The presence of lichen on tree bark often causes concern, leading to the assumption that this growth is harming the plant. The most direct answer to whether lichen is bad for trees is no; these organisms are generally harmless to a healthy host. Lichen is a complex life form commonly found on stable surfaces like tree trunks and branches, especially in areas with clean air and sufficient moisture. Understanding the biological nature of lichen and its relationship with the tree helps to dispel the misconception that it is a parasite.
What Lichens Are
Lichen is not a single organism but rather a complex association involving two or more distinct partners. This unique symbiotic relationship is primarily formed between a fungus and a photosynthetic organism, usually a green alga or a cyanobacterium. The fungus provides the external structure, protecting its partner from drying out and anchoring the body to the surface. The internal algal or cyanobacterial cells contain chlorophyll and produce carbohydrates through photosynthesis, creating food for the lichen. Because the lichen creates its own nourishment from sunlight, air, and rain, it does not extract resources from the tree it grows upon.
The Non-Parasitic Relationship with Trees
The relationship between lichen and a tree is classified as non-parasitic. Lichen uses the bark simply as a substrate, or a stable surface, for structural support. In biological terms, it is an epiphyte, an organism that grows harmlessly on a plant and derives its moisture and nutrients from the atmosphere.
The lichen attaches itself superficially to the outer layer of the bark with root-like structures called rhizines. These structures are used only for mechanical attachment and do not penetrate the tree’s vascular system. Therefore, the lichen does not siphon sap, water, or nutrients from the tree. The vast majority of lichen growth poses no threat to the tree’s overall health and is a natural part of the ecosystem.
Why Increased Lichen Might Signal Tree Decline
While lichen does not cause damage, an unusually heavy coating can indicate that a tree is already in decline. The common misconception is that the lichen is the cause of the tree’s poor health, when the increased growth is merely a symptom of an underlying issue. A healthy, vigorous tree maintains a dense canopy that shades the lower branches and trunk, which limits the amount of sunlight available for lichen growth.
When a tree begins to struggle due to root damage, disease, or old age, its leaf canopy often thins out. This thinning allows more sunlight to penetrate and reach the bark, creating a favorable environment for the light-dependent lichen to thrive and multiply. Furthermore, older or stressed trees may exfoliate their bark less frequently, providing a more stable and less disturbed surface for lichen to colonize. The appearance of abundant lichen should prompt an inspection of the tree for other stresses, rather than immediate concern about the lichen itself.
When and How to Handle Lichen
Removing lichen is unnecessary because it is not harming the tree. Since lichen is generally a sign of good air quality, the focus should be on improving the tree’s overall health and vigor. If the tree is stressed, providing proper irrigation, balancing soil nutrients, and addressing pests or diseases will help the tree grow a denser canopy. A thicker canopy naturally reduces the light exposure on the bark, which can slow the lichen’s growth.
If removal is desired for aesthetic reasons, it should be done carefully to avoid damaging the underlying bark, which could create entry points for pathogens. Manual removal involves gently scrubbing the affected areas with a soft-bristled brush and plain water or a mild soapy solution. Horticultural oils or specialized potassium salt sprays are also available to kill the fungal component of the lichen, but these are rarely necessary.
It is advisable to consult a certified arborist if the tree shows signs of decline. Treating the underlying health issue is far more beneficial than removing the harmless growth.