Lichen often appears as a crusty, leafy, or shrubby growth adhering to tree bark. It is a composite organism, representing a stable partnership between a fungus and a photosynthetic partner, typically algae or cyanobacteria. Seeing these patches of gray-green, yellow, or orange frequently causes concern among property owners who worry the growth may be a disease or parasite harming the tree’s health.
Is Lichen Actually Harming Your Tree?
Lichen is classified as an epiphyte, meaning it uses the tree’s bark purely for physical support or anchorage. It does not possess roots that penetrate the tree’s living tissue or vascular system to draw out water or nutrients, unlike parasitic organisms like mistletoe.
Lichen sustains itself entirely through its own processes. The algal component performs photosynthesis for food, and the fungal component absorbs moisture and minerals from the air and rainwater. The presence of lichen on a tree is harmless to a healthy specimen and often indicates good air quality due to its sensitivity to pollutants.
Lichen is frequently observed on older, slower-growing, or declining trees, which contributes to the mistaken belief that it is the cause of the tree’s poor health. These trees often have a sparser canopy and bark that exfoliates less frequently, creating an ideal, stable, and sunlit surface for the slow-growing organism to colonize. The underlying issue is usually an unrelated stressor like soil compaction, pest infestation, or disease, not the lichen itself.
Distinguishing Lichen from Harmful Tree Growth
Correct identification is important, as several other organisms can look similar but pose a genuine threat. Lichens appear flat, crusty, or three-dimensional with lobe-like or branched structures. They display a wide range of colors from pale gray to bright orange, but they lack true leaves, stems, or any soft, velvety texture.
Moss is a simple plant that should not be confused with lichen, as it presents as a soft, green, velvety carpet. While moss is also non-parasitic, its structure allows it to hold significant amounts of moisture directly against the tree’s bark. In high-humidity environments, this sustained dampness can create conditions that encourage the development of bacterial or fungal pathogens.
Other growths, such as true fungal pathogens or scale insects, require different treatments. Fungal pathogens may manifest as shelf-like structures, mushrooms, or powdery mold, often indicating internal wood decay. Scale insects appear as tiny, immobile, waxy bumps that actively draw sap from the tree’s vascular system, requiring specific horticultural oil treatments.
Step-by-Step Guide to Removal
The safest removal methods focus on gentle, non-abrasive action that preserves the integrity of the bark. The tree’s protective bark layer is delicate, and scraping or cutting can create wounds that become entry points for pests and diseases. The primary tool for removal should be a soft-bristled brush, such as a vegetable brush or an old toothbrush, along with plain water.
Lichen can be dislodged by gently scrubbing the bark surface, taking care not to damage the underlying tissue. For larger areas, a low-pressure stream from a garden hose is effective after the lichen has been softened by rain or manual wetting. If the growth is isolated to smaller, non-essential branches, targeted pruning is the most efficient removal method.
Harsh chemical treatments, including undiluted soaps, copper-based fungicides, or household cleaners, are discouraged due to the risk of phytotoxicity to the tree or harm to the surrounding soil and wildlife. If a mild solution is used, apply diluted dish soap and follow immediately by thorough rinsing with clean water to wash away all residue. Aggressive techniques like pressure washing must be avoided, as the force can easily strip the bark and cause severe damage.
Long-Term Prevention Through Environmental Management
A more effective, long-term strategy for naturally reducing lichen growth is to improve the tree’s environment. Lichen thrives in conditions of high moisture and low light. Judicious pruning to thin the canopy and remove dead or crossing branches will increase sunlight penetration and air circulation to the trunk and limbs. Creating drier conditions on the bark surface will make the area less hospitable for lichen colonization, minimizing its reappearance over time.