Q: I have ordered an English walnut cultivar for my back yard. The only available spot will place it within thirty feet of young crabapple and apple trees. Very recently I learned that English walnut cultivars are grafted to black walnut rootstock. Will such a tree produce soil toxins and harm my apples?
A: This situation could eventually be a problem but I am not sure you should worry about it now. Walnut roots do exude a chemical which is toxic to apple, tomato, hydrangea, azalea and many other plants. However, dozens of other plants are perfectly happy growing near a walnut. The question boils down to how much of the walnut root zone will intermingle with the roots of the apples. In the years while your trees are small in stature I don’t think there will be any contact. Ten years from now, the roots may be in contact.
Here’s an idea: install a root barrier midway between the trees. Simply dig a narrow trench eighteen inches deep and twenty feet long and insert aluminum flashing into it edgewise. By keeping the roots away from each other, your apples can prosper. You’ll have the ingredients for a Waldorf salad in a few years!