The black walnut tree (Juglans nigra) is a common species across the eastern and central United States, recognized for its valuable timber and edible nuts. While its nuts are a food source, other components contain natural compounds that can affect humans, animals, and other plants.
Toxicity to Humans and Animals
Horses are particularly sensitive to black walnut, with exposure to wood shavings, bark, or even roots capable of causing severe reactions. As little as 5% to 20% black walnut shavings in bedding can lead to clinical signs of toxicity within 24 to 48 hours. The primary concern for horses is laminitis, a painful inflammation of the hoof, which can range from mild to extremely severe, potentially leading to permanent lameness or loss of the animal.
Other symptoms include reluctance to move, limb edema, increased heart and respiratory rates, depression, and colic. The toxic compound is juglone, a naphthoquinone present in all parts of the tree.
For humans, the nuts themselves are safe to eat once properly processed, as the toxic compounds are primarily concentrated in the husks, roots, and leaves. Direct contact with the green hulls or wood can cause skin irritation, leading to itching, redness, or rashes similar to poison ivy. Inhaling dust from the tree or husks might cause respiratory difficulties like wheezing or coughing in sensitive individuals. Accidental ingestion of moldy black walnuts poses a risk due to mycotoxins. Symptoms from moldy nuts can include gastrointestinal upset, such as nausea, abdominal pain, or diarrhea, and potentially neurological effects like tremors or confusion.
Dogs and cats generally face a lower risk of toxicity compared to horses, but they can still be affected, primarily by ingesting moldy nuts or hulls. Moldy walnuts can produce neurotoxins that lead to more severe symptoms in dogs, including tremors, seizures, restlessness, vomiting, and incoordination. Cats may experience general signs of illness from ingesting leaves or roots.
Impact on Other Plants
Black walnut trees exhibit a phenomenon known as allelopathy, where they release chemical compounds that can inhibit the growth of nearby plants. Juglone is the main allelopathic compound, released from the roots, leaves, and hulls into the soil. This compound interferes with a susceptible plant’s respiration, photosynthesis, and water uptake.
The toxic zone from a mature black walnut tree can extend 50 to 80 feet from the trunk, with juglone concentration highest within the tree’s drip line. Many common garden plants are particularly susceptible to juglone toxicity. These include vegetables like tomatoes, potatoes, eggplants, and peppers, as well as ornamentals such as azaleas, rhododendrons, blueberries, hydrangeas, and lilacs. Plants sensitive to juglone may show symptoms like stunted growth, yellowing leaves, wilting, and eventual death.
Recognizing Poisoning Symptoms
In horses, classic signs include laminitis, characterized by warm hoof walls, reluctance to move, and a stiff gait. Swelling in the lower legs, increased heart and respiratory rates, depression, and colic can also occur. These symptoms often appear within hours of exposure to black walnut shavings.
For humans, direct skin contact with black walnut hulls or wood typically results in mild irritation, such as itching, redness, or a rash. If moldy nuts are accidentally ingested, symptoms may include gastrointestinal upset like nausea, vomiting, and diarrhea. More severe cases from mycotoxin exposure can involve neurological symptoms such as tremors, incoordination, confusion, or anxiety.
Pets, especially dogs, that consume moldy black walnuts may exhibit general signs of illness. These can include vomiting, diarrhea, lethargy, restlessness, panting, and in more serious instances, muscle tremors or seizures.
Safe Handling and Management
Horse owners should avoid using black walnut wood shavings for bedding, as even a small percentage can induce toxicity. Pastures should be inspected and cleared of black walnut trees or their debris, including fallen nuts, leaves, and branches.
Gardeners should consider the tree’s allelopathic effects when planning landscapes. Planting juglone-tolerant species near black walnut trees is recommended, while susceptible plants should be kept at a safe distance, outside the tree’s root zone. Proper disposal of black walnut leaves, nuts, and wood is important, as juglone can persist in the soil. Composting walnut leaves separately can render the juglone non-toxic over time.
General precautions include wearing gloves when handling black walnut hulls or wood. Prevent pets from consuming fallen nuts or hulls, and ensure children do not play with them, especially if mold is present.