By Barrie Anderson

I can still hear the pop and thump as the vehicles zipping down the country road crushed the hulls of the Black Walnuts that my siblings and I (all hiding in the daylily patch) threw at the oncoming traffic. Most times though, the hulls were so impenetrable the tires would simply send the green, oval glob flying straight back into the ditch.   Even the school bus was unable to crush open these unripened, impervious shells…much to our chagrin! The stains on the road from the few Walnuts that were mashed, were there for weeks. That’s because Juglans nigra or Black Walnut contains a high amount of tannins that stain whatever it touches… clothes, pavement…cars! 

Juglone is the most prevalent biochemical in the Black Walnut. It’s also in Juglans cinerea or Butternut tree as well as Carya ovata, commonly known as Shagbark Hickory. However, these latter two trees have far less juglone in their system compared to the Black Walnut. It is the Black Walnut that carries a negative connotation for gardeners because in the landscape Juglone is the oppressor to many other plants.  This tree,  plays the allelopathy game, which is where one plant inhibits other plants by producing and releasing its own biochemical, and, as such, the Black Walnut floods its system with this evil weapon.  Its highest concentration of Juglone is in the roots and stems of the tree. Similar to a basketball team having backup players on the bench, the Black Walnut also has its bench player, Juglone, hanging out in the buds, and leaves.  So, if the roots and stems fail to deter the other “team”, which is other competing plants, the buds and leaves charge off the bench wiping out any competition. Juglone prohibits respiration, which is the beginning process for germination, cell division, reproduction and water and nutrient uptake. This chemical release is a defense mechanism against herbivory, or herbivores that would otherwise munch the tree down to nothing, destroying it, as well as eliminating other plants.  Because Juglone is not very soluble in water, it remains in the soil for a very long time, thus enabling the Black Walnut to eliminate any future competition. Even after a Black Walnut has been cut down, the stump ground out and completely removed, Juglone will remain in the soil for several years. Gardeners must wait 3-4 years before planting in that area again, and bringing in new soil is a must. 

Fear not though, because there exists plants adept at tolerating Juglone as well as innovative planting methods to aid the garden if a Juglone carrying tree cannot be removed and limited garden space is available. 

One method is to plant in raised beds. This will minimize contact with the roots of a Black Walnut and ease the discomfort on your back if the raised bed is high enough. For raised beds not that high off the ground, gardeners should line the raised bed with some type of landscape fabric to prevent as minuscule an amount of Juglone from entering into the garden bed. Put a load of soil on top of that, and the raised bed is good to go!

Another mode is to clean up any leaves, stems, hulls, and nuts from the Black Walnut so that doesn’t enter into the protected garden.  While this may be quite the undertaking, it is well worth the effort to keep a garden happy, and thriving. 

Also, when mulching anywhere in a garden, avoid mulch containing Black Walnut. Even if the tree has been chopped to bits and ground up for mulch, the toxicity of Juglone will remain for a long time to come. Better off to mulch with shredded Cedar or chunky Pine. 

The best part of this whole Juglone concern is there are actually plants that simply shrug off this chemical and live happily ever after!  See the following for durable plants that can take on the Juglone giants, and please note, this is not a complete list. 

Trees

Acer negundo – Boxelder

Acer saccharinum – Silver Maple

Acer rubrum – Red Maple

Amelanchier – Service Berry

Betula nigra – River Birch

Carpinus caroliniana –   Hornbeam

Castanea dentata – American Chestnut

Cercis canadensis – Redbud

Cretagus crusgalli  –  Hawthorn

Magnolia acuminata – Cucumber Tree

Platanus occidentalis – Sycamore

Quercus alba – White Oak

Quercus rubra – Northern Red Oak

Prunus Americana –   Wild Plum

Salix x. – Willow

Ulmus Americana – American Elm

Evergreens

Juniperus chinensis – Chinese Juniper

Juniperus comunis – Common Juniper

Thuja occidentalis – Arborvitae

Tsuga canadensis – Eastern Hemlock

Vines

Celastrus scandens – American Bittersweet

Clematis x – Clematis

Lonicera sempervirens – Honeysuckle

Parthenocissus – Ivy

Wisteria – Wisteria 

Shrubs

Cornus alternifolia – Pagoda Dogwood

Forsythia x – Forsythia

Hibiscus syriacus – Rose of Sharon

Philadelphus coronarius – Mockorange

Physocarpus opulifolius – Ninebark

Rhus aromatica – Fragrant Gro Lo Sumac

Rhus glabra – Cut leaf Smooth Sumac

Ribes alpinum – Alpine Currant

Sambucus racemosa– Elderberry

Yucca filamentosa – Yucca 

Perennials

(Note: spp indicates multiple species)

Achillia millefolium – Yarrow

Ajuga reptans – Bugleweed

Alcea rosea- Hollyhock

Anemone tomentosa – Anemone 

Asarum canadense – Ginger 

Asarum europaeum – Ginger

Asteracae – Asters

Astilbe chinensis – Astilbe

Astilbe japonica – Astilbe

Campanula spp – Bellflower

Chrysanthemum spp – Mums

Doronicum spp – Leopard’s Bane

Dryopertis spp – Wood Fern

Echinacea purpurea – Purple Coneflower

Epimedium spp – Barrenwort

Eupatorium spp – Joe Pye Weed

Galium odoratum – Sweet Woodruff

Gentiana spp – Gentian

Geranium spp – Cranesbill
Helianthus spp-  False Sunflower

Hellebore spp – Lenten Rose

Hemerocallis spp – Daylily

Heuchera spp – Coral Bells

Lobelia spp – Lobelia

Mentha piperita – Peppermint

Oenothera spp – Evening Primrose

Osmunda cinnamomea – Cinnamon Fern

Phlox paniculata – Upright Phlox

Polemonium reptans – Jacob’s Ladder

Polygonatum spp – Solomon’s Seal

Polystichum spp – Christmas Fern

Primula spp –  Primrose

Ranunculu spp – Buttercup

Rudbeckia spp – Black – Eyed Susan

Sedum spp –  Upright Sedum

Solidago spp – Goldenrod

Stachys byzantina –  Lamb’s Ears

Thalictrum spp – Meadow Rue

Trillium spp –  Trillium

Uvularia spp – Bellwort

Viola spp –  Violet

Bulbs

Galanthus nivalis –  Snowdrop

Iris siberica – Siberian Iris (more of a rhizome than bulb, but threw it on this list)

Muscari spp – Grape Hyacinth

Select Narcissus – Daffodils

Select Tulips – Tulips

Scilla siberica –  Siberian Squill

Vegetables

Beans

Beets

Corn

Melons 

Onions 

Squash

Juglone in the landscape is not insurmountable. With creative planting methods, durable, resistant plants, a garden will still be quite competent in thriving! Just a follow up, yes, we really did throw the unripened walnuts at passing traffic…. a suggestion by one of our parents as we were driving them crazy in the house. Oops.