Green Growth on Tree Trunks: Lichens Are Harmless

Identification

The thin, green, paint-like coating on bark—especially where sap flows—is lichens. They are a symbiotic partnership of fungus and algae (or cyanobacteria).

Key Facts

• Not a disease. Lichens do not penetrate bark or harm the tree.
• Indicate good air quality. They thrive in clean, moist environments.
• No treatment needed. They will not kill branches or cause decline.

When to Worry

If the tree shows other symptoms (dead branches, thinning canopy, fungal conks, bleeding cankers), consult an arborist. The lichens are just bystanders.

Resources

Learn more: British Lichen Society