As the years roll by, I find myself called on to do more and more diagnosis on why certain landscape plants are performing poorly. I am seeing Black Walnut Toxicity on a more frequent basis, enough so that I am training our staff to always keep this factor in mind when advising our customers on selection of trees, shrubs and perennials for landscaping purposes.
Just recently, I evaluated a Whitespire Birch clump that had severe die back. At first, I thought it might be birch borer, but found no evidence of the tunnels that borers create just below the bark, and yet one entire stem had completely died and two others looked as if they would soon follow. As I looked around, I noticed a huge walnut tree in the neighbors back yard, about 60′-70′ away.
When the birch tree was young, its roots were outside the root zone of the walnut tree, but as its root system grew larger, it spread far enough to contact the ever expanding root system of the walnut tree. Walnut roots are the source of the greatest amount of juglone, the toxin released by the walnut tree. The roots of a walnut tree can easily extend to double the width of the branches of the tree, so when planting anywhere near a walnut tree – select from a list of plants resistant to the walnut toxin.