permaculture

[Claude William Genest <genest@pivot.net>]

Good lord but this question come sup often - here for posterity is a 
darn fine list - perhaps include it in an FAQ ?

Ohio State University Extension Fact Sheet
Ohio State University Extension Fact Sheet
Horticulture and Crop Science
2021 Coffey Rd., Columbus, Ohio 43210-1086
Black Walnut Toxicity to Plants, Humans and Horses
HYG-1148-93
Richard C. Funt
Jane Martin

The roots of Black Walnut (Juglans nigra L.) and Butternut (Juglans 
cinerea L.) produce a substance known as juglone 
(5-hydroxy-alphanapthaquinone). Persian (English or Carpathian) walnut 
trees are sometimes grafted onto black walnut rootstocks. Many plants 
such as tomato, potato, blackberry, blueberry, azalea, mountain laurel, 
rhododendron, red pine and apple may be injured or killed within one to 
two months of growth within the root zone of these trees. The toxic 
zone from a mature tree occurs on average in a 50 to 60 foot radius 
from the trunk, but can be up to 80 feet. The area affected extends 
outward each year as a tree enlarges. Young trees two to eight feet 
high can have a root diameter twice the height of the top of the tree, 
with susceptible plants dead within the root zone and dying at the 
margins.

Not all plants are sensitive to juglone. Many trees, vines, shrubs, 
groundcovers, annuals and perennials will grow in close proximity to a 
walnut tree. Certain cultivars of "resistant" species are reported to 
do poorly. Black walnut has been recommended for pastures on hillsides 
in the Ohio Valley and Appalachian mountain regions. Trees hold the 
soil, prevent erosion and provide shade for cattle. The beneficial 
effect of black walnut on pastures in encouraging the growth of 
Kentucky bluegrass (Poa pratensis L.) and other grasses appears to be 
valid as long as there is sufficient sunlight and water.

Gardeners should carefully consider the planting site for black walnut, 
butternut, or persian walnut seedlings grafted to black walnut 
rootstock, if other garden or landscape plants are to be grown within 
the root zone of mature trees. Persian walnut seedlings or trees 
grafted onto Persian walnut rootstocks do not appear to have a toxic 
effect on other plants.

Horses may be affected by black walnut chips or sawdust when they are 
used for bedding material. Close association with walnut trees while 
pollen is being shed (typically in May) also produce allergic symptoms 
in both horses and humans. The juglone toxin occurs in the leaves, bark 
and wood of walnut, but these contain lower concentrations than in the 
roots. Juglone is poorly soluble in water and does not move very far in 
the soil.

Walnut leaves can be composted because the toxin breaks down when 
exposed to air, water and bacteria. The toxic effect can be degraded in 
two to four weeks. In soil, breakdown may take up to two months. Black 
walnut leaves may be composted separately, and the finished compost 
tested for toxicity by planting tomato seedlings in it. Sawdust mulch, 
fresh sawdust or chips from street tree prunings from black walnut are 
not suggested for plants sensitive to juglone, such as blueberry or 
other plants that are sensitive to juglone. However, composting of bark 
for a minimum of six months provides a safe mulch even for plants 
sensitive to juglone.
Plants Observed Growing Under or Near Black Walnut*
Trees

    * Japanese Maples, Acer palmatum and its cultivars
    * Southern Catalpa, Catalpa bignonioides
    * Eastern Redbud, Cercis canadensis
    * Canadian Hemlock, Tsuga canadensis

Vines and Shrubs

    * Clematis 'Red Cardinal'
    * February Daphne, Daphne mezereum
    * Euonymus species
    * Weeping Forsythia, Forsythia suspensa
    * Rose of Sharon, Hibiscus syriacus
    * Tartarian Honeysuckle, Lonicera tatarica, and most other Lonicera 
species
    * Virginia Creeper, Parthenocissus quinquefolia
    * ** Pinxterbloom, Rhododendron periclymenoides
    * **'Gibraltar' and 'Balzac', Rhododendron Exbury hybrids
    * Multiflora Rose, Rosa multiflora
    * Black Raspberry, Rubus occidentalis
    * Arborvitaes, Thuja species
    * ** Koreanspice Viburnum, Viburnum carlesii, and most other 
Viburnum species

Annuals

    * Pot-marigold, Calendula officinalis 'Nonstop'
    * Begonia, fibrous cultivars
    * Morning Glory, Ipomoea 'Heavenly Blue'
    * Pansy Viola
    * Zinnia species

Vegetables

    * Squashes, Melons, Beans, Carrots, Corn

Fruit Trees

    * Peach, Nectarine, Cherry, Plum
    * Prunus species Pear-Pyrus species

Herbaceous Perennials

    * Bugleweed, Ajuga reptans
    * Hollyhock, Alcea rosea
    * American Wood Anemone, Anemone quinquefolia
    * Jack-in-the-Pulpit, Arisaema triphyllum
    * European Wild Ginger, Asarum europaeum
    * Astilbe species
    * Bellflower, Campanula latifolia
    * **Chrysanthemum species (some)
    * Glory-of-the-Snow, Chionodoxa luciliae
    * Spring Beauty, Claytonia virginica
    * Crocus species
    * Dutchman's Breeches, Dicentra cucullaria
    * Leopard's-Bane, Doronicum species
    * Crested Wood Fern, Dryopteris cristata
    * Spanish Bluebell, Endymion hispanicus
    * Winter Aconite, Eranthis hyemalis
    * Snowdrop, Galanthus nivalis
    * Sweet Woodruff, Galium odoratum
    * Herb Robert, Geranium robertianum
    * Cranesbill, Geranium sanguineum
    * Grasses (most) Gramineae family
    * Jerusalem Artichoke, Helianthus tuberosus
    * Common Daylily, Hemerocallis 'Pluie de Feu'
    * Coral Bells, Heuchera x brizoides
    * Orange Hawkweed, Hieracium aurantiacum
    * Plantain-lily, Hosta fortunei 'Glauca'
    * Hosta lancifolia
    * Hosta marginata
    * Hosta undulata 'Variegata'
    * Common Hyacinth, Hyacinthus Orientalis 'City of Haarlem'
    * Virginia Waterleaf, Hydrophyllum virginianum
    * Siberian Iris, Iris sibirica
    * Balm, Monarda didyma
    * Wild Bergamot, M. fistulosa
    * Grape Hyacinth, Muscari botryoides
    * Sweet Cicely, Myrrhis odorata 'Yellow Cheerfulness,' 'Geranium,' 
'Tete a Tete,' 'Sundial,' and 'February Gold'
    * Sundrops, Oenothera fruticosa
    * Senstitive Fern, Onoclea sensibilis
    * Cinnamon Fern, Osmunda cinnamomea
    * Peony, **Paeonia species (some)
    * Summer Phlox, Phlox paniculata
    * Mayapple, Podophyllum peltatum
    * Jacob's-Ladder, Polemonium reptans
    * Great Solomon's-Seal, Polygonatum commutatum
    * Polyanthus Primrose, Primula x polyantha
    * Lungwort, Pulmonaria species
    * Bloodroot, Sanguinaria canadensis
    * Siberian Squill, Scilla sibirica
    * Goldmoss Stonecrop, Sedum acre
    * Showy Sedum, Sedum spectabile
    * Lamb's-Ear, Stachys byzantina
    * Spiderwort, Tradescantia virginiana
    * Nodding Trillium, Trillium cernuum
    * White Wake-Robin, Trillium grandiflorum
    * Tulipa Darwin 'White Valcano' and 'Cum Laude,' Parrot 'Blue 
Parrot,' Greigii 'Toronto'
    * Big Merrybells, Uvularia grandiflora
    * Canada Violet, Viola canadensis
    * Horned Violet, Viola cornuta
    * Woolly Blue Violet, Viola sororia

*These are based upon observations and not from clinical tests.
**Cultivars of some species may do poorly.
Plants That Do Not Grow Within 50 Feet of Drip Line of Black Walnut
Herbaceous Perennials

    * Colorado Columbine, Aquilegia caerulea
    * Wild Columbine, Aquilegia canadensis
    * Asparagus, Asparagus offinalis
    * *Chrysanthemum Chrysanthumum species (some)
    * Baptisia australis
    * Hydrangea species
    * Lilies, Lilium species (particularly the Asian hybrids)
    * Alfalfa, Medicago sativa
    * Buttercup, Narcissus 'John Evelyn,' 'Unsurpassable' 'King Alfred' 
and 'Ice Follies'
    * Peonies, *Paeonia species (some)
    * Rhubarb, Rheum rhabarbarum

Trees

    * Silver Maple, Acer saccharinum
    * European Alder, Alnus glutinosa
    * White Birches, Betula species
    * Northern Hackberry, Celtis occidentalis
    * Apples and Crabapples, Malus species
    * Norway Spruce, Picea abies
    * Mugo Pine, Pinus mugo
    * Red Pine, Pinus resinosa
    * Eastern White Pine, Pinus strobus
    * Basswood, Tilia heterophylla

Shrubs

    * Red Chokeberry, Aronia arbutifolia
    * Hydrangea species
    * Mountain Laurels, Kalmia species
    * Privet, Ligustrum species
    * Amur Honeysuckle, Lonicera maackii
    * Brush Cinquefoil, Potentilla species
    * Rhododendrons and Azaleas, **Rhododendron species (most)
    * Blackberry, Rubus allegheniensis
    * Lilacs, Syringa species and cultivars
    * Yew, Taxus species
    * Blueberry, Vaccinium corymbosum
    * *Viburnum plicatum tomentosum 'Mariesii'

Annuals and Vegetables Transplants

    * Cabbage, Brassica oleracea capitata
    * Peppers, Capsicum species (some)
    * Tomatoes, Lycopersicon esculentum
    * Flowering Tobacco, Nicotiana alata
    * Petunia species and cultivars
    * Eggplant, Solanum melongena
    * Potato, Solanum tuberosum
    * double-flowered cole vegetables

*Cultivars of some species may survive but will do poorly.

The authors express their appreciation to Drs. M. Scott Biggs, 
Department of Horticulture and Crop Science, and Harry Hoitink, 
Department of Plant Pathology, for their review and additional comments.

    All educational programs conducted by Ohio State University 
Extension are available to clientele on a nondiscriminatory basis 
without regard to race, color, creed, religion, sexual orientation, 
national origin, gender, age, disability or Vietnam-era veteran status.

    Keith L. Smith, Associate Vice President for Ag. Adm. and Director, 
OSU Extension.

    TDD No. 800-589-8292 (Ohio only) or 614-292-1868


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