WANDERING around an achingly cool San Francisco coffee shop a few years ago, I was fascinated to see huge, open-topped barrels filled with used coffee grounds and a sign saying they were free for customers to scoop into recycled bags and take home to perk up their plants. I realised that we had reached peak hipster.
Indeed, the claim that coffee waste dramatically boosts plant growth has been a staple of organic gardening books since at least the 1970s, and seems to be seeing a modern renaissance. Proponents everywhere wax lyrical about how the spent grounds are not only rich in nitrogen – a key plant nutrient – but can help lower the pH of garden soils for species whose roots require acidic conditions, like blueberries. It all sounds like such a brilliant idea: upcycling industrial waste into free, organic fertiliser. It is just a shame that in reality it is probably doing the exact opposite. Let me explain…
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Coffee grounds, even after brewing, are still a rich source of caffeine. This compound seems to be produced by coffee bushes – at least in part – as a herbicide to suppress the growth of smaller competing plants. The phenomenon is called allelopathy and is a strategy loads of plants have evolved to help reduce the competition for light, space, water and nutrients around them.
Leaching out of the grounds, the highly soluble caffeine percolates through the soil and has been repeatedly shown to severely stunt the growth of small, neighbouring plants’ roots and slash the rate of seed germination, even at relatively tiny concentrations. Not exactly what the plantfluencers of social media promise.
The weirdest thing about how often this tip is recommended is that we have known about the allelopathic potential of caffeine in coffee plants for decades. The first I could find on this was from 1980, and its conclusions have been echoed by study after study. So effective is caffeine at suppressing plant growth, it has been investigated as a potential novel herbicide for agricultural use, both in the form of direct application of coffee grounds on farms in and even tea leaves (which also contain caffeine) on plantations in .
This effect is so pronounced that over years of intensive cultivation, the accumulated caffeine in the soil of long-established coffee farms can reach levels where it doesn’t just hamper the growth of small weed seedlings, but even the . Studies have been to see if underplanting coffee with more resistant herbs, like sage and oregano, could help reduce the caffeine contamination in the soil by drawing the compound up through their roots, while providing farmers with an extra crop to harvest. I wonder if the world is ready for caffeinated herbs. But that’s another matter.
James Wong is a botanist and science writer, with a particular interest in food crops, conservation and the environment. Trained at the Royal Botanic Gardens, Kew, he shares his tiny London flat with more than 500 houseplants. You can follow him on Twitter and Instagram @botanygeek
For other projects visit newscientist.com/maker.
