What Not To Grow With Walnut Trees

For nearly 2,000 years there has been documented evidence of what not to grow with walnut trees. Pliny the Elder, the famous historian wrote “The shadow of the walnut tree is poison to all plants within its compass”. This allelopathic property is a toxin that helps to preserve the walnut tree from competitors.

Back in the 1800s scientists discovered the compound juglone was responsible for killing near by plants. This compound is found in the leaves, branches, fruits, and is also excreted from the roots. Not all plants are bothered by juglone, but those that are will suffer from yellowing leaves, stem wilting, and death.

Which Types Of Walnut Produce Juglone?

All members of the walnut family produce juglone. Including English walnuts, butternut (white) walnuts, pecans, and black walnuts. With black walnuts having the highest concentration of juglone.

Walnut trees contain the chemical hydrojuglone which is non toxic, but when exposed to air or soil it gets oxidized. It then turns into juglone which actually starves plants of oxygen and water. Juglone doesn’t dissolve very well so it stays highly concentrated at the base of the walnut tree.

For many years after the removal of the tree the area will still contain juglone making it impossible to grow certain plants. This is due to the residue roots releasing more of this compound as they decay in the soil.

Of course juglone is not the only reason plants don’t grow within the drip line of walnut trees. Due to the leaf canopy, light restriction is another factor, as is high water uptake from mature trees.