I have recently installed a new garden in my backyard. My next door neighbour has a mature black walnut tree and I am concerned about a red cut leaf Japanese maple I have planted about 25 feet away from it. I am considering moving it to the front of my house, although I love how it looks where it is and would prefer not to if I knew that the juglone would not affect it. I have peeked over the fence at my neighbour’s garden and she has many diverse plants happily growing there. Would you advise me to move this tree? What is the likelihood of it surviving with this toxic tree neighbour?
Thank you for contacting the Toronto master gardeners with your inquiry concerning growing your Japanese Maple near a walnut tree.
According to Virginia State University’s article Alleopathic Effects and Tolerant Plants “ When certain other landscape plants are planted near or under this shade tree they tend to yellow, wilt, and die. This decline occurs because the walnut tree produces a non- toxic, colorless, chemical called hydrojuglone. Hydro- juglone is found in leaves, stems, fruit hulls, inner bark and roots. When exposed to air or soil compounds, hydrojuglone is oxidized into the allelochemical juglone, which is highly toxic. Juglone is exuded from all parts of the walnut tree. Juglone can affect other plants either through root contact, leakage or decay in the soil, falling and decay- ing leaves, or when rain leaches and drips juglone from leaves and branches onto plants below. Plants located beneath the canopy of walnut trees are most at risk because juglone from the roots and fallen leaves accumu- lates there.”
The harmful effects of juglone on surrounding plants can depend on the soil and its moisture conditions. Good drainage may decrease responses to juglone, thus when planting near a black walnut tree enhancing soil drainage would be beneficial. There are a number of things that one can do to reduce the alleopathic effect of juglone. Regularly clean up all of the fallen leaves and fruit from the black walnut tree, keeping debris away from desired landscape plants. Maintaining high organic matter levels in the soil encourages healthy soil microbial populations that can metabolize toxins. Planting tolerant trees, shrubs, vines, ground covers, flowers and grasses under walnut trees, in areas that might contain walnut roots.“Juglone and black walnut: a Toronto Master Gardeners Guide” – includes lists of plants that can’t tolerate juglone, as well as many that can live in relative harmony with the tree. According to both articles listed above Japanese maples are tolerant of juglone.
Recently, an article published by Linda Chalker-Scott titled Do Black Walnut Trees have Alleoplathic Effects in Other Plants seems to dispute the effect of juglone on plants “the entire body of primary evidence for black walnut allelopathy in the landscape is attributed to two dated Extension publications, one that has been withdrawn from circulation and one that doesn’t exist. These are not reliable sources of information and should not be cited as evidence for juglone toxicity, especially in peer-reviewed journal articles” . What appears to affect plants planted under not only walnut trees but all trees is competition for water, nutrients and oxygen. The denser the planting the more competition. Planting shade tolerant as opposed to sun-loving plants. The addition of mulch to retain soil moisture and to nourish beneficial soil life including mycorrhizae will also be beneficial.
From the above articles it appears that there is a division on whether jugulone really affects plants grown near or beneath the tree. Also, just because the plant is tolerant to the effect of jugulone does not mean that the tree/shrub will grow to its full potential. If your japanese maple was expensive, which some of them can be, for peace of mind I would move your tree. If you liked the look of the dark burgundy leaves then a dark leaf elder or ninebark will have a similar look and both are tolerant of jugulone.
Here’s to continued enjoyment of your japanese maple tree.