Planting Harsh Sites- BLACK WALNUT TOXICITY

Living with Black Walnut Trees

When dealing with mature trees many times we just think about the shade factor and if the tree has dense or dappled areas under it. Also root competition comes to mind as well as dry ground from rain being caught in the canopy before it can get to the soil. But some trees emit substances that can negatively affect near by plants and Black Walnut is one of the most notorious for harming it’s neighbors.

As a popular choice for shade canopy, Black Walnut is also grown for its lumber production and dye properties in its ‘fruits’. The concern lies in the production of Juglone which is in the roots, leaves, nut husks and even carried by water dripping of its leaves during rain. Areas effected by a tree go well beyond the canopy through the root structure and may effect nearby roots as far as ½” away.

The only real solution to black walnut toxicity is to install tolerant plants. Fortunately there a few plants that show little or no effect in relation to Juglone toxins. Many symptoms are a mimic of diseases or physiological disorders so look up before diagnosing a plant. Even cutting the tree won’t solve the problem since the roots still contain the chemical.

Plants to steer clear of are: Nightshade (tomatoes, peppers etc), yews, Pines, Azalea, Potentilla, Hydrangeas and lilacs. Some resistant to Juglone include: Cherry, Crabapple, maples, dogwoods, carrots, onions, snap beans, arbor vitae and daylilies. Checkout the Morton Arboretum  for a comprehensive list of plants.

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