When you look into your garden or backyard, be careful not to ignore your black walnut (Juglans nigra) tree. Lurking inside its leaves, fruits and roots is a toxic chemical made to control competition.

Pros and Cons

Black walnuts are valuable as shade and timber trees. They produce delectable nuts, too. But if your black walnut overhangs your garden, your tree may be out to get you.

The fruit, leaves and roots of black walnut trees contain a chemical, juglone, that can injure other plants. Ingesting even a small amount of pure juglone can cause a serious poisoning effect in humans.

Inside the tree, juglone is a clear liquid (called prejuglone) that's nontoxic. If the tree cells that contain this prejuglone are damaged, cut, or injured, it is immediately oxidized into its toxic form.

You can see this by cutting into the husk of a small walnut. It quickly changes from green to dark brown as it is exposed to the air. The clear prejuglone is rapidly oxidized to dark, reddish-brown juglone.

Insects, diseases and mechanical injury can cause prejuglone to be oxidized into its toxic form. Over time, juglone naturally leaks out of walnut roots, leaves and buds into the soil.

Not Just in Walnuts

By far the highest concentrations of juglone are found in black walnut parts. But many other members of the walnut family contain it, too. English walnut, pecan and the rest of the hickories all have small amounts.

The fruit husks contain the highest concentrations in any species. One of juglone's many purposes is to prevent pests from attacking new seeds.

Juglone's Effects

Some people and animals are susceptible to juglone damage. Consuming too many leaves or using walnut sawdust for bedding can cause a number of problems with animals.

Some people are especially sensitive. Cutting walnut lumber can coat you with sawdust. Everywhere this sawdust lands can produce a red welt on some people.

In the soil, the oxidized juglone will damage many living things, including plant roots. If it leaks back onto a walnut root, though, it is quickly made nontoxic again and stored.

Juglone is a respiration toxin as a herbicide. Once it's released into the soil, small amounts can damage and kill the roots of neighbors.

Beneath the tree, it severely damages annual plants, garden vegetables, fruit trees and some broadleaf perennials. Most grasses seem immune. Wherever walnut roots travel, though, they change the soil they move through by adding juglone.

Take Care in Your Landscape

For very susceptible plants, like tomatoes, potatoes and peppers, even walnut mulch can be damaging. Be sure to age or compost leaves, twigs, fruit husks and wood chips from walnut trees before adding them to a garden or landscape.

An oxidation and aging process converts all the prejuglone into toxic juglone. Then further aging and oxidation, under moist conditions, will break juglone apart into nontoxic pieces. Grind down black walnut stumps or remove them. Take away any chips or sawdust for composting.

Some trees, such as red maple, willow and apple, won't perform well on sites recently occupied by black walnut roots. Many plants won't grow well around living black walnuts or where the trees have recently lived.

One full growing season, though, is usually enough to eliminate most of the juglone from a healthy soil.