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Black Walnut Tree Toxicity - What Plants Are Immune?

Walnut trees (walnut juglans or Black walnut juglans nigra) are fruitful and beautiful. I love to sit in the shade of one of my English walnut trees and look out over the sloping garden. In our video Tricia shows you how to care for and prune walnut trees. walnut orchard

Walnut trees are native to North America. They are large and can grow to over 100 feet tall and are great shade trees. These trees definitely like their own space, and can be bad neighbors to certain plants. Find out the best companion plants for walnuts.

Black walnut tree toxicity

Black walnut trees load their roots, buds, and nut hulls (covering walnut fruits) with the juglone toxin (leaves and stems have smaller amounts of juglone). The toxin seeps into the soil and susceptible companion plants will turn yellow, wilt, and sometimes die. But wait, you say, I don't have a black walnut tree. Actually, most walnut trees are grown on black walnut rootstock these days, therefore the walnut root system is likely to be rich in juglone. The soil under the canopy of the tree will have the highest concentration of juglone due to the combined effects of the roots, along with fallen leaves, hulls, and shells that are lying on the ground. Picking up this litter is good "orchard sanitation" for many Integrated Pest Management reasons, including decreasing the amount of juglone.

Companion plants for black walnut trees

  • Purdue University has informal lists of plants that tolerate juglone and those that are sensitive to it. Choose from the following list for best results in planting near black walnut trees or walnut trees grown on black walnut rootstock. Follow these guidelines for planting within the dripline of the tree and, according to the University of Wisconsin, up to 50'-80' from the trunk. Naturally you need to consider the sun and shade requirements of the plants, as well. Another source for tolerant and sensitive plants is K-State Master Gardener
  • Vegetables: lima bean; snap bean; beet; carrot; corn; melon; onion; parsnip; squash.
  • Fruits: black raspberry, cherry.
  • Landscape plants: arborvitae; autumn olive; red cedar; catalpa; clematis; crabapple; daphne; elm; euonymous (burning bush); 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 creeper.
  • Flowers and herbaceous plants: astilbe; bee balm; begonia; bellflower; bergamot; bloodroot; Kentucky bluegrass; Spanish bluebell; Virginia bluebell; bugleweed; chrysanthemum (some); coral bells; cranesbill geranium; crocus; Shasta daisy; daylily; Dutchman’s breeches; ferns; wild ginger; glory-of-the-snow; muscari (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 aconite; zinnia.

Plants that are sensitive to black walnut tree toxicity

  • Vegetables: asparagus, cabbage, eggplant, peppers, potatoes, rhubarb, tomatoes.
  • Fruits: apple, blackberry, blueberry, pear.
  • Landscape plants: black alder; azalea; basswood; white birch; ornamental cherries; red chokeberry; hackberry; Amur honeysuckle; hydrangea; Japanese larch; lespedeza; lilac; saucer magnolia; silver maple; mountain laurel; pear; loblolly pine; mugo pine; red pine; scotch pine; white pine; potentilla; privet; rhododendron; Norway spruce; viburnum (few); yew.
  • Flowers and herbaceous plants: autumn crocus (Colchicum); blue wild indigo (Baptisia); chrysanthemum (some); columbine; hydrangea; lily; narcissus (some); peony (some); petunia; roses; tobacco.
  • Field crops: alfalfa; crimson clover; tobacco.
  • Tip: This does not mean you can't compost black walnut leaves. According to Ohio State University Extension, "walnut leaves can be composted because the toxin breaks down when exposed to air, water and bacteria. The toxic effect can be degraded in two to four weeks." If you do compost the leaves, put them in a separate compost pile and do not spread the resulting compost on plants that are on the sensitive list.
For more information: The go-to book for anyone growing nut and fruit trees in California is the UC Davis publication, The Home Orchard. Add some walnut trees to your property and create a beautiful landscape with these companion plants that tolerate toxicity.

67 comments

  • Dan, I don’t know of anything that will neutralize the toxicity from the juglone.

    Suzanne at GrowOrganic.com
  • Sue, I would not plant those near your black walnut trees (or walnut trees). They are sensitive to juglone and can be killed when planted near a black walnut tree.

    Suzanne at GrowOrganic.com
  • will japanese lilac trees grow near black walnut trees?

    Sue Spelhaug
  • The beautiful mountain laurel under my walnut tree is dying. Is there any way organically or chemically to neutralize the effects of the walnut’s toxins on the laurel?
    Thank you.
    Dan

    Dan Forsberg
  • Kevin, I would not plant grapes near a black walnut tree. In addition to being sensitive to juglone, grapes need full sun.

    Suzanne at GrowOrganic.com
  • I was wondering about grapes in proximity to back walnut.

    Kevin Harmon
  • Siacr8, the highest amount of the juglone is in the roots, nut hulls, and smaller branches. There is less amount in the leaves and actual bark. Plus the compost has been cooking for 4 years. I think you are ok. Just do not put any old roots, nut hulls and such as mentioned into your pile.

    Suzanne at GrowOrganic.com
  • Mixed some old composted woodchips and bark from various trees from a pile that was at least 4 years old..as im sifting after mixing it into my yard compost i am finding old whole black walnuts..most chips turn to dirt/dust when squeezed..but ive collected this added material from the bottom of the piles..and raspberries and most vegetables are already in a diluted mix of this…have i killed my tomatoes and raspberries? Too much conflicting info available..confused..should i go for it or abort while its early? Im in mendocino county in northern california.

    Siacr8
  • Victoria, What I have read is that the roots can exude juglone as they decay for several years. So if you want to plant right away, you should go with plants that are not sensitive to the juglone. The other option is to wait for a couple of years for the roots to decay.

    Suzanne at GrowOrganic.com
  • Victoria, What I have read is that the roots can exude juglone as they decay for several years. So if you want to plant right away, you should go with plants that are not sensitive to the juglone. The other option is to wait for a couple of years for the roots to decay.

    Suzanne at GrowOrganic.com

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