Walnut - Wikipedia, the free encyclopedia

As garden trees they have some drawbacks, in particular the falling nuts, and the releasing of the allelopathic compound juglone, though a number of gardeners do grow them.^{[3] [4]} However, different walnut species vary in the amount of juglone they release from the roots and fallen leaves - the black walnut in particular is known for its toxicity. ^[5] Juglone is toxic to plants such as tomato, apple, and birch and may cause stunting and death of nearby vegetation. Juglone appears to be one of the walnut's primary defence mechanisms against potential competitors for resources (water, nutrients and sunlight), and its effects are felt most strongly inside the tree's "drip line" (the circle around the tree marked by the horizontal distance of its outermost branches). However, even plants at a seemingly great distance outside the drip line can be affected, and juglone can linger in the soil for several years even after a walnut is removed as its roots slowly decompose and release juglone into the soil.