It is difficult to grow plants under black walnut trees, many people know this. Science-based sources including Cooperative Extension for years have blamed a toxic substance called jugalone, emitted from these tree roots.

However, a recent literature review by horticulture professor Dr. Linda Chalker-Scott at Washington State has determined that this substance isn’t really the culprit.

Knowledgeable people are now returning to this 100-year-old conclusion: it can be difficult to grow many plants near or under black walnut trees due to their dense shade and extensive root system, which absorbs much of the available water. The current problem is, the myths about walnut toxicity have been very widespread, and amplified in the repeated telling. It will take a long time before this myth, included in many reliable publications, will be replaced with current knowledge.

In her recent paper, she shares the story about how this myth survived for so long. Research long ago determined that walnut roots emit this substance which is toxic to many plants. What wasn’t given consideration is the more recent realization that soil can only transport jugalone very short distances. In addition, the conclusions about toxicity were all based on laboratory conditions, not natural conditions outdoors.

Allopathy is the chemical process where plants attack other plants, the process that applies to black walnut trees. In her paper, Dr. Chalker-Scott mentions that most existing alleopathy research has been conducted in the laboratory, not natural conditions. It appears that the lab findings were used to support theories about observations in nature. The concept of considering soil conditions when studying alleopathy wasn’t integrated until 1974.

Our understanding of the truth about many gardening issues can be difficult to pinpoint at times. Also, research findings may change over time as more evidence emerges through repeated research, and focusing on specific variables.

In any case, Dr. Chalker-Scott does have recommendations for growing plants under black walnut trees, which I have blended with my own experience:

n Supplemental water will likely be necessary under these trees. Less dense plantings will require less irrigation.

n Wood chips and old leaves from walnut trees contain no jugalone. They may be used as mulch under black walnuts and elsewhere.

n Use shade-loving plants under these trees. Plants and bulbs that bloom in spring, when there is ample soil moisture, will be most successful. These trees leaf out later in spring, providing some sun for early plants.

She suggests that black walnut trees are worthy of inclusion in your yard. While the nuts do support wildlife, they aren’t my favorite. The squirrels plant those nuts far too often in my garden. The weedy sprouts need to be dug out with a shovel; if you just break the top off, they grow back. The husks stain your hands and clothes when you pick them up; they are too heavy to rake. The nuts are tasty, but it takes serious effort to remove the shells.


To read Dr. Chalker-Scott’s paper, use search terms “WSU alleopathy Linda Chalker-Scott.”

Julie Brocklehurst-Woods has been a Master Gardener Volunteer with Cornell Cooperative Extension of Livingston County since 2002. She enjoys helping all gardeners become successful gardeners, especially helping people identify tools and strategies to prioritize and simplify their gardening tasks. She will answer gardening questions by email:

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