Green Growth on Black Walnut Branches: Lichens, Not Disease

The greenish "fungus" on your black walnut branches is lichens—a harmless symbiosis of fungus + alga. They're common on walnuts because the open canopy lets light reach the bark.

Key Facts

• Not a plant pathogen—they don't penetrate living tissue.
• Indicate good air quality—sensitive to sulfur dioxide.
• No treatment needed—they're benign, even beneficial (nitrogen fixation, wildlife habitat).

Why They're There

• Walnuts have naturally open, coarse bark—great lichen substrate.
• Slow-growing or stressed trees have more lichens (less canopy shade), but lichens didn't cause the stress.

If You Dislike the Look

Gently brush off with a soft brush (winter, when tree is dormant). But they'll return. Accept them as a sign of a healthy environment.

Your walnut is fine. Enjoy the lichens.