Black walnut trees growing in Ohio yards and landscapes can be a plus or minus, depending on where they are located. Although black walnut trees produce edible nuts for people and wildlife, they may also harm nearby plants, gardens, and shrubbery.
Black walnuts (Juglans niagra) are wonderful shade trees. They can grow up to 100 feet high. Walnut trees produce food for humans and wildlife (squirrels, chipmunks, birds, and insects). These critters love them! But, in Ohio (and other states), these trees release a naturally toxic anti-fungal chemical substance called juglone. Nearby grass, gardens, and other plants can die from it. Gardening experts say that not all plants are sensitive to the chemical, however, shrubs, vines, ground covers, perennials, annuals, and gardens may be affected when embedded near black walnut trees.
The juglone chemical spreads itself in all parts of black walnut trees, including; buds, leaves, roots, stems, nuts, and hulls. The severity of the damage varies, but the toxic substance can mostly affect other trees through root contact, falling and decayed plants in the soil, and rainfall (foliage may drip juglone onto vegetation).
Plants, gardens, shrubbery and other greenery that live underneath a mature walnut tree’s canopy—about 50 to 80 feet from the trunk—may be severely damaged and eventually die.
Just like people, plants need oxygen to survive. When various types of vegetation come in contact with juglone, these plants yellow and wilt because they cannot produce carbon dioxide and oxygen needed to breathe. Black walnut trees in Ohio (and other areas of the Midwest) can partially or completely kill off gardens—especially those growing tomatoes, eggplant, potatoes, and peppers. Flowers and ornamental plants are also sensitive to the effects of juglone’s toxicity.
Juglone causes what is known as “walnut wilt"—it may be the reason for limp and decaying greenery if your garden or landscaping is located near a black walnut tree. Yellowing, fading leaves and shriveling, discolored stems are indicators of juglone poisoning. When it comes to some plants, like tomatoes and potatoes, negative reactions can happen quickly and kill off the vegetation within a day or two. Shrubs and trees have similar symptoms – especially on new growth – but older leaves, stems, and branches may cause the death of the plants.
Black walnut tree roots exude the chemical of juglone into the soil; is it possible to control the substance so that it would not kill neighboring plants?
No, but, also … yes. Juglone cannot be controlled by sprays or antitoxins, say gardening experts from Ohioplants.org. The best thing for homeowners to do—avoid planting gardens near black walnut trees, and vice versa. Separating black walnut trees from other vegetation is the best way to keep the toxicity of juglone under control. Tomatoes, apples, pears, berries, potatoes, and various landscaping shrubbery are in danger of being poisoned, as well as rhododendrons, lilacs, and azaleas that are growing too close to tree roots.
Having black walnut trees in your yard does not mean you cannot have a vegetable garden, but you will want to be cautious about where you place it. Dig your garden 100 feet (or more) away from mature trees. The trees' toxic zones are typically within 50 or 60 feet of the trunks, but roots typically extend to 80 feet or longer.
Raised soil beds (in sunlit locations with proper pH balance) will help protect gardens in Ohio landscapes that have black walnut trees. Raised garden beds with amended soil will lessen opportunities for tree roots to grow toward the plot. Black walnut tree twigs, nuts, hulls, leaves, stems, and branches must be kept away from the garden. Remove black walnut trees seedlings as they sprout in unwanted areas.
When it comes to controlling the toxin, cutting the tree down won’t solve the problem. Juglone remains in the wood as the roots are decomposing; this could take five years or longer.
While this list is not a scientific determination, says K-State Research & Extension, these vegetable plants and fruit trees are known to thrive under black walnut trees:
Annuals: begonias, violets, morning glories, impatiens, pansies, marigolds and zinnias. Various shrubs, vines and trees as well as certain bulb flowers (like daffodils) can be planted near black walnut trees.
This vegetation is susceptible to the toxins of juglone:
Annuals, perennials and bulbs: petunias, coral bells (heuchera sp.), Chrysanthemum (morifolium) and Colorado Columbine Aquilegia (caerulea).
Because there are so many fruits, vegetables, vines, ground covers and shrubberies that can die from the toxic effects of juglone, the best advice for planting near black walnut trees is to consult with gardening experts or your county extension service. Keep in mind, however, that any lists available may not be all-inclusive or completely accurate because scientific outcomes do vary; some of these experiments are only based on observation of plants in specific environments.
Ohio's black walnut trees are prolific—they produce their fruits easily (and can sprout up in a number of surroundings). Before planting gardens and landscapes, make note of where the black walnut trees are growing in your yard.
Question: Could juglone be responsible for the death of Japanese Maple, Hydrangea petiolaris and Porcelain Vine?
Answer: Depending on where these trees are planted, the proximity could affect -- although the trees you've noted are not known to become victims. Other factors could play a part in the death of the trees, such as disease, insect infestations, soil, etc.
© 2017 Teri Silver
Teri Silver (author) from The Buckeye State on April 18, 2018:
Hi, April, products on the market are hit or miss. I cannot recommend any but in any case, you'll want to know the soil's pH before treating it. We have a few in our Ohio yard right by the gardens and have not had trouble (yet?). From what I've learned in my research -- the soil is basically "damaged" with these trees around. You can remove the trees but old roots, leaves and compost materials are embedded into the ground. Try taking a soil sample to a testing lab (or test it yourself for alkalinity) and consult the county extension service in the area where you live in Wisconsin for any recommend products that fit your trees' environment. Our trees are so prolific; we can't get rid of them! They grow out of brush piles. Well, the squirrels like them ... ts
april zone5a on April 18, 2018:
I live in Wisconsin, I have a problem with black walnut trees in my yard... sadly I can't move my garden anymore else (My Grandpa planted 2 trees a long time ago, and now we have about 4 in our 2 acre yard)
I remember my Grandma having a garden and I thought it was did good...even under a b.w.t... Is there anything I can put in the garden to help counteract juglone, even a little? like composted manure? (I know nothing will completely counteract it)
Alexander James Guckenberger from Maryland, United States of America on December 15, 2017:
Wow, I was not aware of juglone until just now. :o
Teri Silver (author) from The Buckeye State on November 11, 2017:
Linda Crampton from British Columbia, Canada on November 10, 2017:
This is an interesting and useful article. Thanks for sharing the information about juglone and the helpful tips.