Black walnut (Juglans nigra) and, to a lesser extent, other walnuts (Juglans spp.), as well as their close relatives, the hickories (Carya spp.), release a product called juglone that is allelopathic, meaning toxic to many other plants. This gives the walnut a distinct advantage, as few plants can grow well in its shadow, particularly at its base. Juglone is found in all parts of the walnut tree except the nut itself—leaves, branches, bark, wood, and especially in its roots and nut husks.
Allelopathy: Nature’s Hidden Defense
Allelopathy is nature’s way of reducing competition. Certain plants, like the black walnut, release chemicals that inhibit the growth of others around them. These chemicals, like juglone, interfere with the processes of nearby plants, stunting their growth or even killing them off entirely. This allows allelopathic plants to dominate their surroundings, ensuring they get the resources they need—sunlight, water, and nutrients—without much competition. In the case of the walnut, juglone creates a protective zone where few other plants can survive. But don’t worry, many plants are resistant to these effects and can still thrive alongside walnuts.
Composting Walnut Leaves: Safe or Risky?
But does that mean you should avoid putting “toxic” black walnut leaves in your compost bin? According to Ohio State University, the answer is yes, you can compost them safely. Juglone decomposes in a matter of weeks when exposed to air, water, and microorganisms. If the leaves are shredded, the process is even quicker. Once fully decomposed, walnut leaves apparently make compost of excellent quality, providing rich nutrients for your garden.
What About Walnut Sawdust, Wood Chips, and Husks?
However, it’s a different story when it comes to walnut sawdust, wood chips, and nut husks. These materials decompose much more slowly and retain juglone for longer periods. To be on the safe side, it’s best to let them compost for at least 6 months before using them in your garden. This gives the juglone time to thoroughly break down, ensuring they no longer pose a threat to sensitive plants like tomatoes, peppers, and azaleas.
Growing Juglone-Resistant Plants Near Walnuts
It’s also worth noting that not all plants are affected by juglone—many are resistant. So, if you’re gardening near a walnut tree, choosing juglone-resistant species can help ensure your garden thrives despite the tree’s chemical defenses.
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