Black Walnut Trees and
Juglone

Q. I planted 35 tomato plants in my garden, and they
are all turning yellow and wilting - they appear to be dying. It
does not seem to be a typical tomato problem such as early blight.
Iíve been growing tomatoes for years, and have never seen anything
like this. What could be wrong with them?

A. Further conversation with the writer revealed that
there is a large black walnut tree (Juglans nigra) growing in his
yard. These trees secrete a substance called juglone that inhibits
the growth of other plants near them. This phenomenon is known as
allelopathy. It reduces competition for water and nutrients, and
ensures their long-term survival. Other species of trees, including
tree-of-heaven (Ailanthus altissima), sugar maple (Acer saccharum),
hackberry (Celtis spp.), and American elm (Ulmus americana) also
produce allelochemicals, but their effects pale in comparison to
black walnut. Black walnut trees are so infamous for killing or
damaging plants grown near them that pathologists have coined the
term ďwalnut wiltĒ to describe the problem.

Butternuts,
Walnuts, Hickories and Pecans

Butternut trees (Juglans cinerea), English walnuts (Juglans regia),
hickories (Carya spp.), and pecans (Carya spp.) also produce juglone,
but not as much as black walnut trees. Juglone is found in leaves,
stems, nut hulls, inner bark and roots. It can affect plants growing
under the tree or within the sphere of its root system. Remember
that a treeís feeder roots - those fine, hair-like roots responsible
for absorbing water and nutrients - can extend out two or three
times the circumference of the spread of a treeís branches (drip
line). Although plants growing immediately under the tree are most
at risk, sensitive plants growing in the area of its extended root
system can also be damaged or killed.

During the fall you'll find Black Walnuts
(and usually squirrels) beneath the trees

Susceptible plants are exposed to juglone through root contact, the
decay of fallen leaves and hulls, or even rain leaching juglone from
the leaves and branches to the ground below. Although the writer has
grown tomatoes in his vegetable garden for years with no problem,
the tree has grown over time, and the spread of its root system has
increased as well. While tomatoes are probably the best-known
victims of walnut wilt, 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.

Reducing the
Effects of Juglone

There are steps you can take to reduce the allelopathic effect of
juglone. Clean up fallen leaves, nuts, and other debris from black
walnut trees regularly to minimize their accumulation in the soil.
Incorporate compost, rotted manure, composted grass clippings,
and/or cover crops to maintain a high level of organic matter in the
soil. This encourages a healthy population of soil microbes that can
help break juglone down and minimize damage to sensitive plants.
Avoid using compost that contains black walnut debris unless it has
been broken down thoroughly. If you compost actively - that is, you
turn the pile frequently and keep it moist - the compost can be used
in about eight months. If you use the lazy method of composting -
that is, pile the materials up and let them sit - you should wait at
least a year before using it. You can test the safety of the
finished compost by starting tomato seedlings in it. If they do not
die, the mulch is safe to use.

Juglone and
Tomatoes

The writer is more interested in growing tomatoes than keeping the
black walnut and is considering having it removed. It is a large
tree, and its roots cover so much of his yard that he does not have
another place where he can grow tomatoes. That is an option, but it
will take at least two years - possibly longer - for all of the
roots to die, even if the stump is ground out. If you choose this
route, be aware that black walnuts are valuable trees. Large,
straight-growing specimens can be worth a lot of money. Investigate
options for selling the tree, rather than paying someone to remove
it.

If you are less determined to grow a sensitive crop like tomatoes,
be aware that there are plants that grow well near black walnuts.
Kentucky bluegrass and tall fescue seem to thrive in association
with black walnut and butternut trees, so planting the area in lawn
grass is an option. There are also quite a few other plants that
will grow successfully near these trees. They include:

Trees & Shrubs

black, red, sugar, Japanese maples (Acer spp.)

serviceberry (Amelanchier spp.)

river, cherry birch (Betula spp.)

eastern redbud (Cercis canadensis)

fringetree (Chionanthus spp.)

hawthorn (Crataegus spp.)

daphne (Daphne spp.)

forsythia (Forsythia spp.)

thornless honeylocust (Gleditsia triacanthos var.
inermis)

Carolina silverbell (Halesia tetraptera)

witchhazel (Hamamelis spp.)

rose-of-Sharon (Hibiscus syriacus)

hydrangea (Hydrangea spp.)

American holly (Ilex opaca)

eastern red cedar (Juniperus virginiana)

sweetgum (Liquidambar styraciflua)

black gum (Nyssa sylvatica)

Virginia creeper (Parthenocissus quinquefolia)

mock-orange (Philadelphus coronarius)

oak (Quercus spp.)

black raspberry (Rubus occidentalis)

elderberry (Sambucus spp.)

lilacs (Syringa spp.)

arborvitae (Thuja spp.)

Canadian hemlock (Tsuga canadensis)

elm (Ulmus spp.)

some viburnums (Viburnum spp.)

Perennials

bugleweed (Ajuga reptans)

hollyhock (Alcea rosea)

Jack-in-the-pulpit (Arisaema triphyllum)

European ginger (Asarum europaeum)

false spirea (Astilbe spp.)

bleeding heart (Dicentra spp.)

leopardsbane (Doronicum orientale)

crested wood fern (Dryopteris cristata)

trout lily (Erythronium americanum)

sweet woodruff (Galium odoratum)

perennial geranium (Geranium spp.)

Lenten Rose/Christmas Rose (Helleborus spp.)

common daylily (wild orange type) (Hemerocallis
fulva)

hosta (Hosta ssp.)

Siberian iris (Iris siberica)

lily-turf (Liriope spp.)

lobelia (Lobelia spp.)

bee balm (Monarda spp.)

sensitive fern (Onoclea sensibilis)

cinnamon fern (Osmunda cinnamomea)

garden phlox (Phlox paniculata)

great Solomonís seal (Polygonatum commutatum)

Christmas fern (Polystichum acrostichoides)

primrose (Primula spp.)

lungwort (Pulmonaria spp.)

bloodroot (Sanguinaria canadensis)

live-forever (Hylotelephium (Sedum) spectabile)

lambís ear (Stachys byzantina)

meadow rue (Thalictrum spp.)

spiderwort (Tradescantia virginiana)

trillium (Trillium spp.)

Canada violet (Viola canadensis)

Bulbs

glory of the snow (Chionodoxa lucilae)

spring-blooming crocus (Crocus spp.)

Spanish bluebells (Hyacinthoides hispanica)

winter aconite (Eranthis hyemalis)

snowdrop (Galanthus spp.)

hyacinth (Hyacinthus orientalis)

grape hyacinth (Muscari botryoides)

daffodils
(Narcissus)

Siberian squill (Scilla siberica)

tulip (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 walnut toxicity
publications by Michigan State University Extension, Virginia
Cooperative Extension and Penn State University.