By G. Bosley  a.k.a.  “groundhog”

In 25 years of landscaping and gardening I have never had to deal with planting near a Black Walnut tree, but today I was asked the question.

Most plants of the Walnut family produce a chemical called “juglone” which occurs naturally in all parts of the plant, but can be especially high in the roots and hulls.  Black Walnut and Butternut produce the largest quantity of this chemical which can be toxic to many plants.  The highest concentration in the soil is directly under the canopy of the tree, but can be present anywhere there are roots.  The roots of a mature Walnut tree may reach 50 to 60 feet away from the trunk.  Young Walnut trees do not seem to be toxic until they are 7 to 8 years old.

Sandy well-drained soils tend to have a reduced concentration of juglone and some sensitive plants may not be affected unless their roots make direct contact.  Poorly drained soils tend to have a higher concnetrations of juglone and many plants are sensitive to the toxins.  Plants with shallow roots tend to tolerate being companions with these trees better than plants with deep roots.

Symptoms of toxicity range from stunting of growth, wilting, yellow leaves and death.  The toxic reaction often occurs quickly within one or two days and is irreversible.

To reduce the amount of juglone in the soil keep all debris from the trees cleaned up, especially the fallen leaves and fruit.  Do not compost any of this material.

Maintain high levels of organic mater in the soil by using lots of compost, leaf mould or manure.  This encourages beneficial soil microbes which help neutralize the toxins.

Plants which may tolerate Juglone –  Ajuga, Anemone, Burning Bush, Catalpa, Clematis, Shasta Daisy, Daylily, Forsythia, Hemlock, Honeysuckle, Hosta, Hydrangea, Iris, Juniper, Lilac, most Maples(including Japanese Maple), Meadow Rue, Mock Orange, Pachysnadra, Periwinkle, Phlox, Primrose, Quince, Viburnum, Virginia Creeper, Zinnia

Plants more susceptible to Juglone – Apple, Crabapple, Azalea, Birch, Chrysanthemum, Cotoneaster, Linden, Magnolia, Kalmia, Peony, Pine, Potentilla, Privet, Rhododendron, Norway Spruce

If you have Walnut trees the best solution is to build raised beds with a lining of plastic or fabric to prevent the tree roots from growing into the soil of the raised bed.

I welcome your comments.

www.rootsandshootsdesign.ca

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