Many years ago a quinone called juglone was isolated from the husks of Walnut. Juglone was found to be highly toxic when injected into alfalfa and tomato plants and has even killed apple trees growing near walnuts. Experiments have found that a toxic effect of walnut bark causes a growth failure of tomatoes and alfalfa within the root zone of walnuts. There are, however, some conditions under which these plants may grow near walnut trees without apparent damage.
A scientific term used to describe one plant's suppression of another through the secretion of a chemical in the environment is Allelopathy. It has lent its name to a new branch of science concerned with the interaction between plants and the environment; this branch has done considerable research in the walnut area.
Juglone is a toxic substance, a napthaquinone that has been isolated in many plants in the Walnut family. Some of the plants in the Walnut family containing Juglone are: Persian walnut, J. regia, Black Walnut, J. nigra, and Butternut, J. cinerea, J. sieboldiana, J. mandshurica. There are some hickories which also yield juglone: the Carya ovata, the C. alba, the C. olivae formis and the Pterocarya caucasica.
There is often some confusion in the cause of the wilting, as most of the symptoms are similar. Normal wilting, caused by water deficiency, occurs when: shade and competition for water and nutrients to some degree affect the growth of all plants growing under Walnut. Lowering the percentage of water in the soil affects all of the plants with roots in the same soil profile uniformly. The signs of water deficiency appear slowly, and can be allayed with an addition of water to the soil. The addition of water brings back turgor which persists until soil water is again deficient.
Wilting caused by contact with Walnut roots occurs in a relatively short time, even when there is ample soil moisture. Wilting may occur on only a part of the plant, or the whole plant may be affected. It is wise to detect this early, as plants in the early stages may recover when additional water is applied. Later wilting becomes more severe, there is a browning of the leaves and wilting usually results in the death of the plant. The observed toxic effect of Walnut can also be partly offset by liberal supplies of nitrogen.
This toxic affect on surrounding plants appears to be related to root contact, as walnut hulls and leaves used as mulch have not shown toxic effects on plant growth. Because Walnut roots do not occupy the surface layers in most soil, many shallow rooted plants growing under walnut trees don't come in contact with the roots and are not affected by them.
Tomatoes and alfalfa have been grown normally close to young trees. This suggests that either the toxic substance may not be formed yet in the young trees or that the roots of the young tree are few and do not come into contact with those of the plants beneath it.
Evidence also indicates that the toxic effect does not remain in the soil more than about one year after removal of a walnut tree.
A list of special cultivars that can survive under walnut trees follows:
Ajuga reptans; bugleweed Alcea rosea; hollyhock Asarum europaeum; European wild ginger Astilbe Campanula latifolia; bellflower Chrysanthemum; hardy chrysanthemum Doronicum; leopard's bane Dryopteris cristata; crested wood fern Galium odoratum; sweet woodruff Geranium robertianum; herb Robert G. sanguineum; cranesbill Helianthus tuberosus; Jerusalem artichoke Hemerocallis fulva; common daylily Heuchera xbrizoides; 'Pluie de Feu', coral bells Hieracium aurantiacum; orange hawkweed Hosta forunei; 'Glauca', plantrain lily H. lancifolia H. marginata H. undulata 'Variegata' Hydrophyllum virginianum; Virginia waterleaf Iris siberica; Siberian iris Monarda didyma; bee balm M. fistulosa; wild bergamot Oenothera fruticosa; sundrops Onoclea sensibilis; sensitive fern Osmunda cinnommea; cinnamon fern Phlox paniculata; summer phlox Polemonium reptans; Jacob;s ladder Polygonatum commutatum; great Solomon's seal Primula xpolyantha; polyanthus primrose Pulmonaria; lungwort Sanguinaria canadensis; bloodroot S. canadensis; 'Multiplex', double-flowered bloodroot Sedum acre; gold moss S. spectabile Stachys byzantina; lamb's ear Tradescantia virginiana; spiderwort Trillium cernuum; nodding trillium T. grandiflorum; wide wake-robin Uvularia grandiflora; big merrybells Viola canadensis; Canada violet V. sororia; woolly blue violet
Chioniodoxa lucilae; glory-of-the-snow Crocus Endymion hispanicul; Spanish bluebell Eranthis hyemalis; winter aconite Galanthus nivalis; snowdrop Hyacinthus; 'City of Harlem' Muscari botryoides; grape hyacinth Narcissus; 'Cheerfulness', 'Yellow Cheerfulness', 'Tete a Tete', 'Sundial', and 'February Gold' Scilla siberica; blue squill Tulipa Darwin; 'White Volcano' and 'Cum Laude', Parrot 'Blue Parrot', Greigii 'Toronto'
Acer palmatum; Japanese maple A. palmatum; 'Dissectum', cutleaf Japanese maple Catalpa bignoiides; common catalpa Tsulga canadensis; Canadian hemlock
Vines and Shrubs:
Clematis; 'Red Cardinal' Daphne mezereum; February daphne Forsythia suspensa; Weeping forsythia Hibiscus syriacus; rose of Sharon Lonicera tatarica; Tartarian honeysuckle Parthenocissus quinquefolia; Virginia creeper Rhododendron periclymenoides; pinxterbloom R. Exbury hybrids 'Gibraltar' and 'Balzac'
Begonia; fiberous cultivars and tuberous cultivar 'Nonstop' Calendula officinalis; pot marigold Ipomoea; 'Heavenly Blue', morning glory Viola cornuta; horned violet V. xwittrockiana; pansy
Glechoma hederacea; ground ivy Oxalis corniculata; creeping lady's sorrel Stellaria media; chickweed Taraxacum officale; dandelion Veronica filiformis; creeping veronica