Mistletoe, a flowering plant often associated with holiday tradition, is fundamentally a parasite that lives on the branches of trees and shrubs. Mistletoe can kill trees, but this outcome is not swift. Mortality typically occurs over many years due to a heavy, widespread infestation compounded by existing stress factors, such as drought or disease. A severe infestation progressively weakens a tree to the point of mortality.
The Parasitic Relationship
Mistletoe is a hemiparasite; it performs its own photosynthesis but relies entirely on its host for water and mineral nutrients. The plant establishes this connection using the haustorium, a specialized root structure that penetrates the host’s bark and tissue. This haustorium integrates into the tree’s vascular system, specifically the xylem, which transports water and dissolved minerals.
Once established, the mistletoe effectively taps into the host’s water supply. Mistletoe tissues maintain a higher osmotic potential than the host tree, resulting in a preferential pull of water toward the parasite. This constant diversion creates a significant water and nutrient drain, especially during environmental stress like drought. The host is forced to supply water for the mistletoe’s high transpiration rate, bypassing its normal water control systems.
The resource theft is not always limited to water and minerals. Some mistletoe species also tap into the phloem, the vascular tissue that transports sugars and other organic nutrients. This parasitic drain limits the resources available for the tree’s own growth and defense functions. The infection site often develops a visible swelling, or hypertrophy, on the branch, indicating the host’s reaction to the haustorial growth.
Severity and Impact on Tree Health
The impact of mistletoe depends on the extent of the infestation. The most significant indicator of damage is the number and size of the dense, abnormal growths known as “witches’ brooms.” These brooms are clusters of proliferating branches caused by the mistletoe’s interference with the tree’s hormones, acting as major resource sinks.
A tree with only one or two small clumps is unlikely to decline, but a heavy, widespread infestation severely compromises the tree’s vigor. The constant loss of water and nutrients leads to symptoms such as stunted growth, reduced production of seeds and fruit, and gradual branch dieback. This decline often begins in the outer canopy.
As the infestation progresses, the tree becomes more susceptible to secondary infections. A weakened tree cannot effectively defend against opportunistic pathogens like decay fungi or wood-boring insects. Witches’ brooms also add structural weight to the branches, increasing the risk of limb breakage during storms. For trees already struggling, the added stress of a severe mistletoe infestation can lead to mortality, which may take 10 to 15 years.
Recognizing and Managing Infestation
Recognizing a mistletoe infestation is easiest in deciduous trees during the winter when the host has dropped its leaves, leaving the evergreen clumps clearly visible. The primary sign is the presence of “witches’ brooms,” which are dense masses of growth distinct from the tree’s normal branching pattern. These brooms are often accompanied by a swelling of the host branch at the point of attachment.
The most effective and permanent management method is strategic pruning. Since the haustorium is embedded within the wood, simply removing the visible mistletoe shoots will not eliminate the parasite, which will regrow. For complete removal, the infected branch must be pruned several inches below the point of attachment, typically 12 to 14 inches below the swelling or broom.
If the mistletoe is growing on the main trunk or a large structural branch that cannot be safely removed, chemical control is a temporary option. The plant growth regulator ethephon is sometimes used to cause the mistletoe shoots to drop off. This chemical treatment is generally less effective than pruning for long-term control and must be applied by professionals. Maintaining the tree’s overall health through proper watering and fertilization is a preventative measure, as vigorous trees better tolerate light infestations.