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Broadleafed Shrubs:
Whole Shrub
Walnut Wilt
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.
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
- asparagus
- blueberries
- cabbage
- eggplants
- peas
- peppers
- potatoes
- tobacco
- tomatoes
Perennials, etc.
- amur honeysuckle
- Chionodoxa
- chrysanthemums
- cinquefoil
- crocus
- endymion
- eranthis
- galanthus
- grape hyacinths
- heath
- hyacinths
- narcissus
- peonies
- potentilla
- scilla
- tulips
- meadow rue
Shrubs
- azalea
- lilacs
- mountain laurel
- privets
- rhododendron
Trees
- apple
- basswood
- black alder
- cherry
- hackberry
- loblolly pine
- magnolia
- norway spruce
- pear
- red pine
- silver maple
- sugar maple
- white birch
- white pine
Plants that can be planted near black walnuts:
Crops
- beans
- beets
- black raspberry
- carrots
- corn
- grapes
- onions
- parsley
- parsnip
- timothy
Perennials, etc.
- anemones
- aster
- bittersweet
- bluegrasses
- Clematic virgininiana
- cyclamen
- dandelion
- elderberry
- epimedium
- ferns
- genian
- goldenrod
- hellebore
- heuchera
- honeysuckles
- hosta
- impatiens
- iris
- jack-in-the-pulpit
- kentucky bluegrass
- mayapple
- mint
- narcissus
- orchard grass
- pachysandra
- pansy
- periwinkle
- poison ivy
- solomon's seal
- St. John's wort
- violets
- Virginia creeper
- wild oats
Shrubs
- American barberry
- arborvitae
- autumn olive
- blackhaw viburnum
- daphne
- euonymous
- forsythia
- junipers
- kerria
- maple leaved viburnum
- mock-orange
- rose of sharon
- spice bush
- wayfaring-tree viburnum
- wild hydrangea
- wild rose
Trees
- American holly
- basswood
- bigleaf linden
- black cherry
- callery pear
- black gum
- black locust
- Canadian hemlock
- Canadian redbud
- Carolina silverbell
- catalpa
- crabapple
- common elder
- dogwood
- elm
- goldenraintree
- hackberry
- hawthorn
- hazelnut
- hemlock
- hickories
- hickory black birch
- honey locust
- japanese maple
- Jeffrey pine
- mulberry
- Norway spruce
- oaks
- Ohio buckeye
- pawpaw
- persimmon
- poplar
- red cedar
- redbud
- river birch
- smooth and dwarf sumac
- staghorn sumac
- sugar maple
- sweetgum
- sycamore
- tree of heaven
- tulip poplar
- virginia pine
- white ash
Keywords: juglone, walnut, butternut, allelopathtic
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Black walnut leaves and fruit.
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