Black walnut (Juglans nigra) mulch is toxic to many plants and poses a severe health risk to horses. This toxicity stems from a natural self-defense mechanism the tree employs against competing vegetation. The use of black walnut wood products, including mulch and shavings, introduces this potent chemical into the surrounding environment. Understanding this process is important for gardeners and animal owners who may encounter this common hardwood material.
The Source of Toxicity: Understanding Juglone
The toxicity associated with the black walnut tree is due to an organic compound called juglone. This compound is not initially present in its toxic form within the tree’s living tissues. Instead, the tree contains a colorless precursor, hydrojuglone, which converts into toxic juglone when exposed to air or soil microbes after the plant material dies or is harvested.
Juglone is an allelopathic compound, meaning the tree naturally releases it to inhibit the growth of surrounding plants, reducing competition for resources. It works by inhibiting the respiration process within sensitive plants, depriving them of the energy needed for metabolic activity. While the highest concentrations are found in the buds, nut hulls, and roots, the chemical persists in the wood and resulting mulch, remaining active even after the tree is processed.
Specific Risks to Plants and Animals
The introduction of black walnut mulch into a garden can cause distinct symptoms in susceptible plants. Sensitive plants exhibit signs such as wilting, yellowing of the leaves (chlorosis), and stunted growth, which can ultimately result in death. Garden favorites in the nightshade family, including tomatoes, peppers, potatoes, and eggplants, are highly sensitive to juglone, as are certain ornamental shrubs like azaleas and rhododendrons. The damage is often confused with drought stress because the symptoms—sudden wilting during warm weather—look similar, but juglone-induced wilting is irreversible by watering.
For animals, the most severe risk is to horses exposed to black walnut shavings or mulch. Ingestion or even standing on bedding that contains a small concentration of black walnut wood can cause acute laminitis. Laminitis is an inflammation of the tissue inside the hoof, and it can develop rapidly, often within 24 to 48 hours of exposure. As little as 5% to 20% contamination in a batch of shavings is enough to cause illness, with symptoms including depression, colic, increased heart rate, and swelling in the limbs.
Safety Guidelines and Alternatives
To prevent juglone toxicity in a garden, avoid using black walnut mulch altogether, particularly in vegetable gardens or around sensitive ornamental plants. While juglone does break down naturally over time through microbial activity, moisture, and aeration, this process can take many months. If a wood chip source is unknown, assume black walnut may be present and use the material only around known juglone-tolerant plants or in pathways.
Alternatives for mulching purposes include pine, cedar, or general hardwood mixes, which do not carry the risk of juglone contamination. Other choices include cocoa hulls, shredded bark, or straw, which provide the benefits of moisture retention and weed suppression without introducing toxic compounds. For equine care, ensure that horse bedding is sourced from companies that guarantee the absence of black walnut wood.