Is it safe to use coffee grounds in the garden? Coffee grounds are routinely recommended for the garden but in the last couple of years I’ve seen several articles about the possible harm coffee grounds do to plants and soil. Just because it is free organic material does not mean it is something you should be using.
In this post I will look at several of the claims made agaisnt using coffee grounds in the garden and determine their validity.

Harmful Effects of Coffee Grounds
Coffee grounds are the remaining solid material left after making coffee. Here is a list of the claimed problems with coffee grounds.
- High acidity will change soil pH too much
- High nitrogen level stunts the growth of fruits and flowers
- Caffeine kills plants
- Kills earthworms
- Antibacterial properties kill microbes in soil
This sounds like a horrible thing to add to the garden, but what is the truth?
Coffee Grounds Have High Acidity
I have discussed this in detail in a prior post, called Coffee Grounds Acidify Soil.
Coffee Grounds are not highly acidic. They might be slightly acidic depending on source, but not enough to change the pH of soil, unless you garden on very sandy soil.
Coffee Grounds Have High Nitrogen Levels
Coffee grounds contain 1-2% nitrogen, 0.3% phosphorous and 0.3% potassium along with a variety of micronutrients. They have a C/N ratio of 20-to-1.
They do not have a high nitrogen level. A value of 1-2% is about twice the amount in most organic material, but far less than synthetic fertilizer. Since the grounds are the result of pouring water through them, they will contain very few free ions. All of the nitrogen will be tied up in larger molecules, making it safe to be near plants.
The C/N ratio is ideal for a compost pile, providing a near perfect ratio.
Caffeine Kills Plants
Coffee grounds contain about 6 mg/gm caffeine.
I found the argument that caffeine kills plants surprising since coffee and tea come from plants. These plants and many others produce caffeine as a natural insecticide. Some gardeners even spray it on plants to kill bugs.
Can it really harm plants if plants produce it?
Testing caffeine for allelopathic effects found it stunts the growth of bacteria, fungi, seedlings and plants. Most of these studies use high doses of caffeine and perform the tests in pots. These results can’t be extrapolated to a garden setting.
A study which amended soil with coffee grounds, in both pots and in the ground, tested five horticultural plants (broccoli, leek, radish, viola and sunflower). They found that coffee grounds stunted plant growth, including the growth of weeds. Similar results have been found by others.
This effect is short term. After 12 months the grounds actually produce an increased crop when compared to controls.
There seems to be little evidence that soil amended with fresh coffee grounds will kill established plants.
Coffee grounds in amended soil will affect seedlings or newly planted crops, although the effects are short term, and in the long run they improve plant growth. It is not clear that caffeine is the cause.
It is much less clear how a coffee-ground mulch affects plants. Although some gardeners have reported anecdotal evidence that a mulch harms plant growth, I could find no scientific study that looked at this.
Coffee Grounds Kill Earthworms
People doing vermicomposting regularly, recommend the addition of coffee grounds to the worm bin, provided you do not add too much.
A research study showed, “coffee grounds can be decomposed through vermicomposting and that it improves the quality of vermicompost produced”, but another study that looked at worm populations in three composting systems found that coffee grounds increased worm mortality when coffee grounds were the only food source – which is not what most people do.
Provided coffee grounds are provided in moderation and mixed with other food scraps and/or paper, they should be fine in a worm bin.
I have no idea how coffee grounds affect earthworms in the soil but given the above research it seems likely that when used in moderation, they will have limited if any effect.
More about vermicomposting here.
Coffee Grounds Kill Soil Microbes
Caffeine and several other chemicals in coffee have antibacterial properties so there is no doubt there is some effect on soil microbes. The real questions is, are the effects significant?
Consider that the worms in vermicomposting depend to a great degree on the bacterial activity in their guts. Most of the digestion that takes place inside the worm is actually done by bacteria. If coffee grounds had a significant negative impact on bacteria, worms would either not eat it, or they would die.
Given the fact that in or on soil, the coffee grounds are even more diluted than in a vermicompost bin, they will have an even smaller effect. Also keep in mind that all plants produce antimicrobial chemicals and we have no concern about leaving them in or on the soil.
Coffee grounds probably kill some microbes, but the added organic matter will also feed them. I doubt this is a concern for soil health unless huge amounts of grounds are used.
Coffee Grounds in the Garden – Are They Safe to Use?
As you can see most of the concerns about coffee grounds are unfounded.
There is good evidence that amending soil with coffee grounds can negatively affect plants. So don’t do it.
Mulching with coffee grounds probably will not harm plants, but keep it away from seedlings and new small plants. To be on the safe side keep them out of the vegetable garden. Mulching landscapes should be OK, but don’t use more than an inch at a time.
The best option is to compost coffee grounds first. When mixed with other organic material they decompose in a couple of months and become harmless to plants. It can then be used anywhere in the garden.
Coffe mulching has mabe been the reason for many wild birds death in Sweden. Qtleast it has been concluded that mass death of birds is because of caffeine poisoning and that they have coffe in the dugestive system. Atleast this risk makes me think it’s another reason to only use it in compost if at all.
Interesting comment. I was not able to find a single source that makes this assertion.
Do you have some links?
https://www.forskning.se/2014/03/14/doda-kajor-hade-koffein-i-kroppen/#
They found 40 dead seagulls. They found caffeine in their stomach – but no coffee grounds. No dose mentioned. Caffeine levels in the blood not mentioned.
The ascribed cause of death is birds eating garbage that contained coffee grounds.
The article says there is lots of garbage around and presumably it is around every year and all year long. And yet there was one event that killed 40 birds. You would expect lots of dead birds all year long, and year after year.
Coffee grounds may be the source but I am not convinced.
You asked for a link I gave you one of the first i found in Swedish as it was where I read about it first and it has been recorded a lot of times for different birds.
Then Because i guess you’re not fluent in Swedish one of the first in English. Please if you have specific criteria for the links you wanted gove the criteria beforehand so neither you or I throw our time on unless text that isn’t good enough.
Birds as a rule can’t take lots of caffeine but will get sick and die if the doce is to high. The only thing is how much caffeine is in coffegrounds and can you get the birds to eat enough of it if for instance you use it as a mulch.
I did review both your links – the second one did provide some indication that that this might be a problem. There was no direct link to the use of coffee grounds, but a possible indirect one. The question, do birda actually eat a coffee ground mulch was not addressed.
I agree and looking at pet birds, caffeine is a known no no so that likely wouldn’t be problematic to find some good scientific evidence for how bad it is for birds. But as you say do birds eat coffee grounds (and does the amount of coffe grounds consumed be high enough in caffeine to kill birds) thats really more important part of the argument. And that’s something thats highly dependent on species. But any direct proof I haven’t but if we for the sake of argument are agreeing that caffeine is toxic to birds, what other likely source would there be but coffe grounds? I have a hard time picturing caffeine from any other source.
The studies you linked to seem to indicate the birds got the coffee grounds from garbage dumps where they might have eaten them as a side effect of eating other more tasty things. They might never eat them from a mulch.
This one is a little more convincing. Dead birds were found over a couple of years and over several months in each year. Caffeine was found in various organs indicating that it was in the blood system.
It is interesting that the birds did not die from “toxic caffeine” but by falling out of trees. It is suspected the high level of caffeine in their system caused them to fall. This is more plausible than toxic poisoning.
https://www.google.com/amp/s/www.cbc.ca/amp/1.778807
nice post Thanks for sharing
My worm composter seems to handle coffee grounds well provided they are broken up when added. An espresso puck placed in the composter seems to sit and eventually mold until such time that it’s broken down.
Thank you for this information .Can we grow this on our home garden?
Grow what?
I use coffee grounds in my compost containers, add some leaves and twigs and fork mix them. Then about twice a month or so I add as slurry of ground rock minerals which contains silica carbide, the frit used on sandpaper. This is the slurry from my rock tumblers. The stones are gathered from the local area (like really local) and as they tumble they break down and eventually polish to a glass surface. Each tumbler contains about 1/2 gallon of very thick slurry (like watered down pottery clay) and about 3-4 gallons of rinse water. I dump it in as evenly as I can and leave it until the next time I am going to add something-then I fork mix it. I am trying to dispose of the rock slurry in a way that doesn’t produce a big mound of very dense clay soil somewhere in my garden. After about 3 months of additions and tossing I shovel it out into the wheelbarrow and hand scatter in into the loose litter layer in the landscape garden (fallen leaves, bark grindings. compost, trimmings from the garden). When I dig it out of the compost bins there are entire colonies of worms. When I gather fallen leaves I kind of scrape the soil and pic up some friendly biotic stuff. The clay slurry coats the coffee grounds and loosens the clay slurry, and the coffee grounds gain some density and weight. Been doing this for about 4 years and everything seems the better for it. I have mixed coffee grounds into the veggie soil also and have seen no big improvement nor any adverse side effects in the plants nor flower/fruit production. But the addition of the coffee grounds organic matter to the soil helps it to hold more water and keeps the peaty topsoil mixes from drying out and caking.
My kitchen compost bucket gets hit with about 1lb of brewed coffee a week 2.5 gallon compost bucket. I have a compost been that I dump into once a week sometimes more the been is 30in square 4 feet tall buy the time all that wet garbage makes it to the top of the bin… its FULL of worms and I mean multiple varieties of the creatures and after 6 months the bin is half full. I didn’t put them there…Worms love coffee in moderation and its really good for you. Worms understand coffee like the rest of us. One other thing the worms are GIANT
In reply about Japanese beetles. Midwest (MO) had a massive JB infestation in 2017 ish took out my peach tree and damaged my cherry trees. Had a hedge near cherry trees, it was infested. I used a strong bug killer on hedge. Had to do it twice since they seem to have 2 week re birth cycle. I don’t like bug killer. But hedge I have no idea what type but gets white fragrant flowers. No coffee on it but my pugs fertize it every morning. 😀
Thank you for this article, I had read all the negative study findings about coffee, but decided to experiment in our large school garden as we have access to very large amounts of grounds from a local coffeeshop- and I think coffee grounds are awesome! We use it everywhere- in compost & worm farms, as a feeding mulch on top of straw, on paths with woodchip, and mixed with autumn leaves to make the most amazing leafmold- with no acidification of our soils at all. Free fertility diverted from landfill, and the worms love it- works for us!
I’ve used left over coffee to water my office plants, and have noticed a pleasant improvement in their appearance and growth after a few months. I did this repeatedly, few months on few months off to determine if it was the coffee. Not very scientific but it seemed that the plants like coffee. Black of course.
The more compost is made from coffee grounds the less goes to land fill.
Can only be a good thing hats off to gardeners everywhere