Toxicity of black walnuts towards other plants
Black walnut (Juglans nigra), and to a lesser extent butternut (Juglans cinerea), a closely-related species, produce a toxic substance that can be harmful to nearby plants. The trees produce the toxin juglone, which inhibits respiration in susceptible plants, causing growth to be retarded, stunted, or deformed. This can even kill some plant species.
The extent of the damage depends on how resistant a particular species is to juglone, and the amount of physical contact the plant has with the black walnut roots. Root contact, or close physical proximity to the roots (within 1/4"-1/2"), seems to be necessary before the actual harm is done.
Minor amounts of juglone are supposedly generated in several other ways: by leaf decay on the soil surface, from nut husks, and even from rain water dripping through the leaf canopy. The quantity generated by these other means, however, is small. In addition, juglone is not very water soluble, so it will not move far in the soil.
Symptoms of black walnut toxicity are variable, and can sometimes mimic symptoms of diseases or physiological disorders. Walnut toxicity will often show up as a wilting or drooping of leaves, first partial, then total. This is especially true for fast-growing annual plants, such as potatoes or tomatoes.
Tomatoes, for example, show symptoms of yellowing and discoloration of the leaves, as well as twisting and puckering. Discoloration and darkening of internal stem tissue can also occur.
For woody plants (shrubs, trees, etc.), symptoms on newer tender growth can be similar to that of annual plants. Generally, symptoms on older growth are more subtle, and will appear as an overall decline of the plant, with eventual death possible.
Which plants are harmed by black walnut roots, and which are resistant to damage? The following list shows a number of common plants and how they tend to be affected by black walnuts:
|Resistant to juglone:||Susceptible to juglone:|
|Cherry||Kentucky bluegrass||Wild grape||Basswood (linden)||Azalea||Alfalfa|
|Red Cedar||Green pepper|
|*Possible; evidence not conclusive.|
Plants that are in the Nightshade family (Solanaceae) seem to be especially susceptible to the toxin juglone. Commonly planted vegetables in this family include eggplant, green pepper, potato and tomato. Most research on juglone toxicity and solanaceous plants has focused on tomatoes, partly because they are so widely planted. It is a good idea to be cautious with planting other members of the Nightshade family near black walnuts, also.
The list of susceptible and resistant plants is in no way complete. Research on this subject is limited, and often consists merely of observations of which plants do and do not grow near black walnuts in natural settings. Additionally, some reports offer conflicting information.
There is no chemical control available to stop the potentially toxic effect of black walnut roots. The only practical control is physical separation. Feeder roots from black walnuts extend far beyond the outer edges of the leaf canopy of the tree, so it is best to keep susceptible plants a good distance from it. There is no specific guideline for number of feet away from a tree, but obviously the farther away the better.
Cutting down and removing the tree is of limited help, unless all roots are removed from the soil. This gets to be a tedious and complex job (feeder roots are very tiny), so tree removal is not recommended. Any roots remaining in the soil can continue to give off the toxin juglone, until the roots deteriorate and decay (sometimes several years).
As a last resort, susceptible plants can be grown in above-ground containers. In pots, of course, tomatoes or similar annual plants will not be in root contact with the tree.
As indicated, black walnut leaves probably give off little juglone. But to be on the safe side, it is best to rake up the leaves, and to avoid using the bark or chips as a mulch, except around plants that tolerate juglone. Keep leaves and other black walnut debris out of the compost pile, too.