When Plants Attack: Allelopathy

Everywhere I go lately, I seem to be tripping over the idea of allelopathy. The amaranths in the picture above might look like pretty, harmless and useful plants – but they’re hiding a darker secret. During our class field trip to the Eden Project in December I discovered that they produce chemicals that can inhibit the growth of other plants nearby. This is allelopathy – chemical warfare that gives plants a competitive edge over their neighbours.

In class today we were learning a little bit about plant chemistry, and these allelopathic compounds are considered to be ‘secondary metabolites’ – that is they’re not directly involved in the plant’s growth and reproduction. Plants produce a lot of these phytochemicals, with different functions. Some are toxins to do in the competition or fend off pests, others are colours and odours to attract pollinators and some are protective compounds. Some of them are toxic to humans, others are pleasant and sought after and many have medicinal benefits.

You may well have encountered your own examples of allelopathy. The black walnut (Juglans nigra) is infamous for being a difficult tree to grow things under – and that’s because it produces an allelopathic chemical called juglone, although there are some plants (including beech, birch and some maples) that are immune to its effects.

One of the books I read over Christmas was A Tohunga’s Natural World, by Paul Moon, which is about traditional plant use among the Maori in New Zealand. Hohepa (the Tohunga) notes that nothing grows under pine trees, and that pine needles kill a lot of plants, and it’s true that some pine species are allelopathic.

And if you’ve used Hungarian Grazing Rye as a green manure then you may have been trying to benefit