The Black Walnut tree (\(Juglans\ nigra\)) is a native North American hardwood prized for its dark, durable wood and flavorful nuts. While the tree contains substances that can be toxic, the danger level depends on the specific part of the tree and the species exposed. Understanding the tree’s natural defenses is key to safely coexisting with Black Walnuts, especially for homeowners, gardeners, and livestock owners.
The Toxic Compound: Juglone
The primary chemical responsible for the Black Walnut’s negative effects is Juglone (5-hydroxy-1,4-naphthalenedione). This compound is a derivative of naphthoquinone. Juglone is not distributed equally throughout the tree; the highest concentrations are found in the buds, the outer fruit hulls, and the living roots.
While present in the leaves and stems, it is found there in smaller quantities. Juglone acts as a respiration inhibitor, interfering with the metabolic activity of sensitive organisms. This mechanism disrupts the internal processes needed to produce energy, serving as a powerful natural defense mechanism against competition.
Toxicity Risks for Pets and Livestock
The greatest toxicity risk associated with the Black Walnut tree is to horses. Exposure to the wood or shavings, particularly when used as bedding, can cause severe illness. As little as 5 to 20 percent Black Walnut residue mixed into bedding can lead to toxicosis. The toxic components, including Juglone, are absorbed through the horse’s skin or hooves, or are ingested.
Symptoms of exposure appear rapidly, often within 8 to 24 hours, and include signs of laminitis. Laminitis is an acute inflammation of the sensitive tissues inside the hoof, leading to severe lameness, reluctance to move, warm hooves, and an increased digital pulse. Other systemic symptoms include leg swelling (stocking up), colic, depression, and fever. Since this condition is a veterinary emergency with no antidote, prevention is crucial; Black Walnut wood products should never be used near horses.
The risk to smaller domestic animals like dogs and cats is generally lower for Juglone toxicity. However, the outer hulls of the fallen nuts present a different danger due to mold growth. If the nuts decompose, they can harbor mold that produces tremorgenic mycotoxins, which are highly poisonous if ingested by pets. Signs of this poisoning include tremors, incoordination, and seizures, requiring immediate veterinary intervention.
Allelopathy: The Effect on Other Plants
The Black Walnut tree engages in allelopathy, which is the chemical inhibition of nearby plant growth. The tree’s root system continuously releases Juglone into the soil, creating a toxic zone where many other species cannot thrive. This root zone can extend 50 to 80 feet from the trunk of a mature tree.
The highest concentration of Juglone is found directly beneath the tree’s canopy due to the density of roots and the accumulation of fallen leaves and hulls. Symptoms of Juglone toxicity in susceptible plants include foliar yellowing, stunted growth, wilting, and death. Highly sensitive garden plants include members of the nightshade family (tomatoes, peppers, and potatoes) and shrubs (azaleas, rhododendrons, and lilacs).
Gardeners can avoid this issue by planting tolerant species, such as:
- Corn
- Carrots
- Beans
- Maples
- Ferns
- Hostas
Using raised garden beds and promptly removing fallen leaves and hulls also minimizes Juglone accumulation in the soil.
Safety of the Black Walnut Nut and Timber
The Black Walnut nut kernel is safe for consumption and is a source of protein and healthy fats. The toxicity risk to people is primarily related to direct contact with the outer green hull, which contains a high concentration of Juglone. Handling these green hulls without protection can cause temporary skin irritation, itching, severe blistering, and will stain the skin a dark brown color.
Once the timber has been harvested and properly cured, the wood is safe for use in furniture, cabinetry, and other indoor applications. The Juglone stabilizes in the cured wood, making it a harmless and sought-after material. Woodworkers must exercise caution with the dust created during milling, as sawdust exposure can cause both skin and respiratory irritation. The bark and hulls are sometimes used in supplements or traditional remedies, but consuming the bark daily is unsafe and has been linked to a potential cancer risk.