Black Walnut Toxicity Kills Fish in Backyard Pond

A pond under a black walnut tree killed 27 fish within minutes of transfer. The culprit: juglone, a potent toxin in walnut hulls, leaves, bark, and roots.

Juglone Facts

• Green hulls used historically by Indigenous peoples to stun/kill fish.
• Crushed green hulls in stagnant water = fish kill.
• Toxic to many plants (tomatoes, apples, pines) – allelopathy.
• Highest concentration in buds, nut hulls, roots.
• Rain washes juglone from leaves/hulls into pond.

No Human Toxicity

No reports of human poisoning from walnut parts. But horses, dogs (moldy nuts), and birds can be affected.

Solution

Relocate the pond far from the walnut's drip line and root zone (roots extend 2–3× canopy radius). Or remove the tree. No treatment neutralizes juglone in water.

Historical Note

Walnut poisoning was a widespread fishing method—damming streams to concentrate toxin. Boston Molasses Disaster (1919) is unrelated but similarly dramatic.