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Sandy Feather: Walnut trees' juglone can harm some plants

Saturday, January 04, 2003

Q. I have heard that walnut trees are toxic to neighboring plants. Does this mean that plants nearby will be unhealthy, or that you shouldn't eat anything from a plant growing near a walnut tree? What kind of plants are affected and at what distance? Can you eat blackberries off nearby bushes? How far away from a walnut tree should a vegetable garden be located?

A. Black walnut (Juglans nigra) and butternut trees (Juglans cinerea) secrete a substance from their roots called juglone. Although tomatoes are probably the best-known victims, juglone is injurious to asparagus, azaleas, blueberries, mountain laurel, potentilla, rhododendron and many other plants when they are grown within the root zone of these trees.

 
 

Send questions to Sandy Feather by e-mail at slf9@psu.edu or by regular mail c/o Penn State Cooperative Extension, 400 N. Lexington St., Pittsburgh 15208. Due to volume of mail, personal replies cannot be provided.

   
 

This effect is commonly known as walnut wilt. You also may have heard this called allelopathy.

Allelopathy is a survival technique that reduces the tree's need to compete for water, nutrients and sunlight. Juglone is so effective at killing plants within the root zone of these trees that is has been studied for use as a herbicide.

To avoid walnut wilt, sensitive plants should be planted 10 to 15 feet beyond the drip line (ends of the branches) of black walnut or butternut trees. Roots of sensitive plants must come into physical contact with the roots of black walnuts for injury to occur. That is why tomatoes planted near black walnut trees seem to thrive when they are first planted.

The injury does not occur until their root systems grow large enough to touch the black walnut's roots.

There are actually quite a few plants that will grow near these trees, including blackberries (Rubus spp.). It is perfectly safe to eat the fruits from blackberry bushes or anything else that successfully grows near them. Other plants that will grow within the root zone of black walnut or butternut trees include:

Trees

Japanese maple (Acer palmatum, including cutleaf types); Redbud (Cercis canadensis); Hawthorn (Crataegus spp.); Persimmon (Diospyros virginiana); Eastern red cedar (Juniperus virginiana); Oaks (Quercus spp.); Arborvitae (Thuja spp.); Canadian hemlock (Tsuga canadensis); Elms (Ulmus spp.)

Shrubs and vines

Weeping forsythia (Forsythia suspensa); Rose of Sharon (Hibiscus syriacus); Virginia creeper (Parthenocissus quinquefolia); Mock orange (Philadelphus coronarius); Pinxterbloom azalea (Rhododendron periclymenoides, deciduous); Exbury azaleas (Rhododendron x 'Exbury', deciduous); Brambles (Rubus spp.); Lilacs (Syringa spp.); Viburnums (Viburnum spp.)

Perennials

Hollyhock (Alcea rosea); Jack-in-the-Pulpit (Arisaema triphyllum); European ginger (Asarum europaeum); False spirea (Astilbe spp.); Great bellflower (Campanula latifolia); Bleeding heart (Dicentra spp.); Leopards bane (Doronicum orientale); Crested wood fern (Dryopteris cristata); Trout lily/Dog's tooth violet (Erythronium americanum); Sweet woodruff (Galium odoratum); Bloody cranes bill (Geranium sanguineum); Lenten rose/Christmas rose (Helleborus spp.); Common daylily (Hemerocallis fulva, wild orange type); Hostas (Hosta spp.); Siberian iris (Iris siberica); Great blue lobelia (Lobelia siphilitica); Bee balm (Monarda didyma); Wild bergamot (Monarda fistulosa); Common sundrop (Oenothera fruticosa, tetragona); Sensitive fern (Onoclea sensibilis); Cinnamon fern (Osmunda cinnamomea); Garden phlox (Phlox paniculata); Great Solomon's seal (Polygonatum commutatum); Christmas fern (Polystichum acrostichoides); Primrose (Primula x polyantha); Lungwort (Pulmonaria spp.); Bloodrot (Sanguinaria canadensis); Live Forever sedum (Hylotelephium spectabile); Lamb's ear (Stachys byzantina); Meadow rue (Thalictrum spp.); Spiderwort (Tradescantia virginiana); Trillium (Trillium grandiflorum); Canada violet (Viola canadensis).

Bulbs

Glory of the snow (Chioniodoxa lucilae); Crocus (Crocus spp.); Spanish bluebells (Endymion hispanicul); Winter aconite (Eranthis hyemalis); Snowdrop (Galanthus nivalis); Hyacinths (Hyacinthus orientalis); Grape hyacinth (Muscari botryoides); Daffodils (Narcissus); Siberian squill (Scilla siberica); Tulips (Tulipa spp.)

Annuals

Wax begonia (Begonia semperflorens); Pot marigold (Calendula officinalis); Morning glory (Ipomea purpurea); Pansy (Viola x wittrockiana)

This list has been adapted from a walnut toxicity publication from Michigan State University Extension Service.

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