Answer: Growing plants near black walnuts can be a challenging proposition because some plants are sensitive to the chemical substance ?juglone' produced by walnut roots. This potentially toxic chemical is found in all parts of the tree, although there are higher concentrations in the buds, hulls and roots. Contact with juglone can induce wilting and stunting, the severity of which depends upon the sensitivity
of the plant and its proximity to a walnut tree.
The tree's root zone, which extends outward each year, defines the general area in which juglone is concentrated. Juglone is not very water soluble, does not move far in the soil and is a stable compound which can persist for several years in the inner bark of the roots (something to keep in mind if you decide to remove the tree). Unlike its longevity in the soil, juglone in the leaves is reported to break down in about 4 weeks in a compost pile after being exposed to air, water and bacteria. However, caution would be prudent before using this material around certain plants.
Unfortunately, few plants have been systematically tested for their sensitivity to juglone. In general, information about whether plants are tolerant or sensitive to the chemical are primarily based on observations, often at an arboretum. To further complicate matters, not all reported observations are in agreement and some cultivars of tolerant species do poorly beneath walnuts. A potential explanation for these inconsistencies was found in a Purdue guidesheet on walnuts where they noted that "from observation of native stands of black walnut, decreased toxicity seems to be associated with excellent soil drainage, even among sensitive species."
There does seem to be a consensus that the following plants are sensitive: alfalfa, apple, asparagus, azalea, blackberry, blueberry, some chrysanthemums, lilac, peony, peppers, petunia, potato, rhubarb, silver maple and tomato.
Several plants that I found reported as being sensitive, such as hackberry, hydrangea and wild columbine have been growing for years in our yard within the drip line and sometimes near the trunk of walnut trees. Our well-drained soil may be the reason that these normally juglone-sensitive species are able to thrive under the walnuts. From personal observations in my yard I would add ninebark and New Jersey tea to the list of suspected sensitive species.
On the positive side, walnut trees provide not only delicious nuts but bright, dappled shade which is desirable for many plants, as well as enjoyed by people. Fortunately, plants which are known to tolerate being close to walnuts outnumber
the sensitive ones. Ephemeral spring wildflowers such as Dutchman's breeches, spring beauty, sweet William, dog tooth violet, toothwort, Jacob's ladder and blue-eyed Mary are some early season examples, as are crocus and daffodils. Ground
covers such as ajuga, sweet woodruff and wild ginger offer other landscape possibilities. Redbud, pawpaw, flowering dogwood, sweetgum, river birch plus several species of maple and oak trees have all been observed growing under the canopy of black walnut trees. Some herbaceous perennials tolerant of walnut are numerous ferns, wild geranium, coral bells, coreopsis, many of the hosta and iris species, daylilies and Jack-in-the-pulpit. Various grasses, especially Kentucky
bluegrass, and sedges are reported to do well under black walnut. Although not landscape plants, one list of tolerant plants even included dandelion and chickweed!
My interest in the horticultural phenomena of juglone toxicity has a personal twist to it. We have a number of large walnut trees in our yard and have found that it is possible to grow plants more interesting than grass near them. Additional plants that have been successful in our yard are buckeye trees, and the perennials: ?Luxuriant' bleeding heart (Dicentra eximia), liriope, grape hyacinth,
several coralbells cultivars, tulips, violets of various species, blue lobelia and cardinal flower. I discovered one disadvantage of growing hostas under walnuts - falling petioles can make holes in the larger leaved varieties.
As you can see, gardening is possible around walnuts; you just have to be a little more adventurous.
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