Juglone is a naturally occurring chemical compound, specifically a naphthoquinone, found predominantly in black walnut trees (Juglans nigra). This compound plays a significant role in the tree’s natural defense mechanisms. It acts as a biochemical tool, helping the black walnut tree reduce competition from other plants in its vicinity.
Sources and Release
Juglone is present in various parts of the black walnut tree, with the highest concentrations typically found in the buds, nut hulls, and roots. Lower amounts exist in the leaves, bark, and stems.
The tree releases juglone into the surrounding environment through several pathways. As rain falls, it leaches the compound from the leaves and branches, carrying it into the soil. Juglone is also released when decaying plant matter, such as fallen leaves and nut hulls, breaks down. Furthermore, the roots actively exude juglone into the soil, creating a zone of influence around the tree. This toxic zone can extend an average of 50 to 80 feet from the trunk of a mature tree, with the highest concentrations directly beneath the canopy.
How It Affects Plant Growth
The primary way juglone impacts other plants is through a process known as allelopathy, where one plant produces biochemicals that influence the growth, survival, or reproduction of other organisms. Juglone specifically inhibits the growth of sensitive plants by interfering with their cellular respiration and enzyme activity. This disruption prevents affected plants from properly absorbing water and nutrients, leading to characteristic symptoms.
Plants affected by juglone often exhibit wilting, yellowing of leaves, or stunted growth, even when soil moisture is adequate. In severe cases, these plants may eventually die.
Common garden and ornamental plants highly sensitive to juglone include:
Tomatoes
Peppers
Potatoes
Eggplant
Azaleas
Rhododendrons
Blueberries
Lilacs
Peonies
Conversely, numerous plants can tolerate or resist juglone and can thrive near black walnut trees.
Examples of juglone-tolerant plants include:
Many ferns
Hostas
Daylilies
Various spring-flowering bulbs like daffodils
Smooth hydrangeas
Certain evergreens, such as arborvitae
Understanding these differences helps gardeners select appropriate species for planting near black walnut trees.
Impact on Animals
While juglone primarily affects plants, it can also pose risks to certain animals, with horses being particularly susceptible. Exposure to black walnut wood shavings, often used as bedding, can lead to laminitis in horses, a serious and painful inflammation of the sensitive tissues within the hoof. Even small amounts, as little as 5-20% black walnut shavings mixed with other bedding, can induce this condition.
Other livestock are less affected, though caution is advised. For humans, direct toxicity from casual exposure to juglone is rare. However, handling black walnut husks can sometimes cause temporary skin irritation or staining for sensitive individuals.
Coexisting with Black Walnut Trees
Living with black walnut trees requires specific strategies to minimize the impact of juglone on desired plants. One effective approach is to select plant species known to be tolerant of juglone, allowing them to coexist successfully. Creating physical barriers in the soil, such as root barriers made from sturdy materials, can help prevent the spread of walnut roots into planting areas, thereby limiting juglone exposure.
Managing fallen leaves and fruit hulls is also important. While juglone in leaves breaks down quickly (within 2-4 weeks) in a well-managed compost pile, nut hulls and woody debris require a longer composting period, often six months or more, to ensure the toxin fully degrades. Improving soil drainage and aeration can also help mitigate juglone’s effects, as it is more problematic in poorly drained soils. Cultivating plants in raised beds filled with juglone-free soil provides a contained environment, protecting sensitive species from the compound.