Allelopathy Helps Black Walnuts Compete

A walk through the woods can be one of the most peaceful and calming experiences — a place where you can find quiet for reflection and marvel at the beauty of nature. Little do most people know that some plants, especially one specific tree, wage chemical warfare against other plants to keep away potential neighbors that would compete for nutrients and sunlight. In the Appalachian Mountains, the tree most skilled at chemical warfare is the black walnut.

The black walnut tree (Juglans nigra) is a useful, yet often misunderstood tree. Prized for its excellent wood qualities for lumber and furniture, the nuts it produces are either loved or reviled by those who try them.

The flavor of black walnuts is hard to describe. I would say that they have an almost astringent flavor, mainly due to the high level of tannins in them. They aren’t my favorite, but I don’t mind them either. I’ve learned to accept them, unlike during my childhood when you knew which church lady’s cake to avoid at the potluck because you knew that she put black walnuts in everything she baked.

My appreciation for black walnuts grew the year that I was the official nut judge (no joke) for the Black Walnut Festival in Spencer, WV. It was quite an experience — examining and weighing all the entries with a team of high school FFA students who cracked more than a few inappropriate jokes about the situation.

You could tell when someone was picking or cracking black walnuts, thanks to the tannin stains on their hands that just wouldn’t wash off. Black walnuts are a tough nut to crack (literally), so I also remember my grandmother cracking them “the easy way.” She would just pile them up in the driveway and run over them a time or two with her behemoth of an Oldsmobile (you know, the one that had full seats front and back and could hold half the neighborhood).

Black walnut trees have the interesting ability to excrete a chemical called juglone, which makes it nearly impossible for a number of plants to grow anywhere in its root zone. Juglone works by damaging the tiny root hairs on roots that are responsible for taking up a great majority of the water and nutrients the plants use. Research shows that it also interferes with the interaction of the roots with mycorrhizal fungi that aid the plant in taking up nutrients.

This process is not just specific to black walnuts. There are several other plants that do this. The phenomenon, called allelopathy, occurs when an organism excretes something that inhibits the growth of other things around it. You could equate it to the Penicillium fungus excreting a chemical that kills bacteria around it. We harness that chemical to use as penicillin.

Some plants are especially sensitive to the chemical. Many vegetable plants, especially tomatoes, are sensitive. Some plants, mainly those that would grow wild in the woods, are not susceptible. Many grasses also have a hard time growing beneath black walnut trees (tall fescue and Kentucky bluegrass being the exception, except during periods of drought).

Publication with lists of plants tolerant and damaged by juglone

All parts of the tree produce the juglone chemical, so the effects could spread beyond the perimeter of the tree from fallen leaves and branches. I would also suggest that you make sure any fresh woodchip mulch that you use (specifically that from local tree cutters) is free of black walnut. The juglone may break down after composting the wood chips for six months to a year, but I would still be cautious about its use. The wood will release the chemical, killing susceptible plants for a few years in the area where it is applied. Studies suggest that juglone will break down during the composting process, but I would check to make sure by starting a few tomato seeds on the batch of compost to see what happens.

—Garden Professor John Porter is a county extension agent for West Virginia University and writes the weekly Sunday garden column for the Charleston Gazette-Mail Newspaper.  This article was originally published October 2, 2015.

You can find John’s writing at and on Facebook and Twitter.

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John Porter

John Porter is a horticulturalist located in Omaha, NE. He is a former extension educator with UNL Extension and county agent with WVU extension. In addition to using his horticulture and extension skills in his home garden, he is a sometimes garden writer and lecturer, a trial judge for All-America Selections, and mentor for local school horticulture and agriculture programs.

5 thoughts on “Allelopathy Helps Black Walnuts Compete”

  1. Correct. There has not been a demonstrated effect in most mature landscape/garden plants. Almost all of the research done has been on germination and seedlings, which indicates juglone toxicity inhibits germination and seedling growth in a number of plants. As indicated by you in the group discussion, the mode of action is in root inhibition. So the biggest effect for gardeners would be in direct seeding under/near BW trees or in gardens where uncomposted BW material may be, and in planting small seedlings or direct seeding of crop plants such as tomato, bean, etc (which have been shown to be susceptible). It makes sense, since small herbaceous transplants would have limited roots and germinating seeds would not be able to grow roots.

  2. Where are the actual publications of the studies regarding juglone and its effects published? And when were these done? I was once found a reference to a study done in the 1950’s. I see articles referencing allopathic properties frequently, but I never see an actual scientific study.

  3. As Dr. Chalker-Scott shared above, she has a good review on effects of juglone in landscapes. This article was written around the time that we had a vigorous debate about walnuts and juglone in the Garden Professors blog facebook page and sought sources to confirm or deny the effect. One of the sources in Dr. Chalker-Scott’s review states it pretty well (While the genus Juglans provides what are probably the most widely accepted examples of allelopathic plants, it must be concluded that there still is no unambiguous demonstrations of its effect…… What is clear in the case of walnut is that there are well known (although not well documented) interactions with several plant species.” (p. 39)

    There are some laboratory experiments showing that juglone does have a negative effect on several plants, usually through root initiation (affecting germination/establishment) and photosynthesis.

    But there is not really conclusive evidence that there is an effect in the environment. As the Wills paper points out, juglone is not very mobile in the soil and is rapidly degraded by soil bacteria, so it is unlikely to ever reach a level to create the effects demonstrated in laboratory experiments. So now we understand that the effects are likely to be minimal if landscapes and gardens are managed appropriately.

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