The Black Walnut tree (Juglans nigra) is valued for its timber and nuts, but its leaves, husks, and roots contain a powerful chemical compound that poses a significant risk of toxicity to surrounding plants and certain animals. Understanding this mechanism and its effects is important for gardeners, pet owners, and livestock managers. The danger is most pronounced in the Black Walnut species.
Juglone: The Source of Toxicity
The substance responsible for the Black Walnut’s toxic properties is juglone, a phenolic compound known as a naphthoquinone. This compound occurs naturally in all parts of the tree, including the roots, bark, nut hulls, and leaves, though the highest concentrations are found in the buds, hulls, and roots. Juglone is released into the surrounding soil when the tree’s components decay or the living roots exude the substance.
Juglone functions as an inhibitor of respiration in sensitive cells and organisms, depriving them of the energy needed for normal metabolic activity. The compound starts as a non-toxic precursor called hydrojuglone, which converts into toxic juglone upon exposure to air and soil. This chemical process provides the tree with a natural survival strategy, helping it eliminate competition for sunlight, water, and nutrients.
How Walnut Leaves Affect Other Plants
The process by which the Black Walnut tree stunts the growth of nearby plants is known as allelopathy. The toxic juglone is not highly water-soluble, meaning it does not spread far in the soil, but the dense network of roots can extend well beyond the tree’s canopy. Sensitive plants that encounter juglone often exhibit symptoms like yellowing leaves, wilting, stunted growth, and eventual death.
Juglone accumulates most heavily beneath the tree’s drip line where leaf litter and fallen hulls concentrate. Highly susceptible garden plants include:
- Members of the tomato family (tomatoes, peppers, and eggplants).
- Ornamentals such as azaleas, rhododendrons, and lilacs.
Gardeners can avoid these effects by planting resistant species, which include corn, carrots, beans, and many common perennial flowers.
If gardening near a Black Walnut is necessary, creating raised beds filled with clean, non-contaminated topsoil is a viable solution. It is important to regularly remove all fallen leaves, branches, and nuts to prevent juglone from leaching into the garden bed. Wood chips, bark, or leaves from a Black Walnut tree should never be used as mulch around sensitive plants, as the juglone can remain active in decomposing debris.
Health Risks to Pets and Livestock
Exposure to Black Walnut is particularly hazardous for domesticated animals, especially horses, which are highly sensitive to the toxins. Horses can develop a severe condition called laminitis, or founder, from ingesting parts of the tree or standing on contaminated bedding. Even shavings containing as little as 5% Black Walnut wood can trigger symptoms in horses within hours of exposure.
The clinical signs in horses involve acute lameness, warmth in the hoof walls, and an elevated digital pulse, indicating inflammatory damage to the sensitive laminae inside the hoof. Other systemic symptoms include swelling in the lower legs, reluctance to move, depression, fever, and signs of colic. Since there is no specific antidote, treatment is supportive. This involves immediately removing the horse from the source, washing the legs and hooves, and administering anti-inflammatory medication and cold therapy.
For dogs and cats, the primary danger comes from ingesting moldy nuts or hulls that have fallen to the ground, which can grow neurotoxic fungi. This mycotoxicosis causes severe neurological symptoms, including muscle tremors, incoordination, restlessness, and seizures. While juglone can cause gastrointestinal upset, the mold-produced toxins are the more immediate and serious threat to pets. If an animal is suspected of ingesting walnuts, immediate veterinary attention is necessary, often involving induced vomiting or activated charcoal.
Identifying Black Walnut Versus Other Species
The serious toxicity is specifically attributed to the Black Walnut (Juglans nigra), making correct identification a necessity for safety. This species must be distinguished from the English or Persian Walnut (Juglans regia), which is commonly grown for commercial nuts and is generally considered mildly toxic or non-toxic. The Black Walnut tree is typically much taller, often reaching heights of 100 feet, and features thick, deeply ridged, dark gray to black bark.
The leaves of the Black Walnut are compound, long, and have 12 or more leaflets per leaf, with the terminal leaflet often absent or very small. The nuts are contained within a thick, spherical, green-gray husk that stains hands dark brown or black when opened. In contrast, the English Walnut is a shorter tree with smoother, lighter-colored bark, and its nuts have a much thinner shell encased in a lighter, easier-to-open husk.