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 …