Lichens on Trees in Wet Climates: Harmless Indicators, Not Killers

What You're Seeing

The light green "moss/fungus" covering branches in western Washington is almost certainly lichens—symbiotic organisms (fungus + algae). They don't parasitize trees; they use bark as a substrate.

Why They're There

• Clean air (lichens are bioindicators of good air quality).
• High humidity and rainfall.
• Tree stress (drought, age, poor pruning) reduces canopy density, increasing light on bark—lichens thrive.

Do They Kill Trees?

No. They don't penetrate bark or tap into vascular tissue. Heavy coverage correlates with decline because both result from the same stressors, not causation.

Management Options

1. Improve tree vigor: Water during dry spells, mulch, fertilize based on soil test.
2. Prune for airflow: Remove crossing/dead branches; open canopy.
3. If aesthetics demand: Copper sulfate (Bordeaux mixture) or Kocide 101 suppress lichens. Apply per label; multiple treatments needed.
4. Contact WA DNR Forest Health: forest_health@wadnr.gov for local advice.

Focus on tree health, not lichen removal. Healthy, vigorous trees naturally support fewer lichens.