What is Black Walnut Tree toxicity?

A native Black Walnut tree produces toxic juglone in its roots to reduce competition from other plants and assure its survival.  The juglone can persist in old roots after the tree is cut down as well.

Plants sensitive to juglone show signs of what could be mistaken as drought symptoms. However, the symptoms come on more rapidly and do not dissipate with increased watering

This link to Penn State Extension details more about juglone toxicity and includes plants that are NOT as sensitive to it.