Walnut trees, particularly the native Black Walnut species (Juglans nigra), are known to be toxic to many animals and plants. This toxicity is a natural defense mechanism, and the level of danger depends on the specific species and the part of the tree involved. While the common English Walnut (Juglans regia) poses a low threat, the Black Walnut contains a significantly higher concentration of a chemical compound that can cause serious harm to livestock and inhibit the growth of sensitive plants. Understanding the source of this toxicity is important for property owners who have these trees on their land.
The Chemical Culprit: Juglone
The primary toxic compound is an organic chemical called juglone (5-hydroxy-1,4-naphthoquinone), which the tree uses to discourage competition. The highest concentrations of juglone are found in the Black Walnut’s roots, the hulls of the nuts, and the tree’s buds. Smaller amounts are also present in the bark, leaves, and stems, which can leach into the surrounding soil as debris decomposes. Juglone acts as a respiration inhibitor, interfering with a plant’s ability to absorb necessary energy. While Black Walnut and Butternut trees produce the largest quantities, English Walnut trees and related species like hickory produce amounts so low they rarely cause toxic reactions.
Toxicity Risks to Livestock and Pets
The most severe and widely documented risk is to horses exposed to Black Walnut shavings or sawdust used as bedding. Exposure to the wood can cause acute laminitis, an inflammatory condition of the hoof’s sensitive tissues. Symptoms, which can appear within hours, include depression, swelling in the lower legs, warm hooves, and a stiff gait. If not treated quickly, this inflammation can lead to the rotation of the coffin bone and permanent hoof deformity.
Dogs and other small pets are generally less susceptible to severe juglone poisoning, but they face danger from fallen walnuts. Ingesting moldy Black Walnut hulls or nuts, which often contain Penicillium mold, can lead to mycotoxin poisoning. This mycotoxin, Penitrem A, is a potent neurotoxin that can cause muscle tremors, seizures, restlessness, and hyperresponsiveness to stimuli. The risk is highest when the fallen nuts have been allowed to sit and become dark and moldy.
Allelopathy: Walnut Trees and Surrounding Plants
Walnut trees employ allelopathy, a form of chemical warfare where juglone released into the soil actively inhibits the growth of certain species. The toxic effects are most concentrated directly under the tree’s canopy due to root density and debris accumulation. The root zone of a large Black Walnut can extend up to 50 to 80 feet, potentially causing damage far beyond the dripline.
Juglone-sensitive plants show signs of decline, such as yellowing leaves, stunted growth, and sudden wilting that does not respond to watering. Highly susceptible garden vegetables include members of the nightshade family (tomatoes, potatoes, peppers, and eggplant). Other sensitive plants include:
- Apples
- Blueberries
- Azaleas
- Rhododendrons
- Lilacs
Juglone is poorly soluble in water, meaning it does not move far in the soil. Even after a Black Walnut tree has been removed, the decomposing roots can continue to release juglone, and toxicity may persist in the soil for a year or more.
Safe Management and Removal
To safely manage a Black Walnut tree, preventative measures must limit exposure to the toxic parts. Never use Black Walnut wood, sawdust, or shavings for animal bedding, especially for horses. If a tree is near a pasture, install physical barriers to prevent livestock from chewing the bark or accessing fallen nuts.
For gardening, sensitive plants should be located outside of the tree’s root zone, ideally 50 feet or more from the trunk. If gardening nearby is necessary, use raised beds filled with clean, juglone-free soil. Fallen leaves, nut hulls, and wood chips should be collected and disposed of, as they continue to leach juglone as they decay. These materials should not be used as mulch for sensitive plants, but they can be composted and aged for at least six months to allow the juglone to break down. If a Black Walnut tree is removed, the stump should be completely ground down and the sawdust removed, as the chemical can persist in the soil until the roots fully decompose.