Do Black Walnut Trees Wear a Black Hat?
R.A. Murray, Colts Neck, NJ
Reprinted from Princeton Chapter Newsletter
Black walnut trees produce edible fruit and a most beautiful variety of wood. However walnut trees are not an unmixed blessing. They can be a killer to other plants in your garden.
Black walnut trees produce a chemical (5 hydroxy-1, 4 naphthoquinone) called juglone which is poisonous to many other plants. Other trees, including English walnut, pecan, butternut, and hickory produce juglone but in much lower concentrations than black walnut. The chemical is produced in the leaves during the growing season and subsequently moves to the roots. It is a strong respiration toxin which tends to interfere with a plants ability to utilize energy captured by its leaves. The symptoms range from dwarfing, to wilting and yellowing of leaves, to death of the plant; depending upon the amount of juglone and the relative tolerance or sensitivity of the plant to juglone. Juglone is released from walnut trees in several ways. Falling leaves carry it. Rain dripping from the leaves carry it to the ground. The roots carry it beyond the drip line, sometimes as far as three times the crown radius from the trunk. Mulch containing leaves, twigs, nut hulls, or wood chips of walnut are all sources of juglone. Juglone is poorly soluble in water so it does not move far in the soil (without roots to carry it), however this is not all advantage since it is not leached or rinsed away either.
If you do have walnut trees, you should be alert to their effect on surrounding plants. In nature there may be no obvious effect, because there are a number of species which are tolerant in varying degrees and these would be the plants that would be present. However in a garden where nature has not selected the plants, the effects may be disastrous. If you suspect that your plants may be susceptible, avoid planting them anywhere near a walnut tree. If they are already there and appear to be suffering, move them from under the walnut tree. If you cannot move them too far from the drip line of the tree, cultivate deep and often to keep the tree roots from contacting the roots of the sensitive plant. Only as the last resort should you think of getting rid of the walnut tree. They can be very valuable trees but they do not make good neighbors for many other plants.