Black Walnut trees (Juglans nigra) are well-known for their majestic size and the rich chocolate-brown heartwood that makes them valuable in the high-end furniture market. They also produce very flavorful walnuts and can make a lovely addition to your landscape.
If you’re lucky enough to have these impressive trees on your property, you may find landscaping under and around them a bit of a challenge. Not all plants can thrive in the soil surrounding Black Walnut trees due to the biochemical substance they secrete called Juglone.
In this article, we explain Juglone toxicity and provide sound advice to help you create a healthy, happy landscape around your Black Walnut trees. Read on to learn more.
What You'll Learn Today
- Why Do Black Walnut Trees Produce Juglone?
- Eleven Pretty Perennials That Can Withstand Juglone
- Nineteen Juglone Tolerant Trees and Shrubs
- Absolutely Juglone Intolerant Plants to Avoid
- How Can You Tell If Your Plants Are Reacting Negatively To Black Walnut?
- Five Planting Tips For Healthy Gardening Around Black Walnut Trees
Why Do Black Walnut Trees Produce Juglone?
Juglone is a chemical produced by Black Walnut trees as a protective response to ensure their survival. Unfortunately, this compound can be harmful to many plant species.
Juglone is mostly leached from the trees’ roots into the soil, but it can also drip from the leaves onto the ground.
Contact with Juglone can cause what is known as allelopathy (i.e. “death to others). Luckily, not all plants are negatively impacted by Juglone.
What to Grow Under Black Walnut Trees
Most grasses and many plants are able to thrive under or near Black Walnut trees without succumbing to the effects of Juglone. Here are some options to consider:
Eleven Pretty Perennials That Can Withstand Juglone
- Solomon’s Seal (Polygonatum biflorum) – USDA Hardiness Zone: 3-8. This woodland perennial features arching stems and dangling, bell-shaped white flowers.
- Virginia Creeper (Parthenocissus quinquefolia) – USDA Hardiness Zone: 3-9. A deciduous vine known for its vibrant red fall foliage and five-leaflet leaves. Be aware, however, that this vine can and will completely cover trees if given half a chance. This has the effect of smothering and killing the trees.
- Morning Glory (Ipomoea purpurea) – USDA Hardiness Zone: Annual. These colorful, trumpet-shaped flowers bloom in the morning and come in various hues.
- Shasta Daisy (Leucanthemum x superbum) – USDA Hardiness Zone: 4-9. A classic perennial with white, daisy-like flowers and a bright yellow center.
- Wisteria (Wisteria spp.) – USDA Hardiness Zone: 4-9. A vigorous, climbing vine producing cascades of fragrant, pea-like flowers.
- Hostas (Hosta spp.) – USDA Hardiness Zone: 3-9. Shade-loving perennials with attractive foliage, available in various sizes and leaf colors.
- Aster (Aster spp.) – USDA Hardiness Zone: 3-8. These late-season bloomers offer a burst of color with their daisy-like flowers, coming in a range of shades.
- Vinca (Vinca minor) – USDA Hardiness Zone: 4-9. Also known as periwinkle, this ground cover features glossy leaves and violet-blue, star-shaped flowers.
- Phlox (Phlox paniculata) – USDA Hardiness Zone: 3-8. Tall spikes of fragrant, clustered flowers in vibrant colors make this a garden favorite.
- Ajuga (Ajuga reptans) – USDA Hardiness Zone: 3-10. Low-growing ground cover with colorful foliage and spikes of small, tubular flowers.
- Heucheras (Heuchera spp.) – USDA Hardiness Zone: 4-9. These perennial plants are valued for their vibrant foliage and delicate flowers. There are between 35 and 50 species of these US native plants presenting with various shades of leaves and topped with dainty blooms.
Shrubs to Plant Beneath Black Walnuts
Nineteen Juglone Tolerant Trees and Shrubs
- Viburnum (excluding maresii) – There are more than 150 species and numerous cultivars within the Viburnum genus that provide a great deal of choice in beautiful shrubs or small trees. These hardy plants have various growth habits and flower types. Hardiness zones vary by species. [https://hgic.clemson.edu/factsheet/Viburnum/]
- Flowering Dogwood (Cornus florida) – USDA Hardiness Zone: 5-9. This small tree is known for its showy, pink or white bracts in spring and colorful foliage in the fall.
- American Beech (Fagus grandifolia) – USDA Hardiness Zone: 3-9. This is a stately deciduous tree with smooth gray bark and oval-shaped leaves.
- Eastern Redbud (Cercis canadensis) – USDA Hardiness Zone: 4-9. This small, ornamental tree is famous for its profusion of pink or lavender flowers in early spring.
- Japanese Maple (Acer palmatum) – USDA Hardiness Zone: 5-8. This is a small tree or shrub renowned for its stunning, delicate foliage in various colors.
- Black Cherry (Prunus serotina) – USDA Hardiness Zone: 3-9. This US native tree produces fragrant white flowers and edible cherries.
- Rosa Rugosa (Rosa rugosa) – USDA Hardiness Zone: 2-7. This hardy, low-maintenance rose species has wrinkled leaves and fragrant blooms.
- Arborvitae (Thuja occidentalis) – USDA Hardiness Zone: 2-7. These evergreen trees or shrubs are popular for their dense, pyramidal growth.
- White Ash (Fraxinus americana) – USDA Hardiness Zone: 3-9. This tall, deciduous tree has compound leaves and attractive fall color.
- Red Maple (Acer rubrum) – USDA Hardiness Zone: 3-9. This versatile tree is well-known for its gorgeous red foliage in the autumn.
- Sycamore (Platanus occidentalis) – USDA Hardiness Zone: 4-9. This large deciduous tree has palmate leaves and very distinctive bark.
- Tulip Tree (Liriodendron tulipifera) – USDA Hardiness Zone: 4-9. This tall and stately tree bears tulip-shaped flowers as the common name suggests.
- Euonymus (Euonymus spp.) – USDA Hardiness Zone: Varies by species. Includes shrubs and vines with colorful foliage.
- Forsythia (Forsythia spp.) – USDA Hardiness Zone: 4-9. These early spring bloomers bear cascades of bright yellow flowers.
- Hibiscus (Hibiscus spp.) – USDA Hardiness Zone: Varies by species. Both hardy and tropical varieties are Juglone tolerant. There are nearly 300 member of this genus (Malvaceae). All produce masses of showy flowers. [https://extension.umn.edu/houseplants/hibiscus]
- Catalpa (Catalpa spp.) – USDA Hardiness Zone: 4-9. Large deciduous trees with distinctive, broad leaves and showy flowers which transition into long, dangling beans.
- Oaks (Quercus spp.) – USDA Hardiness Zone: Varies by species. Diverse group of trees with significant ecological importance.
- Privet (Ligustrum spp.) – USDA Hardiness Zone: Varies by species. This group includes both evergreen and deciduous shrubs which are often used for hedges.
- Sumac (Rhus spp.) – USDA Hardiness Zone: Varies by species. These trees are well-known for their compound leaves and red fruit clusters.
Black Walnut (Juglone) Tolerant Evergreens
For an even more comprehensive list of trees, shrubs, vegetables, vines, and flowers that can thrive under and around black walnut trees, you can refer to the detailed information provided by PennState Extension.
Absolutely Juglone Intolerant Plants to Avoid
On the flip side, some plants, trees and shrubs should be avoided entirely when planting near Black Walnut trees, as they are highly sensitive to Juglone. These include:
- Chrysanthemum (Chrysanthemum spp.)
- Tomato Plants (Solanum lycopersicum)
- Rhododendron (Rhododendron spp.)
- Mountain Laurels (Kalmia latifolia)
- White Birch (Betula papyrifera)
- Blackberries (Rubus fruticosus)
- Hydrangeas (Hydrangea spp.)
- Azaleas (Rhododendron spp.)
- Blueberries (Vaccinium spp.)
- Apples (Malus domestica)
- Crocus (Crocus spp.)
- Lilacs (Syringa spp.)
How Can You Tell If Your Plants Are Reacting Negatively To Black Walnut?
Plants experiencing Juglone sensitivity exhibit wilted, yellowed leaves that that occur rapidly. This is especially true when the weather is hot and dry.
It’s easy to confuse drought stress with Juglone sensitivity, but just remember that drought stress takes place slowly when the soil is dry. Juglone sensitivity manifests rapidly, even when the soil is moist.
Black Walnut – Designing Around It
Five Planting Tips For Healthy Gardening Around Black Walnut Trees
Keep in mind that when planting under or near Black Walnut trees, choosing Juglone tolerant plants is just one step toward success.
It’s also essential to consider factors beyond Juglone, such as soil, moisture, temperature, shade, and sunlight. Here are some general tips for successful plantings:
- Create raised beds to reduce root contact, using weed fabric and new topsoil.
- Prevent leaves, hulls, and stems from decomposing near planting areas.
- Avoid mulch containing walnut bark, wood, hulls, and leaves.
- Improve soil drainage with organic matter amendment.
- Locate gardens well away from Black Walnut trees.
Remember to choose the right plants and follow the advice and tips presented here to create a thriving garden under the majestic canopy of your Black Walnut trees.