What chemical or biochemical agents do plants use to inhibit each other's growth?
As with the massive variety of compounds that serve to protect plants against insect and animals, resulting in a wide range of poisons, antibiotics and other natural insecticides, allelopathic compounds are highly varied with different methods of action: some merely prevent germination while others are effectively herbicides.

One example is Alliolaria petiolata. In that species the allelopathic substance is not known, but is thought to ahve a fungicidal effect that impedes mycorrhizal development, which would seriously affect the development of many species.