The Juglone Toxin

• In and around the 1880s, scientists discovered that walnut trees produce a compound called juglone. They first identified the compound in the walnut tree fruit. This material is phytotoxic (it kills plants), and affects specific plant types. Recent studies have found this toxin in the leaves, branches and roots of walnut trees. Concentration of juglone increases in the summer with increased fruit growth. Black walnut trees have the most potent juglone.
  
  The soil surrounding the tree tends to absorb the toxin. Typically, raindrops absorb toxin from the fruit and carry the toxin to the ground surrounding the tree. Moreover, decaying leaves will only increase the soil's toxicity levels. Finally, the roots themselves will secrete this toxin into the soil as well. If you would like to use the walnut tree for compost, you can do so. Juglone breaks down when exposed to air, water and bacteria. This will take at least two months. You can compost it separately and test the toxicity level by planting tomato seedlings in it.
  
  

Plants to Avoid

• Juglone does not affect all plants. If you want to create a garden near the walnut tree, you should avoid certain vegetables. Tomatoes, potatoes, peppers and eggplants are most susceptible to juglone. Affected plants will show limited growth, excessive wilting and, in some cases, the plants may simply die. Of course, the larger the tree, the more toxins in the soil and the added stress to the garden of susceptible plants. The toxin will spread to at least two to three times the "crown radius," which is the distance from the trunk of the tree to the drip line (the edge of widest tree branches).
  
  

Recommended Plants

• There is not a complete list of all plants that are not susceptible to juglone, but research indicates that categories of trees are tolerant of it. Maples, oaks, birches, sycamore and mulberry trees seem to be tolerant of the walnut tree. If you have limited space for your garden and you must plant near the walnut tree, the following vegetables are not susceptible: squash, melons, beans, corn, allium and carrots. You can plant, however, fruit such as peaches, nectarines, cherries, plums, pears and black raspberries near walnut trees.