The Q&A Archives: Walnut Tree Toxins

Question: We planted about 50 black walnut seedlings several years ago without realizing their toxic effect on neighboring trees and plants. (I just discovered this in your Q & A section.) How much space is needed by these trees to not harm nearby maples, pines, spruce and apple trees which we planted, also?

Answer: Juglone is the chemical exuded by the roots, bark, and leaves of walnut trees. Some plants are more susceptible to the growth inhibiting toxin and will not grow directly beneath the canopy of walnut trees. How close you can plant other trees to walnut trees really depends upon soil type and drainage. Most maples are tolerant of juglone, but pine and spruce are sensitive to it, and apple trees are borderline. If you've spaced your trees so they have ample room to grow and are not shading one another out, they should be okay living in your landscape. If the trees are very close and the canopies are overlapping, you can expect they are sharing the same root space, which may cause the other trees to grow poorly, or not at all. If your maples, evergreens and fruit trees are still small enough to move, you may want to consider transplanting them to an area away from the root zones of your walnut trees.

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