Landscaping In Spite of Black Walnuts

If you’ve ever tried to grow tomatoes near black walnut (Juglans nigra), then you know first hand the devastating effect the chemical juglone can have on certain plants. Both black walnut and butternut (J.cinerea) produce juglone in sufficient amounts to cause wilting and yellowing of leaves, and sometimes the death of the entire plant, in susceptible plants. Other trees that produce this chemical in relatively small amounts include English walnut, pecan, shagbark hickory, and bitternut hickory.

Juglone is released from the roots and also occurs in the leaves, nut hulls, bark and wood of walnut, but in lower concentrations than in the roots. Because juglone is not very soluble in water it does not spread very far in the soil. Its effect is worst at the edge of the tree’s crown, or dripline. However, since the roots may stretch beyond the dripline, in general, the toxic zone around a mature walnut tree is within 50-60 feet of its trunk. Juglone is also in the leaves and nut husks that drop to the ground. However, the toxin breaks down when exposed to air, water and bacteria, so these materials no longer pose a threat when completely composted.

Symptoms of juglone toxicity vary depending on the plant, and sometimes mimic diseases or physiological disorders. Affected plants often wilt in a relatively short time after contact with black walnut roots, even when the soil is moist. The entire plant may wilt, or only a part. Later as the wilting becomes more severe, the leaves start to brown and the plant dies. Tomato leaves begin yellowing, as well as twisting and puckering. The internal stem tissue may also become darkened or discolored.

Plant Sensitivity

The following plants are very vulnerable to juglone and should be grown away from a black walnut tree. These are some of the more common species:

Sensitive Plants
Flowers Fruits Shrubs Trees Vegetables
blue false indigo (Baptisia)
columbine
daffodil (some)
lily
peony (some)
petunia
apple
blackberry
blueberry
grape
pear
strawberry
azalea
lilac
potentilla
rhododendron
viburnum
basswood
birch
hackberry
pine
silver maple
saucer magnolia
asparagus
eggplant
pepper
potato
rhubarb
tomato

Fortunately, some plants are much more tolerant of juglone. This is just a partial list:

Tolerant Plants
Flowers Fruits Shrubs Trees Vegetables
astilbe
chrysanthemum (most)
crocus
daylily
hosta
iris
ferns
Monarda (bee balm, bergamot)
pansy
phlox
pulmonaria
sedum (stonecrop)
spiderwort (Tradescantia)
black raspberry
cherry
wild grape
arborvitae
clematis
euonymus
forsythia
honeysuckle
juniper
Rose of Sharon
Virginia creeper
Canadian hemlock
elm
hawthorn
hickory
most maples
oak
red cedar
redbud
beet
carrot
corn
lima bean
melon
onion
snap bean
squash

Overcoming the Problem

  • Of course, the best way to prevent this problem is not to plant a black walnut tree in your yard, particularly on a small lot where tree roots will spread over most of the yard.
  • If you already have a black walnut tree on your property, grow sensitive plants in another part of your yard, well away from the tree.
  • Barriers to prevent walnut roots from advancing into garden areas may be helpful.
  • Because walnut roots often do not occupy the surface layers of the soil, many shallow rooted plants can grow under the trees without contacting the roots and therefore are not significantly affected.
  • Pull out any volunteer walnut seedlings.
  • Don’t use leaves, bark or wood chips of black walnut to mulch plants, even though they have not shown to have the same toxic effects on plant growth as root contact produces.
  • If you already have affected plants under a walnut tree, the toxic effect might partly offset by liberal applications of nitrogen.
  • Cutting down the tree won’t solve the problem for a while because juglone can persist in the wood until the roots are decomposed, which can take five or more years.

– Susan Mahr, University of Wisconsin


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4 comments

  1. Very interesting article. We have some land with many black walnut trees. I now know why the apple trees did not survive. Glad to see that there are many plants that I love that can tolerate the toxic juglone. Thanks

  2. I have been asked this question a few times over the years, what can I plant near my walnut trees? After reading this, I now have answers! Actually, the choices are quite numerous!

  3. Didn’t realize the toxic zone for roots is that extensive, 50 – 60 feet, or that the wood remains toxic for five or more years after cutting. Nice to know some shallow root plants are tolerant of juglone.

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