Garden corner

Walnut trees, including black walnut (Juglans nigra) and butternut (Juglans cinerea) are Kentucky natives and are considered by many as a valuable shade source. Additionally, the nuts produced on the tree are attractive to wildlife and people alike. Problems can occur however when trying to develop a landscape near these trees. Both species contain a chemical called juglone.

Juglone is produced in the leaves, trunk, and stems but is most concentrated in the roots and nut hulls. The area immediately under a tree usually contains a higher quantity of juglone, primarily because of the accumulation of roots and nut hulls. Juglone can be toxic to certain annuals, perennials, vegetables, and woody plants that are planted near walnut trees. This situation is referred to allelopathy. Plants that are susceptible to juglone will exhibit stunting, yellowing, partial to total wilting or even death. This does not mean you will never be able to landscape under these trees, but it does mean your plant palette or landscape methods may be limited.

Most symptoms occur when the roots of your new landscape plants encounter the roots of a walnut. Therefore, raised beds may allow you to plant sensitive plants while reducing risks. Be vigilant about removing leaves and nuts from your raised beds to reduce juglone buildup. Another option may be to plant your landscape beds beyond the drip line of the walnut’s canopy. Although roots still may exist in this zone, the concentration of juglone will be much less. If you decide to remove a walnut tree, you should be able to plant in that area within a year since juglone does not persist long in the soil. Remember not to use leaves, bark, or wood chips of walnut as mulch or compost amendment.

Here is a brief list of sensitive and non-sensitive landscape plants to help you decide what to grow.

Plants Sensitive to Juglone: apple, blackberry, blueberry, pepper, potato, tomato, azalea, rhododendron, lilac, peony, saucer magnolia

Plants Tolerant of Juglone: cherry, black raspberry, snap beans, corn, onions, forsythia, most viburnums, daffodil, daylily, hawthorn, phlox, many hostas, monarda, hibiscus

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