Myth-busting – Coffee grounds in the garden: Friend or foe?

A question I am often asked by new gardeners is: Is it safe to use coffee grounds in my garden? In our caffeine-addicted society, coffee grounds – the ground up powder of coffee beans – are so ubiquitous . Your local cafe produces buckets every day and most households also generate a ready supply from their kitchen. 

In recent times, coffee grounds have gotten a bad rap in the media. I recently heard a popular prime time gardening show warn against using them in your worm farm because they are “acidic”. When I first started growing blueberries many years ago, I was given much similar advice – which is that you should use coffee grounds around blueberry plants to create the acidic conditions that they thrive in. 

You may also have seen some recent articles showing that using coffee grounds in the soil will stunt the growth of plants. As a result of this, many gardeners now think coffee grounds are toxic in the garden. In fact, many gardeners I talk to tell me that they now put coffee grounds straight into their landfill or green waste bins. 

Let me set the record straight.

We’ve been using coffee in our garden for many years. In fact, our entire garden is built on coffee – coffee grounds and coffee husks. We literally use a mountain of coffee in our garden: we get a delivery of 1-2 cubic meters of coffee grounds every year, and our lifeless sandy soil soaks up countless sacks of coffee husks.

No our plants are not suffering because of it – quite the opposite really, they are flourishing. And no our soil is not acidic, if anything our soil suffers with high pH which we need to correct. (Note: the high pH is not due to the coffee, but adding coffee has certainly not helped to acidify our soil to counteract our basic soils).  

The area behind our shed was transformed from lifeless to abundant through the use of coffee grounds

Let’s look at an example. In the area behind our shed, our soil is dry and grey sandy lifeless soil. In order to improve the soil, we piled on over 10cm of coffee grounds in winter. Just some months later in spring, we were able to grow 4 tomato plants, 6 beans and a pumpkin in this tiny space, with the only soil amendments being coffee grounds, coffee husks and horse manure. Quite a remarkable transformation given that even weeds struggled to grow before.

The benefits

I am a huge advocate of using coffee to improve your soil. We are both non-coffee drinkers, so quite ironic really that our entire garden is powered by coffee!

  • Coffee grounds are an incredibly valuable source of organic matter for your garden. They tick all our boxes for a soil amendment – they are rich in nutrients, widely available and sustainable. 
  • Coffee grounds bring life back to your soil – if you don’t believe me, put some coffee grounds into your worm farm or on a patch of soil, check back in a week or so and you’ll find it teeming with worms. 
  • As coffee grounds are so fine in texture, they break down quickly to add nutrients which your plants will lap up.  

Are coffee grounds acidic?

Absolutely not. It is a complete myth that coffee grounds are acidic when decomposed. Most things when decomposed tend towards pH neutral. 

We add coffee grounds in bulk quantities to our worm farm and regularly test the pH. The pH of our worm castings tests towards neutral or even a little basic (and no we do not add lime to our worm farm). Similarly where we add bulk quantities of coffee to our garden soil, our soil pH is still high – and yes there are other factors at play here. I am not saying coffee causes high soil pH. I’m simply illustrating that in the areas where we’ve added coffee grounds, the soil has not become any more acidic than places where we haven’t. [This is a topic I will be writing about later. If you have stunted plants, chances are your soil pH is wrong and likely too high].

Our blueberries thriving not because coffee grounds are acidic but because of the organic matter it adds to the soil

Do coffee grounds stunt plants?

No, provided you use them properly. There is one thing I want you to remember: While coffee grounds look like soil, they should never be treated as such. Rather, coffee grounds need to be thought of as an undecomposed product – just as you wouldn’t mix chopped up fresh food scraps or uncomposted manure into a potting mix, you also wouldn’t use coffee grounds in that way.

Young seedlings are particularly sensitive to soils that are too rich / not fully decomposed. This is the same reason you should only use the liquid from your worm farm on young plants with caution.

The experiment which showed coffee grounds stunting young seedlings was done by mixing coffee grounds into a potting mix. This is an experiment designed for failure. Now you understand what coffee grounds are i.e. an undecomposed raw ingredient, it shouldn’t surprise you that mixing coffee grounds into potting mix would stunt the growth of young seedlings. Had the experiment observed the plants over a period of 6 months to a year, I wouldn’t be surprised if they found that the plants enriched with coffee will eventually outperform those that haven’t been – once the coffee decomposes.

The right way to use coffee grounds

Add coffee to the top layer of your soil like you would mulch

Using coffee grounds is remarkably simple. Coffee grounds can be used as a top dressing on soil i.e. apply on the top layer of your veggie beds or under fruit trees, much like mulch. Gradually the coffee will compost on site and, once nicely decomposed, the worms will bring the goodness down to the plant roots.

We use coffee grounds liberally as alternating layers when creating lasagne beds, and we top dress fruit trees and garden beds with coffee grounds. In this way, they are not getting mixed into the soil (you shouldn’t be digging your soil between seasons anyway). Instead, they are layered on top much like a mulch, or the chop and drop method in permaculture. You will find that wherever you put coffee grounds, the worms will follow.

You can also safely add coffee grounds to your compost bin as a compost accelerator, and of course you can add in your worm farms. Your worms will love you for it.

The verdict

Coffee grounds are most definitely a friend not a foe. Used in the right way, they are extremely beneficial in the garden. They are free, abundantly available, and a powerhouse of nutrients for the soil and the life within the soil. And no they are not acidic.

2 thoughts on “Myth-busting – Coffee grounds in the garden: Friend or foe?

  1. Thank you for this information. Very helpful. Will the coffee grounds keep the snails away?

    1. That’s a great question. In fact we have conducted an experiment on snails using various snail deterrents including coffee grounds, which will be an interesting video that will be up soon on our Youtube Channel. Spoiler alert: Snails did not like the coffee grounds.

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