University of Maryland Extension

Walnut Wilt (Juglone) - Trees and Shrubs

black walnut

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
alfalfapeas
asparaguspeppers
blueberriespotatoes
cabbagetobacco
eggplantstomatoes
Perennials, etc.
amur honeysuckleheath
Chionodoxahyacinths
chrysanthemumsnarcissus
cinquefoilpeonies
crocuspotentilla
endymionscilla
eranthistulips
galanthusmeadow rue
grape hyacinths
Shrubs
azaleaprivets
lilacsrhododendron
mountain laurel
Trees
applenorway spruce
basswoodpear
black alderred pine
cherrysilver maple
hackberry
loblolly pinewhite birch
magnoliawhite pine

Plants that can be planted near black walnuts:

Crops
beansgrapes
beetsonions
black raspberryparsley
carrotsparsnip
corntimothy
Perennials, etc.
anemonesiris
asterjack-in-the-pulpit
bittersweetkentucky bluegrass
bluegrassesmayapple
Clematic virgininianamint
cyclamennarcissus
dandelionorchard grass
elderberrypachysandra
epimediumpansy
fernsperiwinkle
genianpoison ivy
goldenrodsolomon's seal
helleboreSt. John's wort
heucheraviolets
honeysucklesVirginia creeper
hostawild oats
impatiens
Shrubs
kerria
arborvitaemaple leaved viburnum
mock-orange
blackhaw viburnumrose of sharon
daphnespice bush
euonymouswayfaring-tree viburnum
forsythiawild hydrangea
juniperswild rose
Trees
American hollyhoney locust
basswoodJapanese maple
bigleaf lindenJeffrey pine
black cherrymulberry
Norway spruce
black gumoaks
black locustOhio buckeye
Canadian hemlockpawpaw
Canadian redbudpersimmon
Carolina silverbell   poplar
catalpared cedar
crabappleredbud
common elderriver birch
dogwoodsmooth and dwarf sumac
elmstaghorn sumac
goldenraintreesugar maple
hackberrysweetgum
hawthornsycamore
hazelnuttulip poplar
hemlockVirginia pine
hickories
hickory black birch

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