Although you may associate mistletoe with holiday decorations and kissing traditions, it is actually a parasitic plant that lives and feeds on many trees, including the desert willow (Chilopsis linearis). Also known as the desert catalpa and the trumpet flower tree, this native of the southwestern U.S. and Mexico thrives in U.S. Department of Agriculture plant hardiness zones 8 and 9. Identifying and controlling mistletoe can help you keep your desert willow tree healthy.
The desert willow is a large shrub or a small tree hardy enough to thrive in xeriscapes and desert landscapes. The light green, willowlike leaves and low-hanging branches might have earned this plant the "willow" name, but it is part of the to the catalpa and trumpet-creeper family, Bignoniaceae. These ornamental plants bear exotic-looking, fragrant blooms in late spring or early summer, featuring pink, purple or white flowers that often have contrasting yellow throats. Desert willows also add a touch of color to fall landscapes when the leaves turn vibrant shades of gold.
Various mistletoe species thrive within the desert willow's growing range, but just two of these parasitic plants usually live on hardwood trees. European mistletoe (Viscum album), which grows in USDA zones 7 through 10, and American mistletoe (Phoradendron tomentosum ssp. Macrophyllum or Phoradendron serotinum), which grows in USDA zones 6 through 10, are both broad leaf plants that feature slightly hairy foliage, nonshowy flowers and bird-attracting, white berries. Although mistletoe plants bear green stems and leaves, they don't grow any true roots. Unlike other parasitic plants, mistletoes have their own chlorophyll and perform photosynthesis, but they still rely on their host trees for water and minerals.
Mistletoe plants originated in the tropics where they had to compete with numerous other plants for nutrients and space. They adapted to these growing conditions by becoming parasitic plants that live and feed on other plants. Mistletoes use a rootlike structure to directly penetrate into the host tree's xylem, the part of the tree that transfers water and nutrients from the roots to the leaves. Mistletoe grows inside of its host for about 24 months before producing the aerial roots and foliage you can see growing on the outside of a desert willow tree.
Early signs of a mistletoe infestation include slight swellings on the affected wood tissue, you probably won't notice a problem until aerial shoots start growing out of the host tree. These shoots grow leaves, often growing into rounded masses that reach more than 2 feet in diameter. One mistletoe typically won't cause significant harm, but several will rob the tree of water and minerals. This results in weakened wood, stunted plant growth, branch dieback and a gradual decline in the overall health of the tree. Severe mistletoe infestations occasionally cover and kill host trees, but this process takes several decades. Most mistletoe infestations don't kill healthy, mature desert willow trees.
Removing mistletoe from a desert willow tree is a difficult process. Pruning out affected branches, at least 12 inches below the infestation site, can prevent spreading the parasite, but mistletoe roots often survive inside the tree and just keep sprouting new shoots. The University of Arizona Cooperative Extension reports that some gardeners successfully kill mistletoe by wrapping infested trunks or limbs with black plastic so sunlight can't reach the parasitic plant, but this process takes 12 to 16 months to complete. Remove a heavily infested tree to prevent mistletoe from spreading to other desert willows in your yard.
- Cal Poly Urban Forest Ecosystems Institute SelecTree: Desert Willow
- University of Arizona Cooperative Extension: True Mistletoes
- APSnet: What Does Mistletoe Have to Do With Christmas?
- University of California, California Agriculture: European Mistletoe Continues to Spread in Sonoma County
- University of California Statewide Integrated Pest Management Program: Mistletoe
- Photo Credit Spike Mafford/Photodisc/Getty Images