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Black Walnut Tree Toxicity - What Plants Are Immune?

By on January 24, 2013

Glory of the Snow (Chionodoxa forbesii) is a fall-planted bulb that grows happily under any walnut tree.

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” or 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.

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