Coffee grounds in the garden 

What can you do with your old coffee grounds, after they’ve done their job inside your espresso maker? A frequently-asked question on talk-back radio is “Can I chuck my coffee grounds on the garden?” And yes you can – within reason! 

Are they acidic? Ever so slightly, but they won’t make your soil more acid over time. They certainly won’t turn your hydrangea flowers blue if they are normally pink. 

Are they good mulch? Not really on their own, and especially not in large quantities, as the fine particles will reduce the amount of air in the soil and hence increases water retention and “pugging”. The only good way to make good mulch from coffee grounds is by chucking them on a compost heap and incorporating them with other organic material.

Are they a good fertiliser? Yes, but weak in concentration: a couple of percent nitrogen and at most half a percent of Phosphate and Potash. These elements are tied up with larger molecules, so their chemical bonds need to be broken before the N, P and K are released. That makes them a slow-release-weak fertiliser. Nothing wrong with that, but don’t expect too much of this.  

Will they make plants thrive? Yes, mulch is beneficial and weak fertility benefits are also good for the plants and our planet. But we’ve also found out that certain seedlings (especially tomatoes!!) do not like to grow in “coffee grounds habitat”. So best to keep the grounds away from seedlings.

If you add all this together you would come to the scientific conclusion that coffee grounds are best composted with other materials… and then they will contribute positively to the growth and health and happiness of the plants in your garden 

There are some myths around coffee though: 

Myth: Coffee grounds prevent weeds. Any mulch will prevent weeds. But… if coffee grounds are added to this, they will encourage plant growth… and weeds are plants! 

Myth: Coffee grounds repel cats. Yes, but only if you administer the stuff orally! 

Myth: Coffee grounds kill slugs and snails. Absolutely no evidence of that! Not even as a “barrier” around a plant you want to protect. Slugs and snails do not walk over a substance, they slither over that substance with a layer of mucus: the foot of the mollusc never comes in contact with the substrate. What has been shown is that a spray with a high concentration of caffeine will indeed kill molluscs; the grounds have a very low concentration of caffeine and are absolutely useless. 

Myth: Coffee grounds kill insects? Once again: caffeine in high concentrations does; the low concentration in the grounds are harmless. 

Myth: They can suppress fungal problems? The scientific debate is a bit vague, but so far there is no hard evidence that coffee grounds suppress fungal diseases in plants. 

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