How One Plant Can Reign Supreme Over Another
Bang! Zoom! The scent is in the air. And in a process called allelopathy, plants manufacture and discharge volatile molecules like terpenes to shape the behavior of their herbaceous neighbors, in a chemical dance that can have helpful or detrimental outcomes.
It’s like this. All around you, at all times, there is a furious war happening. You probably can’t see it unless you’re cosmic. And you may not be able to smell it. If you were a bug, however, you’d likely be able to inhale what’s happening in your community. You could also be annihilated by a shoe, but that’s a different story.
The battle that’s going on is mammoth, and a sizeable chunk of the instigation and the armaments are coming from the plants. We’ll talk on another day in more depth about the clarion calls placed by plants to predators of herbivores wild enough to eat the plant alive!This article pits floral brethren against one another, as domineering plants aim to capture their piece of the earthen pie.
The monoterpenoid camphor is found in the camphor laurel, evergreen trees, citronella, rosemary, sage, ginger. It flashes a pungent, musky, earthy fragrance.During the 14th century, camphor was used as a fumigant during the Black Plague. Thus, camphor can kill and camphor can purify. The camphor laurel itself presents a resilient, stubborn tree. The effects the tree can have on its surroundings help ensure its survival, via allelopathy. The leaves of the camphor laurel were evaluated against 52 plant species and 27 soil algal groups.  It turned out the leaves delayed the germination of adversarial plants, causing a decrease in radicle and shoot length. Several algal species disappeared from the soil, or at least the herds were thinned out. These effects have led to interest in camphor as a bio-based herbicide.
Once there was this wasteland weed who was interested in taking over the space, and likely the food, of its neighbors. The essential oil extracted from the neighboring wild marigold (aka stinking roger) influenced the ability of the wasteland weed to freely go forth and multiply.  One of the terpenes in the essential oil was cis–β-ocimene, which is also found in lavender, tobacco, mint, parsley, basil, and mango. And in the story of the wasteland weed, isolatedcis-β-ocimene, showing a stronger effect on the weed compared to the essential oil alone, inhibited the overall growth of the test seedlings, stopping the invader in its tracks. It did so by reducing germination, the process by which a plant sprouts from a seed.
Floral colonialization is going on all around you. Plants, just like animals (and us Earthlings), like to set up boundaries. And plants are willing to turn to phytochemical artillery to protect their borders should they need to.
Note: the image complementing this article shows the allelopathic effects of Eucalyptus globulus.
References Chen, W. “Camphor–a fumigant during the Black Death and a coveted fragrant wood in ancient Egypt and Babylon–a review”, Molecules, Volume 18(5): Pages 5434-54. [journal impact factor = 3.098; cited by 80]  Schenk, J. “Phytochemistry, allelopathy and the capability attributes of camphor laurel (Cinnamomumcamphora (L.) Ness &Eberm.)”, PhD thesis, 2009, Southern Cross University, Lismore, NSW. [journal impact factor = N/A; cited by 3]  Arora, R. et al. “Comparative account of allelopathic potential of essential oil of Tagetesminuta L. and its major component cis-β-Ocimene”, Annals of Plant Sciences, 2016, Volume 5(9): Pages 1428-1431. [journal impact factor = N/A; cited by 2]