Black Walnut trees are allelopathic, meaning that they produce a toxic chemical called juglone. Related tree species, like Hickory and Butternut, also produce juglone but at a lower level than Black Walnut. The toxin is present in fruit, leaves and branches and can be excreted from the root system into the soil. Juglone is potent enough to inhibit or kill many plant species surrounding the Walnut tree but many natives are tolerant or resistant. The natives assigned to this category are compiled from our own experience and from research in reputable books and websites.