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The greatest concentration of juglone and hydroxyjuglone (a nontoxic, colorless precursor that is converted into the toxic form juglone by sensitive plants and through oxidation) is found in the vegetative buds, leaves, stems, nut hulls, and roots of the plants. Black walnut (Juglans nigra) and butternut (Juglans cinerea) are the landscape plants most recognized by gardeners as being problems for their other plants. However, English or Persian walnut (J. regia) and hickories (Carya) also produce juglone but to a lesser degree.
The production of juglone is a protective response by the plant to assure its survival. Many plants (e.g., sugar maple, tree of heaven, hackberries, sycamore, cottonwood, black cherry, red oak, black locust, sassafras, fine fescue, and American elm) produce allelochemicals to enhance their survival and reproduction by inhibiting nearby competition. The most common symptoms of juglone sensitivity in landscape and garden plants is the yellowing and wilting of leaves, especially during the hot dry periods during the growing season, ultimately resulting in wilting and death of the plant.
Juglone-induced wilting and wilting due to water stress are often confused. Wilting due to lack of water occurs slowly and can be reversed with watering. Juglone-induced wilting often occurs rapidly even when ample soil moisture is present. Juglone-induced wilting may be partial or may encompass the whole plant. Early wilting symptoms may also be reduced with supplemental water. Later in the season wilting does not respond to additional water, leaves start to brown, and the plant dies. Experimental studies have shown that juglone inhibits plant respiration, depriving sensitive plants of needed energy and cell division as well as water and nutrient uptake.
Sensitive plants located beneath the canopy of a walnut tree are most susceptible to contact with juglone through direct root contact or accumulation of the toxin from leaves and nut hulls in poorly aerated, wet soils with limited microbial activity and organic matter. Juglone is poorly soluble in water and does not move very far in the soil. Well-drained and aerated soils with a healthy population of microbes can accelerate the metabolic decomposition of juglone. Where sensitive plants may survive outside of the canopy of a black walnut, highly sensitive plants may not tolerate small concentrations where decaying roots from a removed tree may still be releasing juglone. Juglone toxicity may persist for years after a tree is removed. So, impatience in replanting an area with juglone-sensitive plants is not advised.
Planting around Walnuts
Understanding the site to be landscaped or developed into a garden is the first step in assuring successful plantings. Identifying your trees and shrubs will help avoid problems with allelopathic toxicity among your future landscape and garden plantings. General tips for planting around black walnuts include:
- Locating gardens well away from black walnuts.
- Creating and plant in raised buds to reduce root contact. This will require lining the bed to reduce root contact using weed fabric and filling the raised bed with new topsoil.
- Improving soil drainage with organic matter additions.
- Preventing leaves, hulls, and stems from decomposing near planting areas.
- Avoiding mulch containing walnut bark, wood, hulls, and leaves
- Plant Tolerance to Juglone
The following lists of plants tolerant to juglone were compiled from published sources. They are based on observation under various settings, but few plants have been experimentally tested for sensitivity to juglone. Many factors affect sensitivity, including level of contact, health of the plant, soil environment, and the overall site conditions. The lists provided here are strictly guides and cannot be considered complete or definitive.
Trees Tolerant to Juglone
- Most maples except silver maple (Acer spp)
- Eastern Red Cedar (Juniperus virginiana)
- Ohio Buckeye (Aesculus glabra)
- Goldenrain Tree (Koelreuteria paniculata)
- Serviceberry, Shadblow (Amelanchier)
- Sweetgum (Liquidambar styraciflua)
- Pawpaw (Asimina triloba)
- Yellow Poplar (Liriodendron tulipifera)
- River Birch (Betula nigra)
- Black Gum (Nyssa sylvatica)
- Hickory (Carya spp)
- Virginia Pine (Pinus virginiana)
- Catalpa (Catalpa bignoniodes)
- Sycamore (Platanus occidentalis)
- Eastern Redbud (Cercis canadensis)
- Black Cherry (Prunus serotina)
- Fringetree (Chionanthus spp.)
- Callery Pear (Pyrus calleryana)
- Flowering Dogwood (Cornus florida)
- Oak species (Quercus spp)
- Dogwood (Cornus alternifolia)
- Staghorn Sumac (Rhus typhina)
- Hawthorne (Crataegus spp)
- Black Locust (Robinia pseudoacacia)
- Persimmon (Diosypros virginiana)
- Sassafras (Sassafras albidum)
- American Beech (Fagus grandifolia)
- Arborvitae (Thuja occidentalis)
- White Ash (Fraxinus americana)
- Canada Hemlock (Tsuga Canadensis)
- Honeylocust (Gleditsia triacanthos)
- American Elm (Ulmus americana)
- Carolina Silverbell (Halesia caroliniana)
- Blackhaw Viburnum (Viburnum prunifolium)