When we first moved to town (2008), I wanted to plant a pecan tree, but the local nursery told me that the only nut trees that would grow here reliably are black walnuts. So with regret, I planted an English walnut grafted onto black walnut root stock. This is only about 15 feet from my pear tree guild, and walnut roots are notorious for secreting juglone, a chemical that stunts many plants' growth, including pears. Rumor (specifically, the book Gaia's Garden) has it that mulberry roots can help neutralize the juglone, so I planted two dwarf mulberries between the walnut and the pear.
Meanwhile, there are many other plants that are known to do just fine under a walnut tree, so I planted them under the walnut to make the best of the situation.
See an album of photos of this guild.
