Allelopathy

The chemical interaction of a tree and its’ components on the surrounding soil, on other plants and on your garden is called Allelopathy. In this way, trees are no different than the plants and herbs in your garden. Your garden may be failing, not because of the soil or your efforts, it may be the nearby trees leaching that chemical in the ground.

One chief source of the most problems agriculturally is the Black Walnut (Juglans nigra) tree. The Black Walnut tree secretes a chemical called hydrojuglone. Ordinarily, hydrojuglone is nontoxic, but once oxidized becomes a highly toxic chemical called juglone to the surrounding plants. With drainage and root growth, plants as far as 5 meters away can suffer from juglone poisoning.

The leaves, husks and bark of the Black Walnut also release the toxic levels of juglone. But the Black Walnut is not the only tree with this chemical. Most walnut trees, pecan trees and even hickory trees produce it to varying degrees. Other trees with a similar effect on the soil include sugar maple, waxmyrtle, cottonwood, black cherry, red oak and the American elm and sycamore trees.

Be sure to locate your garden far from these trees or you will be facing an uphill battle in keeping your plants healthy. If your garden is already planted there are steps you can take to reduce the impact of these trees.

 

Trees that aid in absorbing the juglone include:

 

Although dangerous to many fruit plants some may actually do quite well in this type of soil including grapes and berries.

Just as the Black Walnut affects some plants negatively, the Leucaena trees, dubbed the Miracle Tree, have positive effects on staple crops planted near them.

The Miracle trees planted nearby have been shown to actually increase the crop yields of both corn and rice. For wheat crops, however, the Miracle trees reduce the crop yield through the chemicals they release into the soil. So these interactions between tree and soil and other plants can be positive for some plants and negative for others.

A raised garden however, with a substantial weed shield can help reduce the impact of allelopathy of the surrounding trees on your garden.