Mistletoe is classified as a parasitic plant, meaning it grows on a host tree and derives resources from it to survive. It is detrimental to trees, with effects ranging from mild stress to the death of entire branches or the host tree itself. Although mistletoe contains chlorophyll and performs some of its own photosynthesis, it remains dependent on the host for water and mineral nutrients. This dependency makes it a biological thief that contributes to the decline of an infected tree.
How Mistletoe Attacks the Tree
Mistletoe initiates its parasitic relationship using a specialized root-like structure known as a haustorium. The seed, often spread by birds, germinates on a host branch and forces the haustorium through the bark. The structure penetrates the tree’s tissue layers until it reaches the vascular system.
The parasite specifically targets the host’s xylem, the network of cells responsible for transporting water and dissolved mineral nutrients upward from the roots. By fusing its own vascular cells with those of the host, the mistletoe establishes a direct connection to the tree’s plumbing. This connection allows the mistletoe to siphon off the water and minerals needed for its growth and high transpiration rate.
Mistletoe bypasses the host’s natural control over water loss, forcing the tree to supply water to the parasite. This loss of resources places the host tree under hydraulic stress, especially during periods of drought or heavy infestation. The continuous diversion of water and nutrients inhibits the tree’s growth, reduces leaf function, and weakens its overall health. The haustorium also expands within the host tissue, extending up and down the branch over time, which makes complete removal difficult.
Differentiating the Severity of Infestation
Not all mistletoe species inflict the same level of damage; the potential for harm depends on their degree of parasitism. Leafy Mistletoe (Phoradendron in North America) is considered hemiparasitic. This type has visible green leaves and produces a portion of its own food through photosynthesis.
Leafy mistletoe extracts water and mineral nutrients from the host’s xylem, but producing its own carbohydrates means it is less reliant on the tree for energy. This type of infestation often causes localized swelling at the attachment point and can lead to the dieback of the branch section beyond the parasite. While a heavy infestation can severely stress and weaken an old or unhealthy tree, it rarely kills a vigorous host outright.
Dwarf Mistletoe (genus Arceuthobium) is a more destructive parasite, often considered partially holoparasitic. These species primarily attack conifers and possess small, scale-like leaves, performing very little or no photosynthesis. The parasite thus draws a greater share of its energy, water, and nutrients from the host tree.
The most recognizable symptom of dwarf mistletoe is the formation of dense, tangled masses of wood and branches called “witches’ brooms.” These brooms are a reaction by the host tree to the infection, diverting significant water and nutrients away from the rest of the canopy. Dwarf mistletoe infection causes reduced growth, severe branch dieback, and stunting, ultimately leading to tree mortality over time.
Effective Management and Removal Strategies
The most direct and effective method for managing a mistletoe infestation involves physical removal through pruning. Simply cutting off the visible clump is insufficient because the haustorium remains embedded inside the branch, allowing the parasite to regrow. To achieve complete control, the infected branch must be cut at least several inches below the point of attachment.
Pruning should use a thinning cut, removing the infected branch back to its point of origin or to a healthy lateral branch. For smaller, isolated infestations, homeowners can perform this themselves. However, care must be taken not to remove more than 20 to 30 percent of the canopy in a single year to prevent stressing the tree. If the mistletoe is attached to the main trunk or a major scaffold limb where pruning is not possible, the visible growth can be cut flush with the bark and the area wrapped in black plastic to block light, which may kill broadleaf mistletoe over time.
Chemical control options, such as the plant growth regulator ethephon, exist but are not recommended for the average homeowner. These treatments must be applied during the host tree’s dormant season and can pose a risk of injury to the tree if not applied precisely. For large, inaccessible infestations high in the canopy or when a significant portion of the tree is infected, consultation with a certified arborist is necessary. The arborist can assess whether the tree can be saved or if complete removal is the only viable option.
Preventative measures focus on reducing the spread of sticky seeds and minimizing new infections. This includes removing heavily infested trees that serve as a seed source for surrounding healthy trees. Planting species known to be resistant to local mistletoe varieties is a long-term strategy to avoid future problems.