Beneath the Black Walnut

Growing plants under 11 mature black walnut trees is a complicated puzzle that Liz and Art Williams have spent years solving. Now, they've created a colorful parade of blossoms that progresses from early spring through frost - despite the trees' toxic juglone, which is notorious for stunting and killing susceptible plants as far as 80 feet from the trunk.

"It's really been a challenge," Liz says. She remembers using a machete on the dense tangle of weeds growing under the trees when she began gardening at their home in Delafield, Wisconsin. "Now we plant varieties we know will grow and use containers for other things, like the hardy 'William Baffin' rose that I love."

An age-old problem

More than 2,000 years ago, the Roman agriculturist Varro observed that some plants failed to thrive near black walnut trees. Since then, the problem has perplexed gardeners through the ages. We now know that the culprit is juglone, an allelopathic compound that stunts or kills some plants - even other walnut seedlings. All native walnut trees exude the chemical (which, by the way, is harmless to humans).

Although juglone is found in all parts of the tree, it's most concentrated near growing roots. No wonder plants that are most affected by walnut toxicity are those growing close to the roots of the tree.

"Pruning roots or constructing barriers doesn't seem be an efficient use of time and money, because roots branch out," says Richard Funt, professor and Extension horticulturist at The Ohio State University. "Even if you think all roots are removed, some remnants may remain toxic in the soil for several years."

Complicating matters is the fact that symptoms of juglone poisoning vary, sometimes even mimicking other blights. Juglone inhibits respiration in susceptible plants, so even seemingly healthy specimens may suddenly die. In other cases, the foliage of plants such as tomatoes may wilt and turn yellow, and internal stem tissue may discolor. These symptoms mimic common tomato problems, such as verticillium and fusarium wilt.

Even today, research on this subject is limited, and information about suitable plants is based mainly on observations, some of which are conflicting. "Certain plants may grow shorter simply because of shade, not because of contact with black walnut roots," Funt explains. "Other plants will live under these trees but simply won't do as well as in different locations."