While many gardeners have been sprinkling coffee grounds, making compost tea, and painting pruning cuts, Dr. Linda Chalker-Scott has been busy busting gardening myths like these. One by one, the passionate associate professor and extension horticulturist at the University of Washington has debunked gardening beliefs and practices that don’t stand up to scientific scrutiny. More recently, he has taken aim at a belief that has taken hold like a creeper: the notion that chemicals in walnut trees stunt and kill nearby plants. a paper Chalker-Scott, published earlier this year, claims that the killer walnut theory actually has little basis.
I say “little base” instead of no base. His exhaustive study of the literature revealed experiments that revealed a grain of truth to the idea. in the laboratoryWhen juglone (a chemical produced by walnut trees) was applied to germinating seeds and seedlings, the result was indeed “stunting, wilting, and necrosis” of the young plants. However, in the fieldThis phenomenon (known as allelopathy, which translates as “death to others”) has never been observed in a legitimate scientific experiment. Researchers have concluded that in the landscape, most of the juglone in the root zone of a walnut tree is soon decomposed by soil microorganisms or becomes tightly bound to clay particles. It basically disappears.
Traditional wisdom also holds that hickory mulch is suspect. Many believe that the toxic chemicals in walnut wood chips inhibit plant growth. Chalker-Scott also addresses this concern, noting that there is virtually no hydrojuglone (which converts to juglone in the soil) in walnut wood. It is mainly found in the roots and peels. She advises gardeners: “Use walnut wood chips as mulch if you have them. “They won’t harm plants and work as well as any other woody species.” (If you know her work, you know that Linda Chalker-Scott is a HUGE fan of wood chip mulch.)
In promoting the idea of toxic walnut, gardening experts sometimes cite personal observations of the phenomenon, but without scientific controls, who is to say that it wasn’t simply root competition or too much shade or some other factor that caused their plants had difficulties? Competition for water and nutrients is fierce under a mature walnut tree.
In advocating caution with nuts, other authorities, Chalker-Scott says, cite one of the two Extension publications. This is where it gets really interesting. The first article, from Ohio State University (Funt and Martin 1993), was removed from Ohio State’s website for being out of date. You can’t find it anywhere and read for yourself the claims it makes. The other, from Cornell (Crist and Sherf 1973), is also nowhere to be found and, as far as a Cornell librarian can tell, never existed at all! The original quote was simply a mistake and no one bothered to verify it.
“Do black walnut trees have allelopathic effects on other plants?” by Dr. Chalker-Scott. It was published in March of this year. I was hoping that by now the word would have spread and the garden gurus would have changed their minds. Unfortunately, a quick Internet search for “Walnut Extension Toxicity” reveals the same regurgitated warnings, the same lists of “tolerant” and “sensitive” plants. A partial list of extension sites that still promote this dubious message includes:
State of Pennsylvania
University of Wisconsin
Kansas State
Iowa State
Purdue
Michigan State
University of Delaware
Virginia Tech
university of illinois
University of Nebraska
University of Missouri
University of New Hampshire
Congratulations, however, to the University of Maryland! He cites Dr. Chalker-Scott’s article and agrees with her results.
Now, is Linda Chalker-Scott’s article the last word on nut toxicity? No, that’s not how science works. Many questions remain, and hopefully future research will shed light on them and improve our understanding of the topic. One thing I wonder about are all those anecdotal observations of plants dying under walnut trees. Could there be some truth in the stories after all? Perhaps future studies will show that there are some situations where this happens.
Another thing that interests me is the manufacturing of hydrojuglone/juglone in the plant. If it is not effective at killing competitors in the landscape, then why does the tree produce it? Isn’t it a cost (an investment of energy and resources) for the tree to produce this chemical substance? Is the tree in the process of developing a more effective version of juglone? Or was juglone more effective at one time, but microorganisms have evolved to remove it from the soil so it doesn’t affect the plants they depend on? Or is juglone toxicity simply a side effect? Perhaps the chemical serves some other purpose for the tree.
We don’t know the whole story yet, but in the meantime, please gardeners and horticulture professionals, let’s stop spreading misinformation and rumors.
