Black Walnut Toxicity (Published 1977) (2024)

Within the month a nurseryman and owner of a landscaping service asked for information on plants that can be used as a hedge near or Under black walnut trees. This question relates to a problem that has plagued horticulturists for centuries.

Plinius Secundus in A.D. 37 noted that few plants would grow under walnut trees and believed that it was the shade Of the tree that was in some way toxic to other plants. Nearly two thousand years Later (1977), an article in the American Horticulture Magazine titled “Under the Spreading Walnut Tree”, shows photographs of a variety of flowers growing happily under a black walnut tree. On the other hand, experience reported from Michigan showed growth failure with many ornamentals near black walnut. Both reports are without question, factual.

This situation has led to the recognition of what in a way, might be regarded as a new science called Allelopathy. This term in its most frequent usage is concerned with the injurious effects of substances released by some plants on Other plants. They are chemicals or enzymes, some of them poisonous, Which inhibit growth of other plants by direct Contact or by releasing residues in the ‘AWL Examples are the stunting effect of sunflower seed and buckwheat hulls ‘.used as a mulch and growth failure of peach trees when planted in soil from which older peach trees have been retinoved. The whole matter becomes tech!nically complicated because all plants in a restricted anvironment compete for their basic requirements and it is difficult to separate these from allelopathic ‐influences.

Perhaps the most discussed and certainly a very complicated.allelopathic problem, is black walnut toxicity. This tree (Juglans nigra) is valuable for its wood, for the nuts it produces, and because in its relatively light shade, lawn grasses grow well under the trees when ‘compared with maples or other shallowrooted trees or those with dense shade.

About40 years ago, there was controversy in the Northern Nut ‘Growers Association as to the toxicity of the walnut. This was triggered by research showing that the growth of tomatoes, alfalfa and apple trees was se7xerely stunted or the plants killed by ‘‐'contact with walnut roots, but that many other plants were not affected be

‐Laurence H. MacDaniels is professor emeritus, Cornell University. yond what might be expected from competition for water and nutrients. Chemists isolated a highly toxic material (a Napthaquinone, named juglone) from leaves, bark, husks and roots of walnut, butternut and hickory and this was believed to be the cause of growth failure in affected plants.

On the other hand, there were acceptably reliable reports that under some conditions at least, alfalfa, tomatoes, and apple trees did grow without serious damage, under or within the root spread of black walnut trees. Further, in California, where the Persian walnut (Juglans regia) is grown commercially, the growers recognized no problems with cover crops that could not be solved with ample nitrogen, fertilizer and water. Of interest also was a press release from the U.S.D.A. entitled “Test Clears Walnut's Reputation” which reported no damage to tomatoes or alfalfa growing under walnut trees if the plants were watered and fertilized. Negative results were also secured from carefully conducted greenhouse experiments to which tomatoes, alfalfa, and small apple trees were grown in five‐gallon crocks along with walnut seedlings for comparison.

During the past three decades, through planned research and observaLion, much has been learned about the interaction between growing plants. With the walnut it is generally accepted that under many conditions, the roots do have a toxic effect on many plants but not on all. The most comprehensive study was made over a period of12 years during which detailed data were secured on all the plants growing within a’ 60‐foot radius of three hundred mature black walnut trees in five different states. Overall, 218 species of plants were listed and apparent antagonisms recognized between black walnut and tomatoes, potatoes, alfalfa, blackberry, blueberry, mountain laurel and other heaths, red pine and many other plants of lesser importance. It was also shown that some plants grew better within the root zone of walnut than outside, including Kentucky bluegrass and some other grasses, black raspberries, and some others. The limits of antagonism were sharp corresponding to the spread of the roots of the walnut trees which might be three or four times the area under the tree's crown. It was also noted that within the root spread of walnut trees, the pH of the soil was nearly one point higher than outside it. This difference is also found under many other trees and may be important in its effect on plant

Research has shown that in order to produce toxic symptoms the live roots of walnut must, with the tomato at least, be in contact with the live roots of the af fected plants. Live root bark in contact with alfalfa and tomato seedlings inhibited their growth and pieces of bark in nutrient solution prevented normal root development. On the other hand, walnut hulls and leaves used as a mulch have not produced toxic symptoms in sensitive plants even though they contain the toxic substance juglone.

Of special interest is the fact that when water in which freshly hulled walnuts have been washed is poured on ground containing earthworms, the worms appear on the soil surface within a few seconds and die within an hour or less. The same leachate does not injure the gr‐. ^ Dr adversely affect other plants upon which it has been tried.

The conflicting evidence from research and observation can be explained in a number of ways. It has been shown that the toxic substance juglone, supposedly the cause of walnut toxicity, needs ultra violet light for its synthesis hence it is not formed in the greenhouse.

Observation also suggests the possibility that the toxic agent, presumably juglone, is not formed by young trees or that the roots of young trees do not penetrate all of the area near the tree. Symptoms of toxicity are more likely to occur as the trees become mature.

Evidence also indicates that the toxic substance present in walnut breaks down rapidly in the soil or that there may be a synergic reaction with some other substance to produce antagonism.

In any case, there.is ample evidence to establish the fact that in many soils and under many conditions, walnut toxicity is a problem that cannot be ignored by gardeners. The antagonism is selective causing damage to some plants and not to others.

A question often asked is how long the residual toxic effect of walnut trees remains in the soil after the trees are removed. My experience in the vegetable garden has been that after a full year, signs of toxicity disappear. This is supported by evidence that it is the live roots that are toxic and that the roots of the cut tree die during that time:

Additional evidence is that compost made from walnut hulls and leaves has given no adverse affect on tomato plants grown in it. In the flower garden, however, lilies growing under a small walnut tree removed in the late spring showed stunted growth the following summer. The roots of the walnut were still alive in the ground as shown by living sprouts from the stump.

As a practical matter, gardeners should be aware of the allelopathic antagonism of walnut, butternut, and to a lesser degree hickory with many ornamentals and vegetables. Tomatoes are by far the most sensitive of the vegetables and rarely, if ever, will produce satisfactory crop within the root spread of a mature walnut tree. Other sensitive vegetables are potatoes and sweet peppers. Less sensitive are sweetcorn, beets, parsnips, beans, peas and onions.

Among cluded Rhododendron, mountain laurel and other heaths, blackberry, nine bark (Physocarpus) and lilac. Less sensitive are black raspberry, forsythia, grapes, wild roses, some kinds of dogwood and Euonymus.

There is little evidence regarding trees except with apples and red pine, both of which show antagonism with walnut.

The above lists are far from complete and there is no assurance that any one kind of plant will succeed or fail under walnut in all soils. There is convincing evidence that black walnut trees should not be planted where sensitive plants are to be grown. On the other hand, there is a large selection of plants that will grow satisfactorily provided that they receive an adequate supply of water and nutrients to offset the competition with walnut roots. There still much to learn about antagonisms between plants, which hopefully, research in the field of allelopathy will shed light.