A Common-Sense Guide to Using Coffee Grounds in the Garden

, written by Ann Marie Hendry gb flag

Coffee grounds and coffee beans

Coffee shops often give coffee grounds away free to gardeners, as they’re a waste product they would normally have to pay to dispose of. For coffee-loving gardeners like me, this freely available resource sounds like a real boon. But some gardeners suggest that using coffee grounds could be ineffective or, worse, harmful to plants.

I decided to sort the facts from the hype and find out just how beneficial – or otherwise – coffee grounds are in the garden.

Using Coffee Grounds as Mulch

Mulching is incredibly beneficial but it’s notoriously difficult to come by compost, straw or other organic matter in large enough quantities at a low enough price. Using free coffee grounds seems like the perfect solution, but some gardeners have found that using coffee grounds directly on the soil has had a disastrous effect on plants. However this seems to be linked to using thick blankets of it to mulch around plants and over seeds.

“Pouring
With care, used coffee grounds can be added to the vegetable garden soil

The reason for this could be that coffee beans contain caffeine, which is said to suppress the growth of other plants to reduce competition for space, nutrients, water and sunlight. How much caffeine actually remains in used coffee grounds is debatable, and some plants will be more sensitive to caffeine than others. It would be sensible to avoid spreading coffee grounds around seeds or seedlings as they may inhibit germination and growth.

There is a more obvious reason why using coffee grounds alone for mulching could be detrimental. Like clay soil, coffee grounds consist of very fine particles that are prone to locking together. This turns them into a barrier that will resist water penetration and eventually result in plants dying of thirst.

The solution is to mix coffee grounds with other organic matter such as compost or leafmold before using it as a mulch. Alternatively, rake your coffee grounds into the top layer of soil so that they can’t clump together. Variable particle sizes is key to good soil structure.

Coffee grounds are often said to be acidic but this can vary a lot, from very acidic to slightly alkaline, so don’t expect them to acidify higher pH soils.

“Fertilising
Sprinkle used coffee grounds around plants as a slow-release fertiliser

Using Coffee Grounds as Fertilizer

Many of us will have dumped the cold remains of a forgotten coffee in a plant pot at some point, and then perhaps wondered if it was the wrong thing to do! But it turns out that coffee grounds contain a good amount of the essential nutrient nitrogen as well as some potassium and phosphorus, plus other micronutrients. The quantity and proportions of these nutrients varies, but coffee grounds can be used as a slow-release fertilizer.

To use coffee grounds as a fertilizer sprinkle them thinly onto your soil, or add them to your compost heap. Despite their color, for the purposes of composting they’re a ‘green’, or nitrogen-rich organic material. Make sure to balance them with enough ‘browns’ – carbon-rich materials such as dried leaves, woody prunings or newspaper. Your compost heap’s tiny munchers and gnawers will process and mix them effectively, so using coffee grounds in this way is widely accepted to be safe and beneficial.

Many vermicomposters say that their worms love coffee grounds, so small quantities could also regularly be added to a worm bin if you have one. Paper coffee filters can go in too.

“Ingredients
Used coffee grounds can safely be added to the compost heap

Coffee Grounds as a Natural Pesticide

An oft-repeated nugget of advice is to spread used coffee grounds around plants that are vulnerable to slug damage. There are two theories why: either the texture of the grounds is abrasive, and soft-bodied slugs prefer not to cross them, or the caffeine is harmful to slugs so they tend to avoid it.

However in an experiment slugs took just seconds to decide to cross a barrier of coffee grounds! The same researcher also sought to find out if coffee grounds would repel ants, with similar results – ants may not particularly like coffee grounds, but they won’t scarper out of your garden to get away from them.

“Image
Enjoy your daily brew and recycle used coffee grounds in the garden

Coffee Grounds and Dogs

One word of warning though: coffee grounds may not have much effect on pests, but they can be harmful to pets in large enough doses. It’s hard to say what would be a large enough dose to cause poisoning because the amount of caffeine in used coffee grounds varies. But if you have a dog that insists on sampling anything that smells halfway agreeable, it would be wise to avoid laying coffee grounds directly onto the garden. Bury them in your compost heap instead.

Coffee grounds are free organic matter, whether a by-product of your at-home daily brew or collected from coffee shops that are only too glad to give them away for nothing. If used with care and common sense, they are a worthwhile addition your compost heap and your soil.

Have you used coffee grounds in the garden? What was your experience? Share it with us by leaving a comment below!

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Comments

 
"I only use coffee grounds from my own kitchen. I had collected free grounds from a coffee shop chain store until I realized they used artificial flavorings and I did not want to add chemicals to my organic garden."
LYN on Friday 5 January 2018
"I save all coffee grounds from our morning coffee and use it around the bases of my rose bushes. They love it! I also will be using it on the hydrangeas."
Helen Ullrich on Friday 5 January 2018
"I throw all my coffee grounds into the compost bin and also collect them from hotels in the area along with those beautiful vegetable scraps that prep cooks work with on a daily basis. I bring two empty five-gallon bins and walk out with two full five-gallon bins every day, all year long. It's as routine to me as brushing my teeth."
Wendy on Friday 5 January 2018
"I dumpster dive behind Starbucks for garbage bags, remove the mushy buns, coffee filters and banana peels and spread the coffee grounds around the edges of my garden beds. As they seem to hate the smell, coffee grounds deter animals like dogs, cats, raccoons or squirrels that dig in my garden or like the nice soft soil to poop. I use them in early Spring or whenever I have carefully nurtured seedlings that are killed when dug up by pests. I also add them to the compost bin."
Marion on Friday 5 January 2018
"For many of the reasons noted, composting or mixing coffee grounds into the soil or some carbonaceous material is perhaps the best approach. It allows faster breakdown and release of nutrients, incorporation into the soil matrix, and attraction of worms."
Steve on Saturday 6 January 2018
"It's great to hear that so many gardeners are experiencing success with used coffee grounds! Have you noticed any instances where you feel you shouldn't use them?"
Ann Marie Hendry on Tuesday 9 January 2018
"I collect 20 - 25 gallons of used coffee grounds per week from local coffee house.. I mix them with leaves and horse manure and make a big pile and add water and then let compost. I have been using for fertilizer for many years with good success. I don’t advise dumping coffee grounds in a pile by themselves. Coffee grounds by themselves make a gooy mess. Mixing with leaves works well."
Greg on Thursday 11 January 2018
"I just throw the grounds and filter into the compost. As a mulch, be sure to use a thin layer, mix in a little. If you have too thick of a layer of grounds it crusts over."
Sarah on Wednesday 17 January 2018
"For a number of years, I’ve just been putting uncomposted coffee grounds on my rose bushes - just “dumping them” around the base of each bush. Somehow, they disappear into the soil after a good rain or two and the roses thrive. I use 100% Colombian beans ground at my local coffee house."
Helen Ullrich on Wednesday 17 January 2018

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