The Walnut, Juglans nigra, is a beautiful tree, historically planted in pastures to encourage grass and on hillsides, to control erosion. But, everyone who has tried to garden near one knows that many of the plants we commonly grow are susceptible to juglone toxicity. Juglone (5-hydroxy-alphanpthaquinone) is a phytotoxin that affects nearby plants by stunting, withering and finally death. Other species of Juglans also contain juglone, but in lesser consentrations than the Black Walnut. Next in toxicity levels is the English Walnut, Juglans regia, then the Butternut, Juglans cineraria, Hickory, Carya ovata, and Pecan, Carya illinoinensis. It should also be noted that the Black Walnut is often used as rootstock and still produces juglone into the soil. Persian, English and Carpathian Walnuts are often grafted onto Black Walnut roots.

Juglone is present in every part of the Walnut. It can extend out from a mature tree 50 to 60 feet. Plants even farther have been known to be affected by the toxin. Use of Black Walnut leaves, crushed husks, sawdust or any other part of the Walnut in a garden, especially as uncomposted mulch introduces the toxin into the soil even if a tree is not nearby and should be avoided. However, according to the Purdue extension, Walnut leaves can be composted. When exposed to air, water and bacteria the juglone toxin breaks down and the toxic effect can be degraded in two to four weeks. In soil the breakdown may take up to two months. It is suggested that they be composted separately and tested for toxicity by planting tomato seedlings (which are highly susceptible) in it. If composting the bark, it is recommended that 6 months is needed to make it safe for susceptible plants, and that the tomato seedling test be used.

If you have experienced the frustration of Black Walnut toxicity in your garden there are plants that can be grown near your Walnut tree. While knowing which plants are highly susceptible to your Black Walnut tree will help you choose a better location for them. Following are some plant lists. It should be noted that even resistant plants can occasionally have problems with the toxin.