown that there are resistant tree species such as Chinese pistache, gingko, golden rain tree and conifers that can be planted in mistletoe-infested areas.

“The third priority should be the mobilization of volunteer efforts for planting programs to reforest areas devastated by mistletoe, drought, or invasive species.

Residents of Corrales can help fight the mistletoe problem by early identification of mistletoe in their own trees. Because mistletoe starts at the upper branches of older trees, pruning out infected branches should be done by arborists with appropriate equipment, for safety reasons. Severely infected trees should be removed.

“Selecting trees for planting in landscapes should consider trees that are both drought and mistletoe resistant. The Corrales Tree Committee has prepared a brochure listing recommended trees for Corrales and is available at the Village Administration office and on the Village website.

“Individuals can help with reforestation of the Bosque Preserve by volunteering for replanting efforts coordinated by the Corrales Fire Department. For more information on volunteer tree planting projects, contact John Thompson, chairman of the Corrales Tree Committee.

More information on mistletoe can be obtained from New Mexico State University Extension Guide OD-10 “True Mistletoe” and University of California Agriculture and Natural Resources Publication 7437, “Mistletoe: Integrated Pest Management for Home Gardeners and Landscape Professionals.”

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