Allelopathy

Chemical warfare among plant species. Involves the suppression of growth by one plant on another plant due to the release of inhibitory or toxic chemicals. This can occur in several ways including: the release of allelopathic substances into the soil via root systems, the release of chemicals being washed off leaves by rainfall or even blown from plant to plant by the wind. 

Brittlebush practices allelopathy by producing a leaf-born water-soluable toxic chemical which inhibts the growth of neighboring winter plants.