Mention the words "chemical warfare" or "deployed armies" in any conversation, and your interlocutor might immediately assume you're talking about wars between humans. In reality, however, there are other kinds of wars out there where these techniques are employed far more frequently and in a far more intricate manner: those waged in the plant kingdom. We might not normally think of plants this way, but much like humans and animals, they too have to fight for survival on a daily basis. Nutrients, light and water are the three things any plant needs in order to grow; unfortunately, none of these is ample in supply, which means that the competition between plants can grow fierce. Some plants and trees are at an architectural advantage: taller trees have greater access to natural light, while plants with deeper roots have the ability to absorb more water and nutrients. Others, though, manage to defend their territory through "allelopathy", or chemical warfare. So how does this chemical warfare work exactly? As Dr Robin Andrews explains, plants convert the nutrients they absorb from the ground to energy with the aid of a type of organic compound known as metabolites. These metabolites can be divided into two categories: primary and secondary. Primary metabolites are what allows a plant to live, playing a direct role in its growth and development, and are thus present in every plant. Secondary metabolites, on the other hand, can vary from plant to plant and often play the role of a defence mechanism against neighbouring competitors.