By Art C. Drysdale Every year for the last six I've been at the Success With Gardening Show at Toronto International Centre and one question always comes up. In fact, it is often the most-asked and has to do with gardens with black walnut trees in or near them. However, only in 1928 was the compound, juglone isolated in many plants in the walnut and hickory families, and found to be the cause of the death of other plants coming in contact with the roots of black walnuts, as well as butternut and hickories. Although the poisonous compound is found in the roots, leaves, bark and husks of the nuts of the black walnut, toxicity is usually only noted when the walnut's roots come in direct contact with susceptible plants.
Most people ask just what plants are susceptible.
Actually, quite a long list including tomatoes, blueberry, apple, pear, blackberry, azaleas, rhododendrons, cinquefoil, red and white pine, and other evergreens are all likely to show symptoms, and eventually die. As to plants that are not bothered, there is also a good list from which you may choose. Years ago, there was a report of beets with a strong flavour that may have been related to black walnut roots. The explanation would appear to be the amount of soil moisture. If it's flowers you're trying to grow, stay away from peonies.