Do you tip your used coffee grounds into your compost bin or directly into your garden, thinking that you're doing it a favour?
In the latest New Scientist, they claim this is a myth saying used coffee grounds may be doing our plants more harm than good.
NZ Gardener editor Jo McCarroll joined Sunday Morning to discuss pros and cons of putting grounds on the ground.
She cautioned against quick-fix thinking.
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“You hear that coffee grounds could be used wholesale, they're feeding your plants, they're wonderful mulch, they're all these different things.
“And coffee grounds are organic matter. And we love organic matter. We love all organic matter, we look for every opportunity to integrate it into soil.”
But coffee as a plant is allelopathic, she says, which means it produces a chemical that has an effect on other plants – which can be positive or negative.
“In the case of coffee and lots of other plants, it's negative, it releases chemicals, one of which is caffeine actually, which inhibits the growth and germination of other plants, and suppresses soil bacteria.”
It is unlikely, she says, that coffee grounds will boost the growth of young plants.
“Coffee grounds contain nitrogen, so there's always an element of truth, about 2 percent nitrogen and it also contains very fractional amounts of phosphorus and potassium, which as you all know, that's what plants want.
“But the nitrogen in coffee is not bioavailable, it's not in a form that plants can take up.
“Luckily, rather brilliantly, the soil is full of all these beneficial micro-organisms, which convert it to bioavailable nitrogen, they convert it to nitrate or the other kind of form ammonium, which is the other form of nitrogen that plants can take up themselves.”
Work done at the University of Melbourne suggested coffee inhibited growth, she says.
“They looked at I think four or five different plants broccoli, radish, leek, viola, and they tested them under various controls, including fertiliser, including coffee at different ratios, and under control, and coffee inhibited the growth of all of them.”
Coffee grounds won't deter pests either, McCarroll says.
“I'm sorry to say, I know we're all so desperate to get rid of slugs that we get credulous, but we have done some tests at New Zealand Gardener. Slugs and snails, they crossed it [the coffee], they didn't turn a whisker.
“And I would also say just while I'm throwing out truth bombs, eggshells do not work either.”
