Allelopathy is one way in which some plants actively attempt to crowd out other nearby plants. Plants in the immediate vicinity of others compete for nutrients and water, but in some cases they can release chemicals that damage others nearby. These chemicals can change the way neighboring plants absorb nutrients, carry out various life processes, or develop root systems. Allelopathy differs from other types of competition because of the active release of chemicals into the surrounding environment.
The term comes from a Greek word meaning to suffer upon each other. Knowledge of the processes of allelopathy has been around since students of Aristotle discovered the tendencies of certain plants to kill those around them. Since then, researchers have discovered a number of different species capable of this process, and a variety of chemicals released in this biological version of chemical warfare.
Plants use a wide variety of methods to release chemicals damaging to their neighbors. Some release chemicals into the soil, where they are absorbed by neighboring plant roots and inhibit the processes of that plant. These chemicals can be particularly troublesome, as some can remain in the soil for years. Other chemicals are released through the allelopathic plant's seeds, leaves, or flowers; once these plant parts fall to the ground, chemical compounds are then absorbed into the target plants.
Not all plants have this capability, and not all chemicals have the same makeup. Walnut and pecan trees are intolerant of any plants at their base and near their roots; they release a compound called hydrojuglone. The compound is found throughout the entire tree, from roots to leaves, and is released into the soil via a number of delivery methods. The chemical does not tend to travel far from the root base of the tree, and plants can typically survive outside of the radius of the tree's canopy. Other common plants capable of allelopathy include the box elder, red and sugar maples, and hackberries.
A number of plants have developed a resistance to the chemicals secreted by plants capable of allelopathy. Flowers such as the daffodil and hyacinth are not generally affected by the chemicals secreted by the walnut family, yet others like lily of the valley will quickly turn yellow and wilt. When adding new trees and flowers to the landscape or laying out a fresh design, determining if any plants are capable of allelopathy and which plants are tolerant can be key in keeping everything healthy and green.