The Q&A Archives: Black Walnut Toxicity

Question: I know that black walnuts are toxic to azaleas. I can't find info regarding potential toxicity to pieris japonica. I have 2 different container grown pieris japonica and before I plant them, I'd like to know whether black walnuts are likely to harm them. I have several black walnut trees thoughout my .7 acre landscape.

Answer: Black walnut (Juglans nigra) has a reputation for being allelopathic--that is, for inhibiting the growth of other plants. Small amounts of juglone are released by the tree roots, but a far greater amount of this growth inhibiting chemical is found under the canopy of the tree.

Some plants that are tolerant to juglone include: arborvitae, cedar, catalpa, clematis, daphne, elm, euonymous, forsythia, hawthorn, hemlock, sycamore, astilbe, begonia, bellflower, orange hawkweed, hosta, pansy, phlox, marigold, pieris, primrose, snowdrop, sweet woodruff, trillium and zinnia.

Avoid plants that are sensitive to juglone, including apple, blackberry, pear and blueberry, asparagus, cabbage, eggplant, pepper, potato and tomato. Landscape plants sensitive to juglone include azalea, birch, honeysuckle, hydrangea, larch, lilac, magnolia, maple, pine, potentilla, rhododendron, spruce and viburnums.

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