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.
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