English Walnut on Black Walnut Rootstock: Toxicity Risk to Apple Trees

Q: My English walnut (on black walnut rootstock) will be 30 feet from crabapple and apple trees. Will juglone toxicity harm them?

Potentially, but not immediately. Walnut roots exude juglone, toxic to apples, tomatoes, hydrangeas, azaleas, etc. However, many plants tolerate it. While the walnut is small, root zones won't overlap much.

You could install a root barrier: dig an 18-inch-deep trench and insert aluminum flashing vertically between the trees to keep roots separated.

See Black Walnut Toxicity for susceptible/tolerant plant lists.