Compost Magazine

Composting tips, advice and science.

Cartoon of a worm with eyes and a mouth drinking coffee in a backyard garden next to flowers.

The Garden’s Barista? Worms’ Impact On Coffee Grounds

Have you ever noticed all the gardening tips that flood tabloids at the start of the gardening seasoning?

You know the sort of thing –  “This one tip will supercharge your plants says expert” and the like. 

One that made the rounds this year was the use of coffee grounds, with ‘experts’ advising us to add them directly to our pots, vegetable beds, and flower beds.

However, this also prompted a response from other experts, warning that the coffee grounds could harm plants. 

One botanist, James Wong, even conducted his own experiment, dividing his beds into two and applying the coffee grounds to one of the beds. 

The results were not great:

“The crop yield and growth of pretty much everything in the coffee bed became noticeably worse within about two weeks of application. Plant growth slowed, some developed leaf yellowing, others defoliated and died. Seedling germination in some cases was almost completely inhibited.”

What’s the problem with coffee grounds?

Plate of coffee grounds held by gloved hands next to a flower in a pot.

Assuming that Wong added a lot of coffee grounds, his results were not surprising.

For some plants, caffeine can be beneficial and accelerate root growth. 

However, when the caffeine level rises above 0.1%, caffeine slowed down root growth and caused damage to these plants. 

Used coffee grounds contain several times that amount of caffeine.

Low quantities of coffee grounds are unlikely to do any harm – in fact, they can improve the soil. However, large quantities of coffee grounds are likely to do more harm than good.

But what about composting? Is caffeine another one of the long list of things we are always told not to compost?

Maybe not – at least when it comes to vermicomposting. 

Coffee grounds and worm composting: Can worms remove caffeine?

A number of studies have found that worms can successfully compost coffee grounds. 

For example, a study by Noor et al found that coffee grounds improve the quality of worm compost.

Few studies have looked at the impact on caffeine..

However, one recent study by Dume et al found that worms can reduce micro-pollutants in compost, including caffeine, by absorbing them into their bodies.

The study found they can absorb up to 72% of caffeine – along with a host of other micro-pollutants.

Vermicomposting with coffee grounds: Don’t over-caffeinate your worms!

Worms love coffee grounds. 

I also like adding coffee grounds to my own worm bin, as the grounds are already small and do not require further breaking up. 

Worms in a mixture of shredded paper, coffee grounds and mature compost.
My worms seem to like this mixture of coffee grounds, shredded paper and mature compost.

But in one respect worms are just like us – too much coffee is not good for them!

In fact, when Liu and Price tested composting coffee grounds, they found it could kill many of the worms in the compost bin!

However, when they combined coffee grounds with another material (shredded cardboard) worm survival dramatically increased. 

Harnessing the power of worms 

I am continually amazed by the benefits of worms in the compost heap

From killing pathogens to removing or neutralizing heavy metals, they do a lot more than create a high-quality soil amendment!

If you’re interested in creating your own worm farm, check out our guide to getting started with a simple DIY worm bin.


Do worms like coffee grounds?

Yes, they are often attracted to coffee grounds. In fact, some composters put coffee grounds around their compost heap to attract worms. This may be because of the nitrogen in the coffee, while the gritty texture may also help their digestion. 

Can worms survive on just coffee grounds?

No, and in fact the key is not to overdo it – the study by Liu, K. & Price found a large amount of coffee grounds can reduce worms ability to grow and survive. If you set a maximum limit of around 25% you should be safe.

Will coffee grounds make my worm bin acidic?

There is a bit of a myth that coffee grounds are acidic. Oregon State University found that acidity of used coffee grounds is between 6.5 to 6.8 – so close to neutral. In short, coffee grounds won’t make your worm or compost bin acidic!

How do you feed worm bins coffee grounds?

I’ve found that if you leave coffee grounds on the top of the bedding it can attract fruit flies. Instead, bury your compost in the bedding, or cover it with a layer of damp newspaper. 

Read next…

Worms of Steel: How Fast-Evolving Worms Can Remove Metal From Compost
Earth ​​Worms Can Eliminate Antibiotic Resistant Genes Says New Study
How to Build A Simple Worm Composting Bin In Just 20 Minutes


Adi, A. J., & Noor, Z. M. (2009). Waste recycling: Utilization of coffee grounds and kitchen waste in vermicomposting. Bioresource Technology, 100(2), 1027-1030.

Bravo, J., Juániz, I., Monente, C., Caemmerer, B., Kroh, L. W., De Peña, M. P., & Cid, C. (2012). Evaluation of Spent Coffee Obtained from the Most Common Coffeemakers as a Source of Hydrophilic Bioactive Compounds. Journal of Agricultural and Food Chemistry, 60(51), 12565–12573.

Dume, B., Hanč, A., Švehla, P., Michal, P., Pospíšil, V., Grasserová, A., Cajthaml, T., Chane, A. D., & Nigussie, A. (2023). Influence of earthworms on the behaviour of organic micropollutants in sewage sludge. Journal of Cleaner Production, 416, 137869.

Liu, K., & Price, G. W. (2011). Evaluation of three composting systems for the management of spent coffee grounds. Bioresource Technology, 102(17), 7966-7974.

Muratova, S. A., Papikhin, R. V., & Khoroshkova, Y. V. (2020). II International Scientific Conference “Plants and Microbes: The Future of Biotechnology” (PLAMIC2020). BIO Web Conf., 23, 03013.

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