Black Walnut Tree Toxicity - What Plants Are Immune?

Walnut trees are fruitful and beautiful. I love to sit in the shade of one of my 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 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 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 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.
  • 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, pepper, potato, rhubarb, tomato.
  • 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; 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.

52 comments

  • John, growing the transplant near another black walnut should not be a problem.

    Suzanne
  • The squirrels planted a black walnut seed last fall (2019) and has grown to over 10 feet tall (9/7/2020). I want to transplant it to an area that I already have a dozen or so black walnut trees growing. My question is – will the juglone from the existing black walnut trees be harmful to the black walnut transplant?

    JOHN MANTARO
  • Bernadette,
    I was surprised to find a short dark green moss growing under my black walnut tree!
    Also it seems to be the only place on my crew and a half where it grows. It must like tannin

    Julia Burton
  • My herbs are not affected by black walnut tree near my garden but my tomatoes and squash died two seasons before I knew what it was. I can grow collard greens and cucumbers and jalepeno peppers and bell peppers, although they are on the small side. Not a very long list of things you can grow. I live in zone 7

    Shan Morgan
  • Has anyone intentionally grown moss under a black walnut? I would like to try a few species but don’t know if they are tolerant.

    Bernadette
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