The Q&A Archives: Black Walnut trees poison plants & shrub?

Question: We have a black walnut tree that we have been told that it affects various plants and shrubs that are planted near or within 50 feet of.... is this true and what are the plants and shrubs that it would affect so that we can make sure that we don't plant any near the tree? The Black walnut tree is in our front yard and close to our house. We have lost several rhodies and we have been told that the tree is what is killing the shrub that it is posioning it. What can we do to save them or is there anything that we can do?

Answer: Black walnut (Juglans nigra) has a reputation for being allelopathic. 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. Many plants 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. 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, primrose, snowdrop, sweet woodruff, trillium and zinnia. Since your walnut tree is there to stay, try to plant one of the less-susceptible plants in the vicinity of the trees roots or canopy.

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