They are native to the Eastern U.S. and grow as far north as Maine in USDA Zones 4-9. But black walnut trees are not always good companions in the garden and yard. Toxic Juglone . Walnut trees grow throughout Britain and are happiest in free-draining, fertile and moisture-retentive soil in the sunniest position possible. Black walnut trees are prized for their dark colored heartwood. The bark is smooth and olive-brown when young, developing fissures and fading to silver-grey with age. Avoid exposed sites or frost pockets where strong winds and late frosts may damage flowers. The highest concentration of juglone will be within the dripline, but the tree’s roots (and the juglone’s effects) can stretch to a distance that equals the tree’s height. Black Walnuts (Juglans nigra) I have a lot of black walnuts that grow wild on my farm, but if you aren’t so lucky, you can purchase young trees to plant. With plants that are sensitive to the juglone black walnut trees produce, experts recommend they be placed outside of the tree’s dripline, or at minimum, at a distance of 50 feet from the black walnut tree. Black walnut are so valuable in fact forestry officials are often called on to track down black walnut poachers; in 2004, DNA testing was used to solve one such poaching case, involving a 95-foot black walnut tree worth US $28,500. Crushed leaves have a strong smell. Black walnuts contain a chemical called juglone which can be allelopathic to other plants. Black walnut is a large, broadleaf tree which can reach 30–40m high. Heavy clay or sandy soils are not ideal. The north California walnut, also called Hind's black walnut, is a medium-sized tree with a short, bulky look, since the crown is often wider than the height of the tree. Twigs are stout, green and curving. Growing black walnut trees for wood and profit in America. Very often, when sensitive plants are grown near the roots of black walnut trees, the plants die. Identified in winter by: the segmented pith, or spongy tissue, inside the twig. The trees get about 80 feet tall, but they’ve been known to reach as high as 150 feet. In fact, they can be toxic to nearby plants. The tree gets its name from its dark, heavily ridged bark which occurs even when it is still young. They typically have a short trunk and broad crown, though can be narrower if grown in a woodland situation. Look out for: the leaflets which have toothed edges. Walnut is a deciduous broadleaf tree which can grow to 35m. Look out for: crushed leaves that smell like polish. The trunk on mature trees can be 5 to 6 feet in diameter at the base. The leaves are about 1 foot long, with 13 to 21 leaflets with dentate (coarsely toothed) margins.