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 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.

What Makes Black Walnut Trees Unique?

Black walnut trees (Juglans nigra) are unique not only for their striking appearance and valuable wood but also for their toxicity to plants. These trees produce juglone, a natural compound that can adversely affect many species, making them sensitive or even toxic to plants such as viburnum (Viburnum spp.), flowering dogwood (Cornus florida), and various garden plants, including lilacs (Syringa spp.) and azaleas (Rhododendron spp.).

While some plants, like sunflowers (Helianthus spp.) and hostas (Hosta spp.), are tolerant of black walnut roots and fallen walnuts, others, including anemones (Anemone spp.) and chrysanthemums (Chrysanthemum spp.), are considered highly sensitive and can struggle to thrive in their presence.

The Importance of Understanding Plant Compatibility

Understanding plant compatibility is crucial for successful gardening, especially when dealing with species like black walnut trees (Juglans nigra), known for their toxicity. Many plants, including viburnum and flowering dogwood, may be sensitive to juglone, a compound produced by black walnuts that can inhibit the growth of sensitive species like anemones and chrysanthemums.

By selecting plants tolerant of walnut toxicity, such as elderberries (Sambucus spp.) and certain maples (Acer spp.), gardeners can create healthier ecosystems and avoid the pitfalls of planting incompatible species.

Understanding Black Walnut Tree Toxicity

What is Juglone?

Understanding black walnut tree toxicity is essential for gardeners, particularly regarding the role of juglone, a compound produced by black walnuts (Juglans nigra). This juglone can negatively impact surrounding plants, leading to black walnut toxicity to plants such as viburnum shrub species and sensitive plants like anemone spp. and chrysanthemum spp.

While some plants are tolerant of juglone, many others, including flowering dogwood and wild grape, are highly sensitive and can suffer when planted near walnut trees, making knowledge of walnut toxicity crucial for healthy garden planning.

Black Walnut Tree Toxicity Range

The black walnut tree (Juglans nigra) is known for its significant toxicity to plants, primarily due to the compound juglone produced by its roots and fallen walnuts. The toxicity range of black walnuts can extend up to 80 feet from the tree, affecting sensitive plants such as viburnum (Viburnum spp.), chrysanthemums (Chrysanthemum spp.), and azaleas (Rhododendron spp.). In contrast, some plants, like hostas (Hosta spp.) and sunflowers (Helianthus spp.), can tolerate juglone.

Additionally, studies suggest that juglone may persist in the soil, potentially impacting plant species in the surrounding area long after the walnut tree has been removed. This can affect other plants, such as tomatoes (Solanum lycopersicum), lilacs (Syringa spp.), and certain ferns (Polystichum spp.), making it crucial to consider walnut toxicity when planning garden layouts.

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.

Signs of Juglone Toxicity in Plants

Common Symptoms of Juglone Poisoning

Signs of juglone toxicity in plants can manifest as wilting, yellowing leaves, and stunted growth, particularly in sensitive species such as viburnum (Viburnum spp.) and forsythia (Forsythia spp.). Affected plants like narcissus (Narcissus spp.), primula (Primula spp.), and wisteria (Wisteria spp.) often show these symptoms when grown near black walnut trees (Juglans nigra), which produce juglone through their roots and fallen nuts.

Additionally, species such as tomatoes (Solanum lycopersicum), azaleas (Rhododendron spp.), and lilacs (Syringa spp.) can also suffer from juglone toxicity.

How to Confirm Juglone Toxicity in Your Garden

To confirm juglone toxicity in your garden, look for signs such as wilting, yellowing, and stunted growth in affected plants like viburnum and forsythia. Testing the soil around black walnut trees (Juglans nigra), where walnut roots and nut hulls are present, can help identify juglone.

If sensitive species such as trillium, lobelia, and narcissus show poor health near these trees, it’s likely a result of black walnut toxicity affecting plant species that cannot tolerate 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 Penn State.
• 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.

How to Design a Garden Around Black Walnut Trees

Strategic Garden Planning in Juglone Zones

Designing a garden around black walnut trees (Juglans nigra) requires strategic planning to account for black walnut toxicity, as these trees produce juglone that can adversely affect many plants. To create a thriving garden, incorporate plant species that are tolerant of juglone, such as sunflowers (Helianthus spp.) and coral bells (Heuchera spp.), while avoiding highly sensitive plants like wisteria (Wisteria spp.), narcissus (Narcissus spp.), and trillium (Trillium spp.).

Position more resilient plants, like viburnum (Viburnum spp.) and flowering dogwood (Cornus florida), at a safe distance from walnut roots and fallen walnuts. Additional juglone-tolerant options include elderberry (Sambucus spp.), oak (Quercus spp.), and certain varieties of pine (Pinus spp.), ensuring that your garden remains vibrant and healthy despite the presence of juglone.

Creating Barriers and Safe Zones

To design a garden around black walnut trees (Juglans nigra), creating barriers such as raised beds and containers can effectively protect sensitive plants from black walnut toxicity. By isolating vulnerable species like viburnum (Viburnum spp.) and narcissus (Narcissus spp.) in raised beds, you can minimize their exposure to juglone produced by walnut roots and fallen walnuts.

Additionally, incorporating mulching and soil amendments with materials like wood chips can help reduce juglone impact, allowing you to cultivate a variety of plant species, including juglone-tolerant options like sunflowers (Helianthus spp.), coral bells (Heuchera spp.), and elderberry (Sambucus spp.). This approach ensures that affected plants, such as rhododendrons (Rhododendron spp.) and ferns (Polystichum spp.), thrive in a safe environment.

Conclusion

In conclusion, successfully gardening around black walnut trees requires careful planning and understanding of the unique challenges posed by juglone toxicity.

By selecting juglone-tolerant plants and utilizing strategies such as raised beds and mulching, gardeners can create vibrant and healthy ecosystems even in the presence of these majestic trees.

With informed choices, it’s possible to enjoy the beauty of black walnuts while maintaining a flourishing garden. To explore a variety of walnut trees for your landscape, visit Grow Organic Walnut Trees.

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.

FAQs About Black Walnut Tree Toxicity: What Plants Are Immune?

• What makes a plant tolerant to Black Walnut tree toxicity?

  • A plant tolerant to Black Walnut toxicity can thrive near the tree because it is not affected by the juglone chemical produced by the walnut family. These plants have developed resistance to juglone, allowing them to grow despite the tree’s toxic influence.

• Which plants are sensitive to Black Walnut tree toxicity?

  • Plants sensitive to juglone are prone to wilting, stunted growth, or even death when planted near Black Walnut trees. Susceptible plants, like tulipa spp, ribes spp, and viola spp, are more vulnerable to the toxic effects of juglone. Susceptible plants include:

    • Tulipa spp. (tulips)

    • Ribes spp. (currants and gooseberries)

    • Viola spp. (violets and pansies)

• Are there specific shrubs or trees that are tolerant to juglone?

  • Yes, several shrubs and trees are juglone-tolerant, such as:

    • Forsythia spp. (forsythia)

    • Juniperus spp. (junipers)

    • Philadelphus spp. (mock orange)

    • Carya spp. (hickories, particularly shagbark hickory)

    • Eastern Red Cedar (Juniperus virginiana)

• Which other plants can grow near Black Walnut trees without issues?

  • Many plants are immune to juglone, including:

    • Hemerocallis spp. (daylilies)

    • Epimedium spp. (barrenwort)

    • Astilbe spp. (astilbe)

    • Pulmonaria spp. (lungwort)

    • Echinacea purpurea (purple coneflower)

    • Monarda spp. (bee balm)

• Are there any specific flowers that can resist Black Walnut toxicity?

  • Yes, certain flowers resistant to juglone include:

    • Thalictrum spp. (meadow rue)

    • Helleborus spp. (hellebores)

    • Oenothera spp. (evening primrose)

    • Doronicum spp. (leopard’s bane)

    • Erythronium spp. (dog's tooth violet)

• Can ferns grow near Black Walnut trees?

  • Yes, some ferns are juglone-tolerant. Suitable options include:

    • Wood Fern (Dryopteris spp.)

    • Cinnamon Fern (Osmunda cinnamomea)

• What about ornamental plants like chionanthus spp and achillea spp?

  • Both chionanthus spp (fringe tree) and achillea spp (yarrow) are juglone-tolerant ornamental plants, making them suitable for gardens near Black Walnut trees.

• Is rubus occidentalis affected by Black Walnut toxicity?

  • Rubus occidentalis (black raspberry) is generally tolerant of juglone, making it a good option for fruit-bearing plants near Black Walnut trees.

• Are there any native plants that can grow in Black Walnut’s root zone?

  • Yes, native species like:

    • Staghorn Sumac (Rhus typhina)

    • Echinacea purpurea (purple coneflower)
      can grow near Black Walnut trees without adverse effects.

• Does the Black Walnut tree produce chemicals that affect all plants in the area?

  • The Black Walnut tree produces juglone, a chemical toxic to certain plants but not to all. While some plants are highly susceptible to it, many others, such as those mentioned above, can tolerate or resist juglone's effects.

• Are all trees in the walnut family equally toxic to plants?

  • While all trees in the walnut family, including Black Walnut (Juglans nigra) and carya spp (hickory), produce juglone, Black Walnut has the highest concentration of this compound. However, shagbark hickory is generally less toxic to surrounding plants.

• Can I plant juniperus spp (juniper) near a Black Walnut tree?

  • Yes, juniperus spp is juglone-tolerant and can be safely planted near a Black Walnut tree without suffering any negative effects from the tree’s toxicity.

• What are some good ground covers that are juglone-tolerant?

  • Juglone-tolerant ground covers include:

    • Epimedium spp. (barrenwort)

    • Pulmonaria spp. (lungwort)

    • Vinca minor (periwinkle)

• How do forsythia spp and hemerocallis spp perform near Black Walnut trees?

  • Both forsythia spp (forsythia) and hemerocallis spp (daylily) are juglone-tolerant, making them excellent choices for planting near or under Black Walnut trees.

• Can philadelphus spp grow under Black Walnut trees?

  • Yes, philadelphus spp (mock orange) is resistant to juglone, allowing it to thrive near Black Walnut trees.

• Are erythronium spp and thalictrum spp sensitive to juglone?

  • No, both erythronium spp (dog's tooth violet) and thalictrum spp (meadow rue) are juglone-tolerant and can be planted near Black Walnut trees.

• Can I grow astilbe spp under a Black Walnut tree?

  • Yes, astilbe spp is juglone-resistant and will perform well when planted near Black Walnut trees.

• Is it safe to grow ribes spp near Black Walnut trees?

  • No, ribes spp (currants and gooseberries) are generally sensitive to juglone and may struggle to grow in close proximity to Black Walnut trees.

• What should I consider when planting near Black Walnut trees?

  • When planting near Black Walnut trees, prioritize juglone-tolerant species. Be mindful of the tree’s root zone, and avoid growing susceptible plants like tulipa spp, ribes spp, and viola spp directly under or near the tree.

• Can chionanthus spp and pulmonaria spp grow in Black Walnut-affected areas?

  • Yes, both chionanthus spp (fringe tree) and pulmonaria spp (lungwort) are juglone-tolerant plants that can thrive in areas affected by Black Walnut trees.