‘Used coffee grounds – left over from using a coffee maker – contain a substantial amount of nitrogen, as well as potassium and phosphorus,’ says coffee expert Lewis Spencer of Coffee Direct (opens in new tab).’ These properties make them perfect for garden activities, such as composting. It’s an innovative way to make use of something that would otherwise have ended up in landfill.’  However, its power doesn’t end there. Here’s everything you need to know about this organic garden idea – according to those in the know.  

Are coffee grounds good for plants? Everything you need to know

So, we’ve established that coffee grounds are good for plants. But what does the process involve? And does it work on all greenery? You can boost your plants with our expert tips below.

How to use coffee grounds as fertilizer

Did you know that your coffee grounds can be used as a slow-release fertilizer? ‘I always use coffee grounds as fertilizer,’ says James Gray, founder of Barista & Co (opens in new tab). ‘Some sizes of grounds can’t go down the sink, so giving them to your plants is a great way to reduce waste.’ Lewis Spencer adds: ‘To use coffee compost, simply sprinkle the grounds directly onto your soil and lightly rake it in. Coffee grounds add organic material to the soil, helping water retention, aeration, and drainage. ‘Leftover diluted coffee can create a liquid plant fertilizer, too. Simply mix two cups of brewed coffee grounds with five gallons of water in a bucket overnight.’

How to make compost with used coffee grounds

If you are investigating how to make compost, the experts recommend adding coffee grounds to your ingredients.  ‘Scientists state that a balance of ‘greens’ and ‘browns’ is needed to create the proper environment for composting to occur,’ say the plant doctors at Patch Plants (opens in new tab).  ‘Greens’ are nitrogen-rich materials that are used by microorganisms in the soil for their growth and reproduction, and ‘browns’ are carbon-rich materials used to feed microorganisms and give them energy.   ‘Greens’ include items like fruit and vegetable peels, and used coffee grounds. ‘Browns’ include items such as dried leaves, twigs and newspaper.    ‘When mixing green and brown together you should remember the ratio 1:4 (1 part green, 4 parts brown). If you have too much green material your compost pile will start to smell (a bi-product of microorganism reproduction is ammonia). If you don’t have enough green material, the compost pile won’t heat up because the microorganisms don’t have enough energy to do their thing.  ‘After about three months your old coffee grounds will have been transformed into nutrient-rich compost giving your plants a much needed boost.   ‘Remember to mix your compost thoroughly. If you leave coffee grounds on the surface of the soil, without raking them in and exposed to the air, they can dry out. Dried-out coffee compacts and will act as a barrier, preventing water from reaching the soil beneath. So mix-mix-mix and wait.’ If you practise vermi-composting with a worm bin, coffee grounds are a must as worms love them. For a small bin, add a cup of grounds per week to feed their addiction. Avoid adding too much at once because the acidity could negatively impact your worms. Paper coffee filters can even go in too.

Which plants like coffee grounds most?

Acid-loving plants in particular love coffee grounds because they will lower the pH level of soil, causing it to become more acidic,’ says Jason White, the CEO at All About Gardening (opens in new tab). ‘Your roses, azaleas, lilies, and hollies because they are acid-loving plants that will thrive best in the acidic characteristic of coffee grounds,’ he adds.  The plant doctor at Patch Plants adds that many plants will benefit from a coffee compost that follows the correct 4:1 method. ‘[Ensure] it’s not just put on the top of the soil where it’s left to harden and prevent water from entering the soil.’ However, while coffee grounds have their undeniable benefits, Lindsey Hyland from  Urban Organic Yield (opens in new tab) warns that this trick does not work for every plant.  ‘Tomatoes and rhododendrons are coffee-ground sensitive and may develop brown leaf tips if they come into contact with the grounds,’ she says. So, if you’re looking growing tomatoes, it may be better to keep remnants of your brewed beverage away.  Similarly, Kate Russell from the Daily Gardener (opens in new tab) urges you to keep coffee grounds away from freshly seeded areas, as they can reduce germination.

Are coffee grounds good for hydrangeas?

Your hydrangeas will definitely get a bloom-boost from your recycled coffee grounds. James Gray says: ‘Coffee makes the soil more acidic and is packed with nitrogen, which hydrangeas go wild for, making them become super bright and colourful.  ‘Essentially coffee is a fruit, so think of the amount of nutrition the soil gets from things like dropped apples and berries as this works in the same way.’

Are coffee grounds good for grass?

Your grass could be greener – and longer – with the addition of coffee grounds in the soil. James Gray comments: ‘Try mixing them through the soil in your indoor plants, or if you collect a larger amount, sprinkle them in grassy areas for a little growth boost.’

Are coffee grounds good for roses?

The high nitrogen content makes used coffee grounds a good growing companion for roses, as it helps take the pH from neutral to acidic – you can find out about how to test the pH of soil in our guide. Some experts suggest that you can sprinkle your coffee grounds in the soil next to the plant, but others warn that you should be careful not to put too much on as the high nitrogen content could actually burn – and kill – them. Do not use more than a cup for each bush. Alternatively, you could mix one cup of grounds with one gallon of water per bush and use this mix to water the plants  so your roses are particularly bright and beautiful. And you can also use your coffee compost if you have made some.

Do coffee grounds deter slugs?

Coffee grounds are a great repellent for getting rid of slugs and snails. Simply spread the grounds around vulnerable plants to create a barrier against the insects.  Lewis Spencer says: ‘Research shows that caffeine is effective in repelling slugs and snails when applied to foliage or the growing medium of plants. This is because of the naturally abrasive properties of coffee: soft critters tend to avoid rough surfaces.’

Where can you get coffee grounds?

It is possible to enjoy coffee’s benefits, even if you’re not an avid drinker. Kate Russell suggests asking for used coffee grounds from neighborhood coffee houses, as they are likely to pass them along for free.  ‘Coffee grounds contain up to 10% nitrogen after brewing. They also contain cellulose (carbohydrates), structural lignin (woody plant parts), triglycerides (oils), lipids (fats), protective phenolics (flavonoids/non-flavonoids), and essential oils,’ she says. Now you have every excuse to call into your coffee house at the next opportunity.