Walnut (Hickory) Family (Juglans spp.)
Walnuts require two trees to be close to one another in order to produce their mast. As mentioned earlier, black walnut trees are allelopathic. Black walnut contains a toxin called juglone. Its allelopathy wards off many other plant species. Planting near a black walnut tree should be avoided unless you are absolutely certain the juglone will not affect that particular plant species.
*photo courtesy of Norman G. Flaigg*