Q: I have heard from a number of people that mistletoe does not kill a tree. That might be true, but it weakens the tree so much that anything that comes along after the infestation of mistletoe will kill the tree—like our drought. I think people want to hear that it doesn’t kill the tree so they don’t have to act on ridding the tree of it. Or maybe people don’t care that much about their trees. Even the Rillito River Path trees have a lot of it in them, and the city pays to plant and water the trees.
A: There are many species of mistletoe and they are parasitic plants. In general, the species we are familiar with in the low elevations of the Sonoran Desert attach to their host plant to absorb water and nutrients. The healthier the host plant, the healthier the parasitic mistletoe that depends on it. If the host plant is suffering from other problems, such as drought, the mistletoe can take enough water and nutrients to weaken the host and that can lead to the death of the host. A common species in our area is called mesquite mistletoe (Phoradendron californicum). This is a native plant that has a place in our desert ecology in that it provides food and shelter for some animals. The birds that eat the seeds will spread mistletoe by pooping them out in other trees along with a dollop of their own brand of fertilizer. It would be impossible to eradicate mistletoe from our environment because it of the way it is spread by birds. It would also be hard to generalize about the relationship between people and mistletoe. I am fairly certain there are a few different reasons why mistletoe is allowed to thrive on landscape trees. Depending on the person, these may include the knowledge that it is food for birds, that it’s not always easy or affordable to cut it out of every tree, and that there is an endless supply of this plant in the open desert from which it can spread. I am familiar with the city’s lack of resources and I expect they are doing the best they can with limited staff and funds to manage trees and other plants on public property.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Questions may be emailed to tucsongardensage@gmail.com
