The majority of inhibiting chemicals are produced as secondary substances by plants and released into the soil through the roots or leaf wash. The suppression of growth through the release of chemicals by a higher plant is known as allelopathy. Thus, alleopathy (allelon = each other + pathy = suffering) means chemical control of distribution among plants.
The release into the environment by an organism of a chemical substance that acts as germination or growth inhibitor to another organism. Typical substance include alkaloids, terpenoids and phenolics (secondary metabolites). The phenomenon was described originally for health and scrub communities, notably the Californian chaparral, but is now thought to be a wide spread anti-competition mechanism in plants(e.g., barley inhibits competing weeds by means of root secretions). It is however, extremely difficult to demonstrate in natural ecosystems
Alleopathy is also found in other organisms (Example: Antibiotics may be produced by fungi to inhibit competing bacteria, when the term antibiosis may be used)
walnut tree produces a non toxic substance, juglone, which is found in its leaves, fruits and other tissues. When the leaves or fruits fall on the ground, juglone is released into the soil, where it is oxidised to a substance that inhabits the growth of certain under story species and garden plants such as broad leaf herbs, and favors others such as blackberries and blue grass.
In Souther California, Salvinia leucophylla leaves emits some volatile oils which reach the soil surface and ihibit the germination of seeds of other species and the inhibitory effects persist for several months.
Clerodendrum is also exert allelopathic potential on several weeds such as Abutilon indicum, Amaranthus spinosus, Cassia tora, etc