Question: My wife and son planted three tomato plants under our
walnut tree this summer. As they grew, then withered, I remembered
that walnuts and tomatoes aren't compatible. We did get one tomato,
about the size of a walnut. Are tomatoes that grow beneath a walnut
tree safe to eat? Will they taste like that black yucky stuff that
gets on your hands from walnuts?
Also, my wife now wants me to cut down a second small walnut tree
growing amid cedars and bunch of other trees in the back of our
wooded lot. She's tried to plant ferns and other shade-loving things
beneath it but nothing's ever grown well beneath it. Anyway, when
I noted the tomato-walnut tree problem, she immediately deduced
that that's the problem with the things she's tried to grow underneath
this other walnut tree. I'd rather not cut it down. Can the walnut
tree toxin affect these things too?
Answer: Plants adversely affected by being grown near black walnut
trees have foliar yellowing, wilting and eventually death. Tomatoes
are particularly susceptible. The causal agent is a chemical called
juglone, which occurs naturally in all parts of the black walnut.
The largest concentrations of are in the walnut's buds, nut hulls,
and roots. However, leaves and stems do contain a smaller quantity.
Juglone is only poorly soluble in water and thus does not move
very far in the soil or into the plant. The flavor of your tomatoes
is not likely to be affected as much as their existence!
Since small amounts of juglone are released by live roots, particularly
juglone-sensitive plants may show toxicity symptoms anywhere within
the area of root growth of a black walnut tree. However, greater
quantities of juglone are generally present in the area immediately
under the canopy of a black walnut tree, due to greater root density
and the accumulation of juglone from decaying leaves and nut hulls.
This distribution of juglone means that some sensitive plants may
tolerate the amount of juglone present in the soil near a black
walnut tree, but may not survive directly under its canopy. Alternatively,
highly sensitive plants may not tolerate even the small concentration
of juglone beyond the canopy spread. Because decaying roots still
release juglone , toxicity can persist for some years after you
remove your tree.
Species survival near or under black walnut trees is further complicated
by the fact that the amount of juglone present in the soil depends
on soil type, drainage and soil micro-organisms. Competition for
light and moisture under the canopy also greatly affects which
species survive where. I can grow columbine, often listed as juglone-sensitive,
on my glacial outwash under a walnut, but I still don't appreciate
being pelted with walnuts in the fall!
There are lists of juglone-sensitive and juglone-tolerant
plants in the Purdue Extension publication "Black Walnut Toxicity" (HO-193-W).
Ask Purdue Extension office in your county for a copy or find it
on-line at http://www.hort.purdue.edu/ext/HO-193.pdf . See the
links at the end of the publication for more plant lists. Following
is a list of plants you might try growing under walnuts. If you
see them decline, move them or lose them!
Landscape plants: arborvitae, autumn olive, red cedar, catalpa,
clematis, crabapple, daphne , elm, euonymous , forsythia, hawthorn,
hemlock, hickory, honeysuckle, junipers, black locust, Japanese
maple, maple (most), oak, pachysandra, pawpaw, persimmon, redbud,
rose of sharon , wild rose, sycamore, viburnum (most), Virginia
Flowers and herbaceous plants: astilbe , bee balm, begonia, bellflower,
bergamot, bloodroot, Kentucky bluegrass, Spanish bluebell, Virginia
bluebell, bugleweed, chrysanthemum (some), coral bells, cranesbill
, crocus, Shasta daisy, daylily, Dutchman's breeches, ferns, wild
ginger, glory-of-the-snow, grape-hyacinth, grasses (most), orange
hawkweed, herb Robert, hollyhock, hosta (many), hyacinth, Siberian
iris, Jack-in-the-pulpit, Jacob's ladder, Jerusalem artichoke,
lamb's-ear, leopard's-bane, lungwort, mayapple , merrybells , morning
glory, narcissus (some), pansy, peony (some), phlox, poison ivy,
pot marigold, polyanthus primrose, snowdrop, Solomon's-seal, spiderwort,
spring beauty, Siberian squill , stonecrop, sundrop , sweet Cicely,
sweet woodruff, trillium, tulip, violet, Virginia waterleaf, winter
-- Beverly Shaw, Advanced Master Gardener, Purdue
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for the state of Indiana.
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