You’ve read the ‘coffee grounds are magic’ but also ‘coffee grounds are rubbish’. So what is the reality of using coffee grounds in your garden?
Like other organic and artificial materials you add to your garden, used coffee grounds do not suit all methods or all plants. But I’ve done my best to give you facts and tips so you can decide if its right for you and your garden – read on!
Garden Teacher Tip
Old, or to be more precise, used, coffee grounds are those which have had hot water passed through them to make a cup of freshly brewed coffee.
The type of coffee – espresso, filter, pod – is not important. Neither, for the most part, is the variety of coffee bean.
And despite being called ‘decaffeinated’ this type of coffee will have usable grounds, although better suited to some methods than others.
Caffeine levels vary between beans and the type, content and method of brewing. This variability has hindered the amateur gardener from making the best use of scientific research on the use of coffee grounds. I’ve read much of the science for you and then related that to the way in which most of us actually garden in the real world.
But I don’t drink coffee!
Now there are a few regular tea drinking readers who may be thinking “Using coffee grounds in your garden is all well and good for you, Marie, you drink lots of coffee! But what about me?”
Would I forget you? No! Did you know you don’t have to be a coffee drinker in order to get hold of used coffee grounds? The easiest places and people to ask are: –
- Coffee drinking friends and family without gardens and / or a compost bin
- Plenty of coffee shops have bags of their used grounds for customers to take, for free or a donation to charity. These may be small independent cafés or national chains
Btw you can use tea leaves in the garden too – more on that another time, for now just compost them.
Why Would Using Coffee Grounds in Your Garden be beneficial?
Coffee grounds contain approximately 2 percent nitrogen, 0.06 percent phosphorus, and 0.6 percent potassium by volume. These three essential nutrients for plant growth are those represented by the initials NPK in the fertilisers you use. They also contain many micronutrients including calcium and magnesium. The nitrogen is released slowly as the coffee grounds are broken down by soil microorganisms.
Coffee grounds are slightly acidic, pH5.5 to pH 6.8. making them suitable for use when you grow acid loving plants. They’re not going to alter the pH across your whole garden though!
A healthy soil and productive compost bin require a carbon to nitrogen ratio 20:1 to 30:1. This level stimulates the soil microbes to release the essential nutrients from the soil to the plant roots. Coffee grounds have a ratio of 20:1 to 24:1
Of course, using old coffee grounds for plants is not a cure-all, there are some caveats to be made. But many of the common issues can be resolved, as we shall see below.
Using Coffee Grounds in Your Garden – Mulch
This use is where many people have a lack of success, which is why I’m tackling it first. There are two main reasons why a mulch can be problematic: –
Caffeine is allelopathic
Basically, this means it may release chemicals which suppress the growth of nearby plants.
Not all plants will be affected and sometimes it may be the acidity not the allelopathy that’s the problem. However, tomatoes, seeds and seedlings will have their growth affected.
A fine crust forms on top of the soil
Coffee grounds are, by their nature, fine particles. If you’ve tried to scoop out wet grounds from the bottom of a cafetière you’ll know how they stick together! When used as a mulch on the surface of the soil, this can create a solid barrier which forms a water proof cap that prevents moisture from reaching plant roots. Not what was wanted!
Solutions –
- Lightly rake the coffee grounds into the soil so it isn’t lying directly on the surface.
- Mix the coffee grounds with another organic material, for example, by chopping up leaves and using the two together.
Acidic coffee grounds
Fresh grounds are more acidic than those which have been composted. Most mature acid loving plants will be fine with a thin mulch even from fresh grounds – but let them cool! Better to let them compost.
NB The caffeine in coffee is toxic to dogs. If your pooch eats everything it can, then this is not a suitable use.
Using Coffee Grounds in Your Garden – Add to your Compost Bin or Wormery
Despite its brown colour, coffee is a green element in the compost bin, having a higher level of nitrogen than carbon. No more than 20% of your compost mix should be coffee grounds.
Compost Bin
Hot coffee grounds can kill beneficial organisms, but once cool, you can add remaining water and grounds from the coffee pot. As part of a mixed compost this is the easiest way to recycle them. Some have declared the coffee acts as a compost activator, others have noticed no difference.
Wormery
Start slowly with adding coffee grounds. Worms seem to treat them like marmite! This is experiential evidence from my own wormery and those of clients. Due to the ‘caking’ effect, it is best to break up dried grounds and sprinkle them on, or mix with cut up banana skin.
Using Coffee Grounds in Your Garden – Turn them into a Liquid Fertiliser
Strictly speaking, a liquid fertiliser is made from fresh plant material so coffee grounds are more like a cross between that and a compost tea. Be that as it may, the resulting liquid can be used to give your plants a nutrient drink!
However, choose your plants carefully, as some will just curl up and die. Others will lap it up. Yes, we’re back onto acidity and allelopathy! For example, most Clematis hate it, Azalea are happy, Hostas don’t care.
My advice is to keep the brew weak to begin with as the coffee strength and water (tap, rain) vary. Try a small cup of grounds in a bucket of water.
Using Coffee Grounds in Your Garden – Pest Control
Slugs and Snails
I’ve used this myself when the slugs and snails are heading towards my Hostas and it’s too cold for nematodes. Lightly raked into the soil coffee grounds have worked as a deterrent. However, this was less successful on the allotment. But that could be down to, for example, playful foxes leaving gaps for molluscs to crawl through.
Cats, Foxes, Rabbits
Some people have found coffee grounds deter cats from using parts of their garden as a toilet.
Hasn’t worked for me; my cats like the smell of coffee (!) And it didn’t deter foxes either. Although it did put off some pet rabbits of my acquaintance.
Uses for Old Coffee Grounds in the Garden – Blooming Roses and Bananas
Brewing up a tea of old coffee grounds and banana skins to water your roses generally seems to result in a mass of blooms! Make small quantities at a time and use promptly. I use 1 cup of grounds, 1 chopped banana skin in a bucket of rain water.
A variation of this is to add the coffee grounds and chopped banana skin to the base of planting hole or trench when planting new rose bushes. Mix them with the soil to work as a slow release fertiliser at root level.
Uses for Old Coffee Grounds in the Garden Why not Colour your Garden Furniture?
Not necessarily a long-lasting fix, but a fun – and free – way to give your untreated wooden furniture a vintage look. There are various recipes and techniques out there, some involve vinegar, some use the grounds, some just the coffee water.
Try the colour out a couple on a spare bit of wood first. Use the same wood as the garden bench you want to stain, as different woods have different absorbency rates.
I have only tried this on a piece of wood, and it came out a pleasant sepia tone. I’m now eyeing up an old deckchair…
Uses for Old Coffee Grounds in the Garden – Use on your BBQ
Use on your BBQ or burn in your Chimenea. Not the loose grounds, but a ‘fire log’ made from compressed old coffee grounds. Now, before you start complaining about the amount of effort this might involve, may I just remind you that you can collect bucket loads of free coffee grounds from your local cafe.
Admittedly, I haven’t actually made a log from coffee grounds, although I have burnt coffee logs. Because, if you don’t want the faff of making them you can buy them. I used those from a British, London based company, and I do know that there’s at least one American company as well.
And finally
NB Although you can store coffee grounds for a couple of days, they will start to go mouldy, but are still fine in the compost bin.
I’ve hopefully given you something to think about. Using coffee grounds in your garden is certainly a different way to look at that coffee you’ve been drinking whilst reading this! As a beneficial organic kitchen waste, it makes sense to use it not lose it.
Whatever you do with your coffee grounds, have fun, enjoy your garden and if Plews can help you with anything, please do get in touch. For example –
Why not ask about our bespoke Gardening Lessons and Courses where your classroom is actually your own garden? You could learn how to grow an orchard of citrus fruits in your conservatory. We can help with both gardening basics and more ‘expert tasks’, carry out worm and other experiments and for example, also show you how to plan a vegetable plot. Have a read through this pdf download for info.
For further gardening advice and inspiration, check out Plews Potting Shed blogs, including the selection below Plus we have a monthly Tipsheet (currently changing our provider, so there may be a delay) You could come and find us on Instagram Pinterest and Facebook too.
Feeling nosey? You can have a peek at the progress of my garden renovation, Spitfires and Slow Worms, on Instagram and Facebook
You’ll get to see it in person if you come to the Plews Gardening Workshops Although later dates and topics still need to be finalised, there are current ones on Eventbrite for you to book into. Follow for updates on topics and dates on Instagram, Eventbrite, Facebook
Plus get your local garden club, allotment, WI, U3A or other group to ask me along to give a talk (and bring plants 😉)
Related Gardening articles you may enjoy from our Award Winning Blog
Could You Grow Your Own Coffee?
Your Own Tea Garden – Camellia Sinensis, the Tea Plant
Holly, Yerba Mate, Coffee – What’s Growing in Your Garden?
5 tips for dealing with Slugs and Snails
Making Liquid Fertiliser from Weeds
