What Not to Plant Near a Black Walnut Tree: A Guide to Choosing Black Walnut Companion Plants

Understanding What Not to Plant Near a Black Walnut Tree and Finding Black Walnut Companion Plants

Walnut trees (Juglans or Black Walnut Juglans nigra) are both fruitful and beautiful. I love sitting in the shade of my English walnut trees, overlooking the sloping garden. However, when selecting walnut tree companion plants, it’s important to consider what not to plant near a black walnut tree due to the toxin juglone it releases.

In our video, Tricia shows you how to care for and prune walnut trees while also highlighting black walnut companion plants and plants tolerant of juglone that thrive around them.

Walnut trees, native to North America, can grow over 100 feet tall and provide excellent shade. However, they require plenty of space and can be tough neighbors to certain plants due to their allelopathic properties. If you're in the United States and interested in nut production, it’s important to choose companion plants that thrive alongside walnuts. Female flowers of walnut trees, which produce the nuts, need specific conditions for successful pollination. Be sure to select plants that won't be negatively affected by the tree’s unique characteristics.

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 with the juglone toxin, which can affect companion plants like red maples, causing yellowing, wilting, and even death. While you might not have a black walnut tree, keep in mind that many walnut trees are now grown on black walnut rootstock, meaning their root systems can still be rich in juglone. The soil under the tree canopy will have the highest concentration of this toxin, due to the combined effects of roots, fallen leaves, hulls, and shells.

Additionally, black walnut grows can be difficult to transplant due to the juglone in the soil. This makes proper orchard sanitation crucial, as removing fallen debris helps reduce juglone levels and prevents issues like fall webworm (Hyphantria cunea) infestations and the spread of thousand cankers disease. While the black walnut's edible nut is highly valued, managing these challenges is key to maintaining a healthy environment for both the tree and its surroundings.

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. It also helps manage pests like fall webworm Hyphantria cunea, which can harm your plants if left unchecked.

Conclusion

In conclusion, gardening around black walnut trees requires careful planning and an understanding of juglone toxicity. By selecting walnut tree companion plants and plants tolerant of juglone, gardeners can thrive even near these majestic trees. It's important to know what not to plant near a black walnut tree to avoid damage, and choosing the right black walnut companion plants ensures a vibrant, thriving garden.

With informed choices and strategies like raised beds and mulching, it's possible to enjoy the beauty of black walnuts while maintaining a flourishing garden. And for a fun twist, why not enjoy some ice cream while you plan your landscape? To explore a variety of walnut trees for your garden, visit walnut trees for sale.

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 What Not to Plant Near a Black Walnut Tree: A Guide to Choosing Black Walnut Companion Plants

• What are walnut tree companion plants?
  • Answer: Walnut tree companion plants are species that can grow well near black walnut trees without being harmed by juglone, a toxic compound produced by the tree's roots. These plants are tolerant of juglone and can thrive in the presence of walnut trees, creating a harmonious garden environment.
• What not to plant near a black walnut tree?
  • Answer: It’s important to avoid planting certain species near a black walnut tree due to juglone toxicity. Some plants, such as tomatoes, potatoes, and certain types of roses, are sensitive to juglone and can be harmed by the tree’s roots. Understanding what not to plant near a black walnut tree helps prevent damage to your garden.
• What are some good black walnut companion plants?
  • Answer: Black walnut companion plants include species that are tolerant of juglone and can thrive under the canopy of walnut trees. Examples include plants like ferns, certain types of grasses, and some wildflowers. These plants are well-suited to grow in areas where juglone toxicity is present, ensuring a healthy garden.
• What are plants tolerant of juglone?
  • Answer: Plants tolerant of juglone are those that can grow near black walnut trees without suffering from the toxicity of juglone. Some examples include hostas, azaleas, and certain groundcovers like creeping thyme. Choosing plants that tolerate juglone ensures the health and vitality of your garden around walnut trees.
• How do I choose the right walnut tree companion plants?
  • Answer: When selecting walnut tree companion plants, it’s crucial to look for species that are resistant to juglone toxicity. These plants are well-suited to growing near black walnut trees and will thrive without being harmed by the tree's natural chemicals. Opt for plants like ferns, certain types of perennial herbs, and other juglone-tolerant varieties.
• Can I plant vegetables near a black walnut tree?
  • Answer: Vegetables are typically not recommended near black walnut trees because many are sensitive to juglone. It's important to know what not to plant near a black walnut tree, such as tomatoes, potatoes, and peppers, which are highly vulnerable to the toxin. Instead, opt for black walnut companion plants like certain herbs or ornamental plants that can tolerate juglone.