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 from its allelopathic effects – the decaying foliage inhibits the germination of small seeds for a while, although larger seeds and transplants are unaffected. And so Hungarian Grazing Rye can help you with your weed control, as well as breaking up heavy soil and adding organic matter.

I think it’s fair to say that allelopathic effects are probably more widespread than we realise, and it’s an area of ongoing research. Garden Organic have an interesting factsheet on the potential of using allelopathy for weed managment. They note that possible allelopathic effects (e.g. poor seed germination or impaired root and shoot growth) can be caused by other factors, and it’s not always clear whether a weed-suppressing effect is being caused by allelopathy or something else (although if the weeds are suppressed, in some ways it doesn’t matter!).

And whilst we’re investigating using allelopathy to solve some of our weed problems, it is creating others. It seems likely that the success of many of the truly invasive weeds that are causing problems in different parts of the world is at least partly due to their allelopathic chemicals affecting the native flora. For something like Japanese knotweed control, for example, allelopathy is not our friend, it’s our enemy.

It’s easy to think of the animal kingdom as red in tooth and claw, and the plant kingdom as much more passive and cooperative, but the actual picture is more complicated. Wherever you look, you can find examples of organisms working together, and organisms trying to get the upper hand.

Time permitting, I will write a few more posts in my “When Plants Attack” series in due course, looking at some of the other ways plants get rough.