When I first started writing this, I thought, “Of course!” I grew up hearing that you should put your used coffee grounds around your roses, and across the Internet, garden bloggers rave about the nitrogen properties of coffee.
You can’t imagine my surprise when I quickly found articles that warned me against using spent coffee grounds — they’ll kill your whole garden. But if spent coffee grounds so universally killed plants and earthworms, how on earth does this myth keep getting spread?
It turns out, whether you should use spent coffee grounds in your garden is a lot more complicated than the emphatic yes or dire no that I’d seen so far. There’s no simple answer and a lot more questions.
In this article, I’ll help you navigate the facts from the myths by examining the research.
Studies show that spent coffee beans prevent the germination of Chinese mustard, broccoli, leek, radish, viola and sunflower. (And that’s just what was tested in these studies. The list is probably a lot longer.)
This is presumably because of the caffeine. Coffee trees didn’t evolve their caffeine content survival mechanism in order to get humans to plant them everywhere.
Caffeine actually reduces the plant growth of competitors, much like the allelopathic compound juglone which keeps a wide variety of plants from growing near black hazel trees. If few plants can grow near them, they don’t have to compete.
Even spent coffee grinds still contain enough caffeine to reduce plant growth. This study found in the Journal of Agricultural and Food Chemistry found that caffeine ranged from 3.59 to 8.09 mg per gram of spent coffee. That’s almost as much as a cup of earl grey!
But that’s not the entire story. Some plants adapted to grow alongside black hazel trees, and you can confidently grow squash and beans underneath these trees.
Another study found that spent coffee grounds improved germination rates for sugar beet root and soybeans.
One research study by the University of Melbourne published in 2016 found that applying spent coffee grinds to the soil in urban vegetable gardens results in the vegetables growing poorly, no matter what variations they made with compost and other amendments.
For an anecdotal experience, botanist and science writer James Wong reported in the Guardian that, “The crop yield and growth of pretty much everything in the garden bed became noticeably worse within about two weeks of application. Plant growth slowed, some developed leaf yellowing, others defoliated and died.” Yikes.
Caffeine may be the culprit once again. Caffeine reduces the ability of plants to form roots. Even at lower levels of caffeine, there’s a significant decrease in the number of roots and root length.
The exceptions are cabbage and soybeans, which showed improved growth and yield, and there are probably others.
Here’s an incomplete list of what studies have shown to improve or deteriorate with spent coffee grounds directly applied to the soil:
Improved with spent coffee grounds
Deteriorated with spent coffee grounds
Earthworms are a required part of the soil food web, as they aerate the ground and break down organic matter — the very thing that we organic gardeners are trying to encourage.
This anti-coffee grounds article on Discovery pointed to a study by the Nova Scotia Agricultural College, arguing that this study had “inadvertently found that compost spiked with coffee grounds kills earthworms.”
Seems pretty damning. Or is it? It seems weird that thousands of vermicomposters would add coffee grounds without noticing mass deaths.
It turns out that feeding a vermicompost bin only with coffee grounds will kill earthworms. So don’t do that. By following common advice of providing different food sources and a lot of carbon bedding, survival and breeding increase.
Another study on vermicomposting coffee grounds [PDF] found that there wasn’t a big difference in earthworm survival in a vermicompost, whether you add ⅛ of the total volume or ¼ or no coffee grounds.
Another common myth is that coffee grounds can suppress diseases in plants, and that actually turned out to be true.
Studies found that when coffee grounds were added to a compost mix (even containing as little as 0.5% of the total volume), some suppression occurred of common pathogenic fungal rots and wilts like Fusarium, Pythium, and Scloertinia species as the coffee grounds decomposed.
Researchers suggested that this is because other non-pathogenic funguses and molds prevent the above pathogenic fungal rots and wilts from establishing.
Another study found that covering ripening cheeses with coffee grounds prevented bacterial pathogens like E. Coli and Staphylococcus spp. from establishing too.
But don’t get too excited yet. This disease suppression has only been demonstrated under controlled laboratory conditions with a handful of vegetable crops. We don’t know if this works in the garden or even works outside of these few vegetable crops.
So, in the meantime, don’t depend on coffee grounds to suppress disease.
Coffee grounds have a pH around 6.2 to 6.8. That’s not very acidic (7 is neutral), and it’s not enough to change the pH level of your soil.
Studies on composting coffee grounds have measured pH levels everywhere from mildly acidic 4.6 to a neutral 7.7 to even mildly alkaline 8.4 [PDF], and even that the pH level can go up and down during decomposition.
That’s good news for the microbes stabilising your compost pile, as they do best with material that’s between pH 6.5 and 8.0.
But if you’re trying to make the soil more acidic for acid-loving plants like blueberries, you’ll need to find another method.
Used coffee grounds are a great free resource to put to work in your garden. This video shows you the reasons to use them:
If you’re looking for an organic method of keeping slugs out of your vegetable garden, coffee grounds are not it.
Robert Pavlis of Garden Myths did a quick experiment in his garden by placing three slugs in a circle of coffee grounds. Two of the slugs immediately crossed the circle with no ill-effects.
You’re better off trying beer traps and copper barriers.
In another 3 experiments, Robert Pavlis tried to get rid of ants nesting in inconvenient places by sprinkling coffee grounds around ant nest entrances.
He found that the ants avoided the grounds in the short term, but they’d move the grounds overnight. The only time that he had moderate success was in moving ants starting up a new nest — and he also included a mix of sugar and borax which does work on ants.
Outside of the possibly killing your plants thing, they’re a terrible mulch. They’re ground up and easily compacted, making it difficult for oxygen and water to get through. Go for another mulch instead.
Outside of a few definites (don’t use as mulch, it won’t kill slugs or ants), there’s a lot of suggestive research, and a lot we still don’t know about the effect spent coffee grounds have on plants and soil health.
If you want to use spent coffee grounds, following these general guidelines will help avoid the worst effects:
- Don’t apply coffee grounds directly to the soil that your plants are growing. If you’re going to use them, compost them first.
- If you compost, keep coffee grounds to 20% or less of the total compost volume, whether you’re hot composting or vermicomposting.
- Allow coffee grounds to cool before adding them to any compost system. The hot temperature can burn beneficial microbes and earthworms.
- Don’t depend on coffee grounds to suppress diseases in the garden. These results have only been found in the laboratory.
- Only use spent coffee grounds, not fresh. Fresh coffee grounds will have a more disastrous phytotoxic response.
In the end, the best advice I can give you: don’t believe everything about gardening you read on the Internet, especially if it doesn’t cite sources. And always check to see what the up-to-date research is as something new is always coming up in horticulture.