Coffee grounds can be good for plants — but be careful
There's a good way and a bad way to use coffee grounds in a garden. Here's what to know.
A popular gardening tip involving coffee grounds has some truth to it, but be careful.
Coffee grounds aren't "magic pixie dust," but in the right circumstance they can help improve your soil according to Linda Brewer, a soil scientist with Oregon State University's Department of Horticulture.
"A little is fine, a lot is probably not fine," Brewer said. "As with everything, the dose makes the poison."
A good way to use coffee grounds in your garden
In small doses, coffee grounds can add carbon and bulk to the soil, but not much else. Despite some internet claims, they're not a substitute for well-rotted compost or fertilizer.
"A good way to use them in the garden is to put no more than half an inch of grounds on the surface – and I would keep them away from the stems of plants," she said.
"It’s a good carbon source for soil bacteria," she said. "But coffee grounds are very low in nitrogen, potassium, phosphorous, the big three."
What happens if you go overboard with the coffee grounds?
Too many grounds could be harmful to the very insects that keep soil healthy. "The coffee grounds contain caffeine and caffeine is a chemical, it’s exposure to a drug. Too much caffeine can be toxic," Brewer said.
The grounds might keep slugs and snails away – good – because they are coated in a mucus membrane that separates them from the outside world and they wouldn't like the caffeine. But it might also drive earthworms away – bad.
She visited one home gardener who had gone overboard. She found that as much as 40% of the soil in his raised beds was coffee grounds.
It did not go well for the plants.
How to prepare your coffee ground soil
The best way to deal with coffee grounds, Brewer says, is to compost them along with your kitchen and yard waste. There's a reason gardeners swear by the benefits of composting. "Just monitor it for water, give it an occasional turn," she said. "That's all you have to do."
But if that's too much, and if you find yourself with a surplus of coffee grounds, then by all means give your garden a sprinkle – no more – now and then.
"Remember that soil microbes are going to benefit from a diverse diet," she said. "Anything – anything – can be overdone."