When selecting plant material for a home garden it is important to first take inventory of the trees on your property and neighbouring properties. Trees like walnut can present a real challenge for gardeners. Not only do the plants you select have to deal with shade and strong competition for water and nutrients, (as with planting near any large tree), they additionally have to contend with a potentially toxic substance called juglone. Juglone is a chemical given off by the roots, nut hulls and leaves of black walnut trees, butternut trees, English walnut, pecan and hickory trees. It acts as a growth inhibitor, which in essence is nature’s way of suppressing the competition in order to ensure its survival. Some plants however are sensitive to the juglone and will die if grown near nearby.

The area directly under the walnut canopy has the highest concentration of juglone but because of the expansive root system of walnut trees, juglone sensitive plants grown within 20 to 30 meters can be adversely affected.

Symptoms

Symptoms of juglone toxicity appear as wilting, yellowing leaves, stunted or slow growth and eventually death. Certain plants are more sensitive to juglone than others. Plants highly sensitive to juglone may die within a few months.

Identifying Black Walnut Trees

The Leaves:

• The leaves are oddly pinnate containing 5-25 leaflets.
• These leaflets have serrated edges.
• The leaves are arranged alternately on the branches.
• The leaves, when crushed smell like spiced citrus.

The Bark and Branches:

• The bark is furrowed and dark brown to deep gray in colour.
• The bark has rounded ridges and deep crevices running vertically up and down the trunk.
• There are leaf scars, (that are in the shape of a shamrock), along the twigs, where the leaves pulled away from the branches.
• The shoots and twigs have chambered pith.

The fruit:

• Look for round, green fruit casings on the ground or in the tree about the size of a tennis ball.

Flower:

• Yellow-green catkins. The female flowers have pink or reddish pistils. 

Overall Shape and Form:

• Walnut trees develop broad canopies reaching 18 m width and 30 m in height.
• They have a strong central leader with dense, heavy branches.

Note: English walnut trees look almost the same, but they are typically grown in orchards. The butternut walnut is not of concern here as it does not produce enough juglone to be a problem.

Plants Sensitive to Juglone

 (These plants are sensitive to the effects of juglone and will not perform well near a walnut tree). The names in bold are particularly sensitive.

• Alder, black (Alnus glutinosa)
• Apple and Crabapples (Malus spp)
• Asparagus (Asparagus officinalis)
• Ash, white (Fraxinus americana)
• Aster spp
• Autumn Crocus (Colchichum autumnale)
• Birch, White (Betula papyrifera)
• Blackberry (Rubus allegheniensis)
• Blueberry (Vaccinium corymbosum)
• Cabbage (Brassica oleracea)
• Chrysanthumum spp (some)
• Clover, crimson (Trifolium incanatum)
• Columbine (Aquilegia)
• Cotoneaster (Cotoneaster pannosa)
• Daffodils, some (Narcissus spp)
• Egg Plant (Solanum melongena)
• European Alder (Alnus glutinosa)
• Hackberry, sugar (Celtis laevigata)
• Honeysuckle, Amur (Lonicera maackii)
• Hydrangea spp (most)
• Huckleberry (Gaylussacia sp.)
• Larch, Japanese (Larix kaempferi)
• Lilac
• Lily (Lilium spp) Asiatic, Oriental and Trumpet lilies
• Linden aka Basswood (Tilia heterophylla)
• Magnolia, saucer (Magnolia × soulangeana)
• Mountain Laurels (Kalmia latifolia)
• Northern Hackberry (Celtis occidentalis)
• Norway Spruce (Picea abies)
• Pine (most) (Pinus ssp) an exception is Virginia Pine (Pinus virginiana)
  • Eastern White pine (Pinus strobus)
  • Mugo Pine (Pinus mugo)
  • Red Pine (Pinus resinosa)
  • Scotch Pine (Pinus sylvestris)
  • Loblolly Pine (Pinus taeda)
• Peas (Pisum sativum)
• Pear (Pyrus spp)
• Peony (some) (Paeonia sp.)
• Peppers (Capsicum annuum)
• Petunia spp
• Potentilla aka Cinquefoil (Potentilla fruticosa)
• Potato (Solanum tuberosum)
• Privet (Ligustrum spp)
• Red Choke Cherry (Aronia arbutifolia)
• Rhododendron and Azaleas (most) (Rhododendron spp) except Exbury Hybrids Azalea “Gibraltar” & “Balzac”)
• Rhubarb (Rheum rhabarbarum)
• Silver Maple (Acer saccharinum)
• Tomato (Solanum lycopersicum)
• Viburnum plicatum tomentosum ‘Mariesii’
• Yew (Taxus spp)

Plants Tolerant to Juglone

(These plants are somewhat tolerant to tolerant of juglone.

• Actinida, bower (Actinidia arguta)
• Ajuga spp
• Allium spp
• American Holly (Ilex opaca)
• American bladdernut (Staphylea trifolia)
• Anemone (Anemone spp)
• Arborvitae aka Western Red Cedar (Thuja plicata)
• Aster spp
• Astilbe spp
• Azalea Exbury Hybrids “Gibraltar” & “Balzac”
• Barberry (Berberis spp)
• Beauty Bush (Linnaea amabilis)
• Bee Balm (Monarda spp)
• Beech
  • American Beech (Fagus grandifolia)
• Beets (Beta vulgaris)
• Beans (Phaseolus vulgaris)
• Begonia spp
• Big Bluestem (Andropogon gerardii)
• Birch
  • Yellow Birch (Betula alleghaniensis)
  • Sweet birch (Betula lenta)
  • River birch (Betula nigra)
• Bittersweet (Solanum dulcamara)
• Bloodroot (Sanguinaria canadensis)
• Black gum (Nyssa sylvatica)
• Black Locust (Robinia pseudoacacia)
• Bleeding heart (Lamprocapnos spectabilis)
• Blue fescue (Festuca glauca)
• Boneset (Eupatorium perfoliatum)
• Bradford pear aka Callery pear (Pyrus calleryana)
• Bugleweed (Ajuga reptans)
• Burning bush aka Winged Euonymus (Euonymus alatus)
• Buttercup (Ranunculus spp.)
• Catalpa (Catalpa bignoniodes)
• Carolina Silverbell (Halesia caroliniana)
• Carrot (Daucus carota)
• Cherry
  • Black Cherry (Prunus serotina)
  • Pin Cherry (Prunus pensylvanica)
  • Chokecherry (Prunus virginiana)
  • Sour Cherry (Prunus cerasus)
• Chestnut, American (Castanea dentata)
• Clematis spp
• Coralbells (some spp.)
• Corn aka Maize (Zea mays)
• Cranesbill (Geranium spp)
• Cucumbertree magnolia (Magnolia acuminata)
• Current (Ribes spp)
• Daffodil (Narrcissus pseudonarcissus)
• Daphne (Daphne spp)
• Daylily (Hemerocallis spp)
• Devil’s walking stick (Aralia spinosa)
• Dogwood
  • Flowering Dogwood (Cornus florida)
  • Pagoda Dogwood (Cornus alternifolia)
  • Red Osier Dogwood (Cornus sericea)
  • Silky Dogwood (Cornus amomum)
  • Bunchberry (Cornus canadensis)
• Eastern Redbud (Cercis canadensis)
• Eastern Red Cedar (Juniperus virginiana)
• Eastern white cedar (Thuja occidentalis)
• Elderberry (Sambucus Canadensis)
• Elm (Ulmus)
  • American Elm (Ulmus americana)
• Evening primrose (Oenothera spp)
•  False cypress (Chamaecyparis)
• Ferns
• Forsythia (Forsythia spp)
• Fringe Tree (Chionanthus spp)
• Garden phlox (Phlox paniculata)
• Geum spp
• Geranium (Pelargonium spp)
• Goldenrain Tree (Koelreuteria paniculata)
• Grape Hyacinth (Muscari spp)
• Harebell (Campanula rotundifolia)
• Hawthorn ((Crataegus spp)
• Hazelnut (Corylus americana)
• Hellebore (Helleborus spp)
• Hemlock, poison (Conium maculatum)
• Hemlock, Canada/Eastern (Tsuga canadensis)
• Hickory (Carya spp)
• Hollyhock (Alcea rosea)
• Honey-locust (Gleditsia triacanthos)
• Honeysuckle (Lonicera spp) (except Amur honeysuckle (Lonicera maackii)
• Hornbeam, American (Carpinus caroliniana)
• Hosta spp
• Indian Grass (Sorghastrum nutans)
• Iris
  • Bearded Iris ((Iris x germanica)
  • Blue Flag Iris (Iris versicolor)
  • Siberian Iris (Iris sibirica)
• Ironweed (Vernonia spp)
• Jack-in-the-pulpit (Arisaema triphllyum)
• Jacob’s Ladder (Polemonium caeruleum)
• Juniper (Juniperus spp)
  • Chinese Juniper (Juniperus chinensis)
  • Common Juniper (Juniperus communis)
  • Eastern Red Cedar (Juniperus virginiana)
• Kentucky Blue Grass (Poa pratensis)
• Kerria, double flower (Kerria japonica ‘pleniflora’)
• Lamb’s Ears (Stachys byzantina)
• Large-flowered Bellwort (Uvularia grandiflora)
• Leatherwood (Dirca palustrus)
• Leopard’s bane (Doronicum spp)
• Lily (some)
  • Michigan Lily (Lilium michiganense)
  • Wood Lily (Lilium philadelphicum)
• Lily-of-the-valley (Convallaria majalis)
• Lily Turf (Liriope muscari)
• Little Bluestem (Schizachyrium scoparium)
• Lobelia spp
• Lungwort (Pulmonaria officinalis)
• Maples (most Acer spp) except Silver maple
  • Red Maple (Acer rubrum)
  • Sugar Maple (Acer saccharum)
  • Japanese Maple (Acer palmatum and cultivars)
  • Boxelder Maple (Acer negundo)                         
• Marsh Marigold (Caltha palustris)
• May Apple (Podophyllum emodi)
• Meadow Rue (Thalictrum spp)
• Melons
• Mistflower (Conoclinium coelestinum)
• Mock Orange (Philadelphus spp)
• Mulberry (white) (Morus alba)
• Nectarine (Prunus persica)
• New Jersey tea (Ceanothus americanus)
• Ninebark, common (Physocarpus opulifolius)
• Oak species (Quercus spp)
  • Black Oak (Quercus velutina)
  • Northern Red Oak (Quercus rubra)
  • Scarlet Oak (Quercus coccinea)
  • Shingle Oak (Quercus imbricaria)
  • White Oak (Quercus alba)
• Obedient Plant (Physostegia virginiana)
• Ohio Buckeye (Aesculus glabra)
• Onion (Allium spp)
• Ox-Eye Daisy (Leucanthemum vulgare)
• Pachysandra spp
• Pansies (Viola spp)
• Parsnips (Pastinaca sativa)
• Pawpaw (Asimina triloba)
• Peach (Prunus persica)
• Peppermint (Mentha piperita)
• Periwinkle (Vinca minor)
• Persimmon (Diosypros virginiana)
• Phlox
  • Woodland Phlox (Phlox divaricata)
  • Creeping Phlox (Phlox stolonifera)
  • Garden Phlox (Phlox stolonifera)
• Pine, Virginia (Pinus virginiana)
• Plum (Prunus domestica)
• Poplar (Populus spp)
  • Yellow Poplar (Liriodendron tulipifera)
• Prickly ash (Zanthoxylum americanum)
• Primrose (Primula spp)
• Purple leaf sandcherry (Prunus cisterna)
• Quince (Cydonia oblongata)
• River Birch (Betula nigra)
• Raspberry
  • Black Raspberry (Rubus occidentalis)
  • Purple-flowering Raspberry (Rubus odoratus)
• Robinia (Robinia pseudoacacia)
• Rose of Sharon (Hibiscus syriacus)
• Rose
  • Carolina Rose (Rosa carolina)
  • Smooth Rose (Rosa blanda)
  • Swamp Rose (Rosa palustris)
• Rudbecia spp
  • Brown-eyed Susan (Rudbeckia triloba)
  • Black-eyed Susan (Rudbeckia hirta)
  • Cutleaf Coneflower (Rudbeckia lanciniata)
• Rye Grass (Elymus spp)
  • Bottlebrush Grass (Elymus Hystrix)
  • Virginia Rye (Elymus virginicus)
  • Canada Rye (Elymus canadensis)
• Sassafras (Sassafras albidum)
• Sedum Spp.
• Service Berry Shadblow (Amelanchier canadensis)
• Shasta Daisy 
• Siberian squill (Scilla sibirica)
• Siberian Bugloss (Brunnera macrophylla)
• Side-oats Grama (Bouteloua curtipendula)
• Silverbell (Halesia carolina)
• Slipper Elm (Ulmus rubra)
• Snow Drops
• Snowball Hydrangea (Hydrangea arborescens) the best-known cultivar being ‘Annabelle’
• Solomon’s Seal
• Southern Catalpa
• Spanish Bluebell (Hyacinthoides hispanicus)
• Speedwell (Veronica spp)
• Spiderwort (Tradescantia spp)
• Spicebush (Lindera benzoin)
• Squash (Cucurbita spp)
• St. Johnswort (Hypericum prolificum)
• Sumac
  • Fragrant Sumac (Rhus aromatica)
  • Shining Sumac (Rhus copallina)
  • Smooth Sumac (Rhus glabra)
  • Staghorn Sumac (Rhus typhina)
• Sunflower (Helianthus spp)
  • Sunchoke (Helianthus tuberosus)
  • Oxeye (Heliopsis helianthoides)
  • Pale-leaved Sunflower (Helianthus strumosus)
  • Tall Sunflower (Helianthus giganteus)
  • Woodland Sunflower (Helianthus divaricatus)
• Sweet Woodruff (Galium odoratum)
• Sweetgum (Liquidambar styraciflua)
• Switch Grass (Panicum virgatum)
• Sycamore (Platanus occidentalis)
• Toothwort (Cardamine spp)
• Trillium (Trillium spp)
• Trout Lily (Erythronium americanum)
• Tulip, some (Tulipa spp)
• Tulip Tree (Liriodendron tulipifera)
• Tupelo (Nyssa sylvatica)
• Viburnum (some spp.)
  • Maple-leaved Viburnum (Viburnum acerifolia)
  • Blackhaw Viburnum (Viburnum prunifolium)
  • Korean Spice Viburnum (Viburnum carlesii) 
  • Wayfaring tree (Viburnum lantana)
  • Southern Arrowwood (Viburnum dentatum)
  • Smooth Arrowwood (Viburnum dentatum)
  • Hobblebush (Viburnum lantanoides)
  • Nannyberry (Viburnum lentago)
  • Highbush Cranberry (Viburnum trilobum).
• Violets (Viola spp.)
• Virginia Creeper (Parthenocissus quinquefolia)
• Wayfaring tree (Viburnum lantana)
• White Snakeroot (Ageratina altissima)
• Wild Plum aka American plum (Prunus americana)
• Wild Grape (Vitis)
• Wild ginger (Asarum spp.)
• Willow (Salix spp) 
• Wisteria spp
• Witch Hazel (Hamamellis spp)
• Windflower (Anemone spp.)
• Yarrow (Achillea spp)
• Yellow Buckeye ((Aesculus octandra)
• Yucca spp
• Zinnia spp

For those who wish to grow vegetables near a walnut tree try constructing raised beds and line them well to keep out walnut roots, keep leaves and nuts cleaned up in the fall. Container gardening is also an option.

References:

Booth S. (Updated Dec 01, 2020). Juglone (Walnut) Tolerant Native Plants, for Ontario Gardens. https://www.inournature.ca/juglone-tolerant-native-plants

Cullen M., (2020). Juglone Tolerant Plants. http://markcullen.com/juglone-tolerant-plants/

Dana M.N. and Rosie Lerner B., (Rev. 2/94). BLACK WALNUT TOXICITY. Purdue University Cooperative Extension Service. https://www.extension.purdue.edu/extmedia/HO/HO-193.pdf

Feeley C., (n.d.). Black Walnut: The Killer Tree. Iowa State University Extension and Outreach. https://www.extension.iastate.edu/news/2005/jul/070701.htm

ISU Extension and Outreach, (n.d). What plants are sensitive to the juglone produced by black walnuts? Iowa State University Extension and Outreach. https://hortnews.extension.iastate.edu/faq/what-plants-are-sensitive-juglone-produced-black-walnuts

Johnson’s Nursery, (Updated 2015). Black Walnut Toxicity. https://kb.jniplants.com/black-walnut-toxicity/

Joy A. and Hudelson B., (Revised:  5/7/2010). Black Walnut Toxicity. UW-Madison. Horticulture. Item number:  XHT1017 https://hort.extension.wisc.edu/articles/black-walnut-toxicity/

Lien J., (2017). What Plants Will Grow Near Black Walnut Trees? https://www.gardenguides.com/97747-plants-grow-near-black-walnut-trees.html

Morton Arboretum, (n.d.). Black Walnut Toxicity. https://mortonarb.org/plant-and-protect/tree-plant-care/plant-care-resources/black-walnut-toxicity/

OMAFRA, (2016, Modified 2022-12-02). Walnut Toxicity. http://omafra.gov.on.ca/english/crops/facts/info_walnut_toxicity.htm

Sellmer J., Ph.D. (2007). Landscaping and Gardening Around Walnuts and Other Juglone Producing Plants. https://extension.psu.edu/landscaping-and-gardening-around-walnuts-and-other-juglone-producing-plants

Updated Jan. 5, 2023

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