Black Walnut + Apple

Black walnut and apple are incompatible companions. Black walnut produces juglone (5-hydroxy-1,4-naphthoquinedione), a chemical released through roots, fallen leaves, and decomposing nut hulls that is toxic to a wide range of plants. Apple is among the most juglone-sensitive fruit trees and will show decline symptoms (wilting, yellowing, dieback) when planted within 50 to 80 feet of a mature black walnut.

The toxicity zone extends roughly to the drip line and beyond, and persists in soil for years after a walnut tree is removed. Replanting apple into former walnut ground requires waiting 2 to 5 years and testing soil juglone levels before assuming it is safe.

There is no practical workaround for this pairing at close range. If both trees are priorities on a given property, site them at opposite ends with maximum distance between them. Do not compost black walnut leaves or hulls in beds where apple trees will root.