One of the gardener's biggest bugaboos is trying to grow plants in the shadow of the black walnut tree. If they could, many plants would rather pull up their roots and hightail it out of town than face the wrath of the black walnut!
The nasty chemical in question is commonly known as juglone and while it has been isolated from most parts of the walnut tree, it seems that the toxic effects of juglone only occur when the roots of susceptible plants make contact with the roots of walnuts.
There are a few things that affect the level of toxicity of black walnuts. First it seems that the effect is greater in poorly drained soils and that in this case, direct contact of one root to another may not be necessary for the toxic effect to occur. It also seems that younger walnuts do not cause toxicity problems - the problem seems to start when the walnuts are seven to eight years old. Finally, removal of the walnut tree will not stop the problem right away. Apparently, the roots must break down completely before all of the juglone they contain will disappear and the risk of toxicity passes - a process that can take several years!
What are the symptoms of walnut toxicity? The plant leader(s) will turn down and look wilted. This will be followed by a wilting affecting most or all of the plant. The wilting itself often occurs very rapidly and is in effect, a one way street. Even though this wilting can be quite rapid, it doesn't mean that every plant that is susceptible will die right away. In many cases, plants will grow for a year or more before any damage becomes visible. Those plants that show some tolerance to juglone will often be stunted, weak and have little or no flower. It seems that juglone causes excessive cell growth in the xylem vessels of susceptible plants (the xylem vessels are the transport systems for water and nutrient within the plant). As these growths multiply, they block the xylem which in turn stops water movement and in severe cases leads to the wilting of the plant.
While gardening around a black walnut is a challenge, all is not lost. Removal of the tree is an option, but it is not a very good one as these stately trees are quite beautiful in their own right (also, you will still have the lingering problem with the roots). Since the highest concentration of roots occur out at the dripline of the tree and the roots closer to the trunk are often deeper and far less numerous, you can try planting shallow rooted plants closer to the trunk of the tree. Raised beds are also an option when planting in the shadow of a walnut. However, I wouldn't suggest bringing in a load of topsoil with the thought of raising the grade all the way around the tree. Changing the grade in such a manner can sometimes have adverse effects on the tree itself.
The most obvious option is to grow plants that are seemingly tolerant to black walnut. Of this known listing (OMAF and Rural Affairs has a publication with a list of plants showing resistance to walnut induced toxicity), a great many plants are either native ones or are ones that occur naturally in woodlands. A few examples, trillium, anemone, meadow-rue, primroses, jack-in-the-pulpit, periwinkle, may apple, Solomon's seal, dog tooth violet, ostrich fern and the Christmas fern. A few ornamentals - honeysuckles, mockorange, forsythia, burningbush, lilac, hostas, lilies and the viburnums. I am sure there are far more....