An allelopathic organism is an organism which can produce chemicals which interfere with the growth of other organisms. This term was originally used specifically in reference to plants, but researchers have learned that fungi, bacteria, algae, and corals also demonstrate allelopathic traits. Using allelochemicals, these organisms can actually shape their surrounding environment.
This term comes from a Greek root meaning “harm,” and many allelochemicals are, in fact, harmful. In a classic example of allelopathy at work, a walnut tree produces chemicals which inhibit growth beneath the tree. People who have seen walnut orchards or standalone walnut trees may have noted that nothing grows directly under the tree as a result of the chemical secretion from the walnut.
Plants can use allelopathy to kill off or contain competition, ensuring that they get more resources. In other cases, allelopathy can actually be beneficial, promoting growth or health in some way. For example, marigolds produce allelopathic chemicals which make them resistant to insects, which means that other plants growing around marigolds benefit from these chemical secretions even though they don't produce them.
Researchers have identified a number of allelopathic organisms and researched the chemicals they produce and their effects. This research is of special interest in the realm of agriculture, as allelopathic plants can be planted or avoided, depending on the situation, to increase crop yields or to keep crops healthier. For example, organic gardeners use allelopathic flowers which control insects and weeds in lieu of pesticides and herbicides.
Being aware of the allelopathic nature of some trees and plants is also important in landscaping. Some species cannot be planted together, or must be planted with care to ensure that they do not interfere with the surrounding landscaping. Eucalyptus trees, for example, tend to suppress growth below them, which can make them unsuitable for some kinds of gardens, or desirable in others.
Alleopathy is only one method plants have for shaping the environment around them. Plants also compete in other ways, ranging from producing roots which snatch nutrients before other plants can get them to growing in a way which shades the ground, limiting access to sunlight and water for plants growing directly under them. Many organisms can also create symbiotic relationships which are mutually beneficial. Plants, fungi, algae, and bacteria are actually like little gardeners, creating optimal conditions for success with a variety of techniques and actively changing the landscape around themselves.