University of Maryland Extension

Walnut Wilt (Juglone) - Trees and Shrubs

black walnut

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Some plants growing near black walnut trees may show reduced growth, wilt and eventually die. Walnut wilt should be suspected when wilting occurs even though there is sufficient soil moisture. A substance called juglone causes these symptoms. Juglone is a compound that can have toxic effects when it interacts with susceptible nearby plants. When one plant injures another through the production and release of chemical compounds, the plant is said to be Allelopathic. The black walnut is the most well known allelopathic plant, due to high levels of juglone. Other trees such as Persian walnut (grafted onto black walnut rootstock), Siebold walnut, Manchurican walnut, caucasian walnut, Shagbark hickory, Mockernut hickory, Butternut, and pecan are also allelopathic. Butternut, hickory, and pecan are only mildly allelopathic because they produce relatively small amounts of juglone.
walnut leaves
Photo: Leaves of black walnut tree

Many factors influence the amount of damage susceptible plants will sustain. Juglone has been found in the fruit, leaves, branches, and roots. Most damage to nearby plants appears to be the result of root contact.  Decomposition of plant material by microorganisms releases juglone to living roots in the soil. Juglone breaks down quickly in well-aerated soil with lots of microbial activity.  Damage to susceptible plants is greatest in poorly drained soils.  Some black walnuts seem to produce more juglone and are more deadly than others.  Juglone seems to increase when the tree is under stress.  Keeping the tree well watered might make it easier to grow susceptible plants underneath.

Black walnut leaves can be added to the compost pile, as toxins will break down in two to four weeks when exposed to air, water, and bacteria.  Rain leaches juglone from buds, leaves, and twigs. The concentration of juglone in the tree varies with the season.  In spring juglone is concentrated in the leaves.  The amount is high in the roots all summer.  Concentration of juglone in the hulls increases as the nuts mature.

Plants that are incompatible with walnut may be grown one and one half times the distance of the outermost limbs from the black walnut.  Keep in mind that walnut trees grow over one foot a year.  Therefore susceptible plants should be planted fifty to eighty feet from the tree.

The tables below list plants which are most susceptible to damage from juglone and those that have been observed growing under black walnuts.

Plants susceptible to damage from Juglone:

Crops
alfalfa peas
asparagus peppers
blueberries potatoes
cabbage tobacco
eggplants tomatoes
Perennials, etc.
amur honeysuckle heath
Chionodoxa hyacinths
chrysanthemums narcissus
cinquefoil peonies
crocus potentilla
endymion scilla
eranthis tulips
galanthus meadow rue
grape hyacinths
Shrubs
azalea privets
lilacs rhododendron
mountain laurel
Trees
apple norway spruce
basswood pear
black alder red pine
cherry silver maple
hackberry sugar maple
loblolly pine white birch
magnolia white pine

Plants that can be planted near black walnuts:

Crops
beans grapes
beets onions
black raspberry parsley
carrots parsnip
corn timothy
Perennials, etc.
anemones iris
aster jack-in-the-pulpit
bittersweet kentucky bluegrass
bluegrasses mayapple
Clematic virgininiana mint
cyclamen narcissus
dandelion orchard grass
elderberry pachysandra
epimedium pansy
ferns periwinkle
genian poison ivy
goldenrod solomon's seal
hellebore St. John's wort
heuchera violets
honeysuckles Virginia creeper
hosta wild oats
impatiens
Shrubs
kerria
arborvitae maple leaved viburnum
mock-orange
blackhaw viburnum rose of sharon
daphne spice bush
euonymous wayfaring-tree viburnum
forsythia wild hydrangea
junipers wild rose
Trees
American holly honey locust
basswood Japanese maple
bigleaf linden Jeffrey pine
black cherry mulberry
Norway spruce
black gum oaks
black locust Ohio buckeye
Canadian hemlock pawpaw
Canadian redbud persimmon
Carolina silverbell    poplar
catalpa red cedar
crabapple redbud
common elder river birch
dogwood smooth and dwarf sumac
elm staghorn sumac
goldenraintree sugar maple
hackberry sweetgum
hawthorn sycamore
hazelnut tulip poplar
hemlock Virginia pine
hickories
hickory black birch

 

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