Black Walnut and Juglone Toxin
james...@gmail.com
The black walnut tree emits a toxin that hinders the growth of many
plants in its near proxcimity. Though many plants are effected here are
some plants which tend to do well growing near this tree.
Perrenials:
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
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
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.