Mistletoe is an evergreen plant recognized for its role in holiday traditions, but its unique growth habit links it permanently to other woody plants. It does not grow freely in the soil. Instead, it grows exclusively on the branches and stems of trees and shrubs, forming dense, rounded clumps that become especially noticeable in winter when host trees have shed their leaves.
Global Distribution of Mistletoe Species
Mistletoe is a common name for over 1,000 species of hemiparasitic plants found across every continent except Antarctica. These species are generally separated into two main groups: Viscum and Phoradendron. The genus Viscum, which includes European mistletoe (Viscum album), is native to Eurasia, extending from Europe into central and southern Asia, and is typically found in temperate climate zones. Phoradendron, or American mistletoe, is restricted to the Americas and boasts around 240 species. Phoradendron species thrive in warm temperate, subtropical, and tropical regions, and are common across the eastern and southern United States.
The Essential Growing Condition: Host Dependence
Mistletoe is an obligate hemiparasite, meaning it must attach itself to a host tree or shrub to survive. It does not develop a typical root system in the soil. Instead, it develops a specialized structure called a haustorium. This haustorium acts as a modified root that penetrates the bark and wood of the host plant. The penetration creates a direct vascular connection, allowing the mistletoe to extract water and mineral nutrients from the host’s xylem sap. Although it performs some photosynthesis, mistletoe relies heavily on the host for water and dissolved inorganic compounds. This means mistletoe can only establish itself where a suitable host is present.
Specific Habitats and Preferred Host Trees
Mistletoe is not generally a deep forest plant; it favors hosts in open situations that receive plenty of light. Common habitats include orchards, parklands, churchyards, and gardens. The evergreen clumps are particularly noticeable in winter against bare deciduous branches. Host specificity varies considerably, and the mistletoe species determines which host trees it can grow on. European mistletoe (Viscum album) prefers cultivated apple trees, poplars, limes, and hawthorns. American mistletoe (Phoradendron leucarpum) infests about 60 tree species in North America, commonly targeting hardwoods like oak, maple, elm, and walnut. While certain mistletoe species can be found on conifers, the leafy mistletoes primarily target deciduous trees. The presence of mistletoe is governed by the availability of a compatible host tree that allows for seed dispersal and establishment.
Seed Dispersal and Establishment
For mistletoe to establish itself, its seeds must be moved from an existing plant onto a new host tree branch. This process relies almost entirely on birds, which are attracted to the bright, fleshy berries. Birds, such as the phainopepla in the American Southwest and the mistle thrush, consume the fruit. The seeds possess a sticky, gelatinous coating called viscin, which is not fully digested as the seed passes through the bird’s digestive tract. When the bird deposits the seed, the viscin adheres it firmly to the host’s bark. The sticky material dries, cementing the seed in place, allowing it to germinate and send out its specialized haustorium to begin the parasitic connection with the host tree. This bird-mediated dispersal explains why mistletoe often appears high up in the canopy and on outer branches.