Plant Friends of the Black Walnut

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The following plants typically grow well with the root zone of the black walnut:

Perennials:

Ajuga reptans, bugleweed                   
Alcea rosea, hollyhock
Asarum europaeum, european wild ginger
Astilbe
Campanula spp., the bellflowers
Chrysanthemum, hardy chrysanthemum
Doronicum, leopards bane
Dryopteris cristata, crested wood fern
Galium odoratum, sweet woodruff
Geranium robertianum, herb robert
Geranium sanquineum, cranesbill
Helianthus tuberosus, Jerusalem artichoke
Hemerocallis fulva, common daylily
Heuchera x brizoides Pluie de Feu, coral bells
Hieracium aurantiacum, orange hawkweed
Hosta fortunei, Glauca
Hosta lancifolia
Hosta marginata
Hosta undulata Variegated
Hydrophyllum virginianum, virginia waterleaf
Iris sibirica, siberian iris
Monarda didyma, bee balm
Monard fistulosa, wild bergamot
Oenothera fruticosa, sundrops
Onoclea sensibilis, sensitive fern
Osmunda cinnamomea, cinnamon fern
Phlox paniculata, summer phlox
Polemonium reptans, jacobs ladder
Polygonatum commutatum, great solomon's seal
Primula x polyantha, polyanthus primrose
Pulmonaria, lungwort
Rosa banksiae, Lady Banks' rose
Sanquinaria canadensis, bloodroot
Sanquinaria canadensis multiplex, double-flowered bloodroot
Sedum acre, gold moss
Sedum spectabile, showy stonecrop
Stachys byzantina, lamb's ear
Tradescantia virginiana, spiderwort
Trillium cernuum, nodding trillium
Trillium grandiflorum, white wake-robin
Uvularia grandiflora, big merrybells
Viola canadensis, canada violet
Viola sororia, woolly blue violet

Bulbs:

Chionodoxa luciliae, glory of the snow
Crocus
Endymion hispanicus, spanish bluebell
Eranthis hyemalis, winter aconite
Galanthus nivalis, snowdrop
Hyacinthus, city of haarlem
Muscari botryoides, grape hyacinth
Narcissus 
Scilla siberica, blue squill
Tulipa Darwin
Tulipa Parrot
Tulipa Greigii

Trees:

Acer palmatum, japanese maple
Acer palmatum dissectum, cutleaf japanese maple
Catalpa bignonioides, common catalpa
Tsuga 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 periclymeniodes, pinxterbloom
Rhododendron exbury hybrids, gibraltar and balzac

Annuals:

Begonia, fibrous cultivars and tuberous cv nonstop
Calendula officinalis, pot marigold
Ipomoea heavenly blue, morning glory
Viola cornuta, horned violet
Viola x wittrockiana, pansy

Courtesy of the American Horticultural Society, Box 0105, Mount Vernon, VA
22121 

In addition to the above, I have had good results with Achillea ptarmica (Sneezewort), Chrysanthemum leucanthemum (Oxeye daisy), Datura metel (thornapple), Hypericum sp. (St. Johnswort), Leonurus marrubiastrum, and Rhamnus sp. (buckthorn) since many weeds will grow anywhere.


 The following is excerpted from Ohio State University  Extension Factsheet HYG-1148-93
Black Walnut Toxicity to Plants, Humans and Horses

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.


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Etching of Black Walnut Tree.