Black Walnut (Juglans nigra) and Allelopathy
From Virginia Cooperative Extension

Though grown primarily for its wood and nuts, black walnuts are often found growing on landscape sites where they serve primarily as shade trees. When certain other landscape plants are planted near or under this shade tree they tend to yellow, wilt and die. This decline occurs because the walnut tree produces a non-toxic, colorless chemical called hydrojuglone. Hydrojuglone is found in leaves, stems, fruit hulls, inner bark and roots. When exposed to air or soil compounds, hydrojuglone is oxidized into the allelochemical juglone, which is highly toxic.

Several related trees such as English walnut, hickories and pecan also produce juglone, but in smaller amounts compared to black walnut. Juglone is one of many plant-produced chemicals that can harm other plants in a process known as allelopathy. (Additional common landscape trees with allelopathic properties: sugar maple, tree-of-heaven, hackberries, southern waxmyrtle, American sycamore, cottonwood, black cherry, red oak, black locust, sassafras, and American elm.)

Juglone in the Soil
Juglone is exuded from all parts of the walnut tree. Juglone can affect other plants either through root contact, leakage or decay in the soil, falling and decaying leaves, or when rain leaches and drips juglone from leaves and branches onto plants below. Plants located beneath the canopy of walnut trees are most at risk because juglone from the roots and fallen leaves accumulates there.

Although juglone has low water solubility and does not move far in the soil, small amounts may be injurious to sensitive plants. Plant roots can encounter juglone when they grow within 0.5 - 0.25 inches from a walnut root. Walnut roots can extend in the soil well beyond the crown or drip line of the tree, affecting susceptible plants far from the black walnut.

The accumulation and depletion of toxins in the soil is affected by factors such as soil type, drainage, aeration, temperature and microbial action. Soil microorganisms ingest allelochemicals as energy sources, and metabolic decomposition can render the chemicals non-toxic to plants. When soils are well drained and aerated, a healthy population of aerobic microorganisms can accelerate this progress.

Wet, poorly aerated soil, very common in many urban areas, discourages microbial growth. Plants sensitive to the walnut tree's toxic effect may be at a higher risk when planted in heavy urban soils that lack organic matter. Toxins adhere to organic matter rather than being absorbed by plants, and organic matter also encourages a healthy soil microbial population.

Mycorrhizal fungi are commonly associated with forest tree roots and are considered necessary for normal uptake functions. Allelochemicals can disrupt the uptake process by damaging the root hairs or by inhibiting mycorrhizal populations in the soil. These different soil factors all have an effect on the accumulation or depletion of juglone produced by the black walnut tree.

Reducing Allelopathic Effects
Regularly clean up all fallen leaves and fruit from the black walnut tree, keeping debris away from desired landscape plants.

Compost plant debris to degrade any toxins present in the compost pile, and to detoxify the compost. If composting is impractical, do not use any part of the walnut tree as compost or mulch for other plants.

Maintain high organic matter levels in the soil because organic matter encourages healthy soil microbial populations that can metabolize toxins.

Plant tolerant trees, shrubs, vines, ground covers flowers and grasses under walnut trees, or in areas that might contain walnut roots.

Tolerant Grasses
Tall fescue and Kentucky bluegrass grow well near black walnut except during drought conditions when soil moisture is low. When moisture is adequate these grasses may grow better under walnut trees than in other parts of the lawn, possibly because the soil may be more basic. Soil under black walnuts tends to be alkaline, with the pH often 0.7 points higher than beyond the roots, thus influencing the growth of many different plants.

Tolerant Trees and Shrubs
arborvitae, Americanglobeflowerpine, Virginia
ash, whitegum, blackprivet
barberryhawthornred cedar, eastern
beech, Americanhemlock, Canadianredbud, eastern
birch, black: 'Heritage' riverhibiscussassafras
boxelderhickoryserviceberry
buckeye, Ohioholly, Americansilverbell, Carolina
catalpahoneylocustspruce, Norway
cherry, blackhoneysuckle, amur; tatariansumac
crabapplehydrangeasweetgum
daphnelilacsycamore
dogwood, floweringlocust, blacktulip tree
elderberrymaple, red; sugar; black; Japaneseviburnums (some species)
elm, Americanninebarkwitchhazel
forsythiaoak, white; red; scarlet 
fringetreepawpaw 
goldenraintreepear, callery 

Tolerant Vines, Ground Covers and Flowers
goldenrodraspberry, black
grape, wildrose, wild
hollyhockrudbeckia
hostascilla
hyacinth, grape; orientalsedum
iris, Siberianspeedwell
ironweedspiderwort
jack-in-the-pulpitSt. John's wort
lamb's earsunflower
liriopetrillium
lobeliatulip
May appleviolet
morning glorywisteria
mulleinyarrow
phlox 
primrose 

Plants Damaged by Juglone
applecrocus, autumnpeony
azaleaforget-me-notpine
birch, whitegrape, domesticpotato
blackberrylily-of-the-valleyrhododendron
blueberrylindenthyme
chrysanthemummountain laureltomato

References:

Brooks, M. G. 1951. Effect of black walnut trees and their products on other vegetation. Bulletin 347, West Virginia University, Agricultural Experiment Station, Morgantown, WV.

Coder, K. D., 1999. Allelopathy in trees. Arborist News 8(3):53-60.

Rietveld, W. J. 1983. Allelopathic effects of juglone on germination and growth of several herbaceous and woody species. J. of Chemical Ecology 9(2):295-308.@

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