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 crabapple and apple?
A: This 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 apples, tomatoes, hydrangea, azalea and many other plants. However, many 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. While your walnut tree is small I don’t think there will be any contact.
You could even install a root barrier between the trees. Simply dig a trench eighteen inches deep and insert aluminum flashing into it edgewise. By keeping the roots away from each other, you’ll have the ingredients for a Waldorf salad in a few years.