Coffee Grounds have a reputation for solving all kinds of garden problems. It is claimed that they reduce the number of diseases, ward off insects, fertilize the soil, and even keep slugs from eating your plants. Let’s have a look at the truth behind coffee grounds.
Coffee Grounds in Garden
Coffee has become a very popular drink, and the process of making coffee results in something called coffee grounds. It is the solid dark brown material left over after making coffee. Households produce small quantities of the stuff, and if you want more, many coffee houses will gladly give gardeners their grounds. After all, it is just garbage to them.
Since grounds are free and organic, they have generated a lot of myths for gardeners.
The following is a list of the benefits ascribed to coffee grounds.
Coffee Grounds Acidify Soil
I have discussed this in more depth in Coffee Grounds Acidify Soil. The short answer is that coffee grounds are only slightly acidic, and in the long term they will not acidify soil. It is pure myth.
Coffee Grounds Make a Good Mulch
Coffee Grounds are organic, and will slowly decompose in the garden–sounds like a good mulch. Coffee grounds are quite fine, and as such they compact easily. Anything that compacts will reduce the amount of water/rain and air reaching the soil. This is not good for your plants or the other soil biota.
Grounds can be added, in small amounts, to other mulch and it will work just fine. Just don’t use it as the only mulch. A sprinkling of grounds on the soil here and there is fine.
As indicated in Coffee Grounds Acidify Soil, there is evidence that uncomposted coffee grounds inhibit the growth of some plants and affect the germination of seeds. It is probably a good idea to compost the grounds before adding them to your soil.
Coffee Grounds are a Good Source of Nutrients
Coffee grounds contain 1-2% nitrogen, 0.3% phosphorous and 0.3% potassium along with a variety of micronutrients. The amounts of P and K reported seem quite variable, but there are low amounts of both of them. These nutrients are tied up in large molecules similar to other types of organic material as discussed in more detail in Organic Fertilizer – What is its Real Value.
Coffee grounds, either in the soil or in your compost bin, will slowly decompose releasing the nutrients. Just like any other organic material, this is a good slow release fertilizer. Don’t expect quick results from this fertilizer, but over time it will provide nutrients for your plants.
Are Coffee Grounds a Green or Brown?
It is important to get your green and brown ratio right for composting! Well, that is not quite true. It is more about the C/N ratio than the brown and greens.
Coffee grounds have a C/N ratio of 20. The target value for composting is around 30, so coffee grounds are slightly green.
Coffee Grounds make Plants Grow Better
That is kind of a big statement without any specific claims. It is hard to argue against such a statement.
As mulch, it certainly benefits plants—any mulch will do that. It is a slow release fertilizer and that is always good for plants.
The grounds also contain a variety of specific chemicals that have been shown to enhance the growth of seedlings in the lab. However, it has also been shown that the grounds inhibit the growth of certain types of seedlings, including tomatoes. These are all lab results using seedlings—not mature plants. It is unclear if any of these observations translate to the garden situation. Nor is it clear what effect these chemicals have on mature plants.
Because of the potential problem of these chemicals, it is probably best to compost coffee grounds before you add them to your soil. It is also a good idea to keep them away from seedlings.
Coffee Grounds Prevent Weeds
I have not been able to find any confirmation of this. Certainly, as mulch, it will reduce the number of weeds—any mulch will do that. It is possible that the chemicals in coffee grounds inhibit some weed seedlings in the same way that they inhibit tomato seedlings, but that is just a guess on my part.
Since coffee grounds also help plants to grow, you can expect that there will be some weeds that will grow better after being treated with the grounds. After all, weeds are just plants. If you believe that coffee grounds make plants grow better, then you have to believe that they will also make weeds grow better.
Coffee Grounds Repel Cats
I found this statement on some sites, but can’t confirm it or deny it.
Coffee Grounds Kill Slugs
Since slugs seem to be a big problem in the garden, I have made two previous posts; Do Beer Traps Kill Slugs, and Does Copper Repel Slugs. So what about coffee grounds?
Studies have shown that caffeine will kill slugs and snails. Spraying plants with caffeine will deter slugs from eating the plant. These findings have probably been misinterpreted and translated into the fact that coffee grounds also kill or deter, slugs and snails. This type of extrapolation happens a lot and causes many of the myths discussed at GarenMyths. The caffeine that was sprayed on plants was fairly concentrated compared to the caffeine found in grounds, which have very little caffeine. In fact, the concentration of caffeine in grounds is so low, it won’t kill slugs or snails.
I have found no evidence of other types of chemicals in coffee grounds that would kill slugs–this is a myth.
Will coffee grounds deter slugs from reaching a plant? Do they dislike crawling over the grounds so much that they leave your plants alone? So far I have found no scientific data that suggests this is true, and unlike the videos for beer and copper, no one has made a video showing that slugs hate to crawl on coffee grounds.
To resolve this more clearly I decided to run a test, the results of which are reported in Slugs and Coffee Grounds. It clearly shows that slugs don’t mind crawling on coffee grounds.
Coffee Grounds Change the Color of Hydrangea
Some hydrangea have pink flowers in alkaline soil and blue flowers in acidic soil. Since blue is the color preferred by most gardeners, it has been recommended that coffee grounds added to soil will make hydrangea blue. As discussed above, coffee grounds will not change the soil pH, so they will not change the color of hydrangea flowers.
Coffee Grounds get rid of Ants
Apparently, ants do not like the smell of coffee grounds and they will avoid them. It is claimed that if coffee grounds are put onto an ant hill, they will leave the area.
This seems easy enough to test, so I decided to run some tests in Ants and Coffee Grounds.
Coffee Grounds Kill Insects
Lots of web sites talk about coffee grounds acting like an insecticide, but they give few details. Most talk about caffeine as the insecticide, and that has some truth if it is used in high enough concentrations–much higher than what is found in grounds.
How would you use grounds as an insecticide? Spray it on the leaves? It is a solid material consisting of fairly large particles. It is not practical to use it on plants for flying insects. What about soil insects? People feed it to dew worms and they don’t get killed.
There does not seem to be any evidence that coffee grounds work as an insecticide.
Coffee Grounds Suppress Fungal Diseases
I found this stated in quite a few places. Here is a quote from one source, “The natural mold and fungus colonies on coffee appear to suppress some common fungal rots and wilts, including Fusarium, Pythium, and Sclerotinia species,”. Interestingly, the underlined section of the quote was attributed to a report written by Dr. Linda Chalker-Scott, someone I greatly admire and trust. So this has to be true—right?
First of all molds are fungi—they are not two separate things as stated in the quote. Secondly, the quote from Dr. Chalker-Scott dealt with coffee grounds, not coffee. These might seem like small errors but in my experience they usually indicate an author who misinterprets information–it is starting to smell like a myth!
What Dr. Chalker-Scott said was (ref 3) “ Researchers suggest that bacterial and fungal species on decomposing coffee grounds, prevent pathogenic fungi from establishing”. The words, “researchers suggest” are interpreted as a “known fact”–coffee grounds suppress fungal diseases. You have just witnessed the birth of a myth.
Dr. Chalker-Scott goes on to say that this work was all done in the lab under controlled conditions and that “their efficacy in gardens and landscapes is unknown”. What that means is that there is no scientific evidence that coffee grounds suppress fungal diseases in the garden. Many things scientists see in the lab, under controlled conditions do NOT translate into the garden.
There seems to be no clear evidence that coffee grounds suppress fungal diseases in plants.
Goodbye Cellulite
I quote, “some celebrities swear by this odd treatment involving old coffee grounds: Mix an egg white with the day’s used coffee grounds, warm it up in the microwave, and then spread the gooey concoction on your problem areas. Wrap tightly in saran wrap. Boom—you’ve just saved $700 at the spa.”
If it works for celebrities it has got to work. Give it a try, and post before and after pictures 🙂
References:
1) The Truth About Coffee Grounds in Your Garden: http://www.myearthgarden.com/2012/02/the-truth-about-coffee-grounds-in-your-garden/
2) No longer available
3) No longer available
4) Do Coffee Grounds Really Kill Slugs: http://www.paghat.com/coffeeslugs.html
5) Slugs in Gardens: http://www.jstor.org/discover/10.2307/1386?uid=3739448&uid=2&uid=3737720&uid=4&sid=21104488656997
6) Photo Source: Tristan Ferne
Still don’t know if coffee is beneficial to my flowers and vegetable garden.
Just found your blog very interesting.
Think of coffee as being an organic material just like other food scraps. They all contain nutrients and will decompose to help feed plants. Coffee grounds have no magical properties.
Hi.
I have been using coffee grinds on all my house old plants. I have seen a tremendous amount of growth, leaves started becoming bright green….. But I have noticed that small insects have started coming up?? What steps can be done to eliminate the insects?? Please help. Thank you.
The insects have nothing to do with the coffee grounds. The insects could be lots of things. Try letting the pots dry out a bit. If that does not work, try posting pictures in a gardening group on Facebook.
This has been happening to my plants as well. The problem persists even out in the sun (they’ve been kept in a green room). There are other plants in the green room don’t have small bugs. Any recommendations?
Start by identifying the pest. Either use Google or post your pictures in a gardening Facebook group.
Totally agree with previous comments on the usefulness of your research, the quality of your evidence based analysis and general helpfulness of your work. Thank you for sharing and please keep up the great work!
Regarding coffee grounds: I read with interest your myth busting and appreciate learning before I go off on some heroic effort because the local barista says to. I have a built in coffee maker using decaf instead beans so I have lots of grounds to dispose of. Can you give me advice on what would be a productive way of disposing of my grounds please? I do not have a compost but am interested in learning a best way that would be useful in disposing and am willing to begin comparing or anything else useful. Many thanks and happy gardening, enjoy your info! Martha
Simple – just spread them around your garden – let nature compost for you. An inch of coffee grounds everywhere is no problem. If you have more you might want to think of adding a compost bin.
I stopped composting several years ago. I now use the cut and drop method. https://www.gardenmyths.com/composting-the-cut-and-drop-method/
What is the cut and drop method? You can see I am no gardener but want to become one. Thank you for your info & response. I bet your Easter Egg hunt is interesting!
See the link I posted.
Ver interestfbg reading. Thanks for your knowledge and letting people in on your research.
Hi Robert,
I was almost about to sprinkle some coffee grounds on a few areas where my lawn was amost dead. It is slowly coming back, after a few times of deep watering. So, no coffee sprinkles? 🙂
I can confirm that wet/moist coffee grounds will deter cats. My mom was tired of the neighborhood cats using her flower beds as their litter box. She put the wet coffee grounds on the flower beds a few times, and the cats stopped coming. Cat paws are very sensitive. The grounds are sticky, adhere to their paws and between their toes. Cats don’t care for that. Never mind the taste they must endure cleaning them.
A friend of mine has used citrus fruit peels for the kid’s sandbox. It worked, but the consequences were terribly. Poor cat had a severe allergic reaction, and its face turned into the of a balloon! So, not everything “natural” is safe.
Great post, thank you.
Bea 🐝
I shall stick my neck out a long long way and say, with the evidence of one compost heap, that coffee grounds are a BRILLIANT compost accelerator!
We realised, early in the year, that our group would need a large volume of compost this autumn. Coffee grounds were mentioned by one of the members, with the words “free” and “large amounts” so I did a bit of Google research and started collecting from the four coffee shops in town. Once a week, on the day I was in town anyway – each collection gave about 20 litres. Spread this in layers on my usual slow-chilly compost heap and it was AMAZING. In only a few days it was too hot to touch. I took the cover off yesterday and it is full of mouseholes and smaller migglies, so presumably it has cooled down and i will be starting to sieve it shortly.
My heaps are usually very slow, being full of bracken and other stuff too “brown” to feed to the goats or chickens, but not hard enough to put on a bonfire – all sorts of garden waste. (No pernicious roots or seeds).
I suspect the coffee grounds are very “green” and just start things up nicely, laid in a half inch layer over a four foot square heap.
regards, Deb.
What is ‘very green’? When you are talking about browns and greens in compost, you are really talking about the C:N ratio. The amount of carbon to nitrogen. things less than 30:1 are green because then have a lot of nitrogen compared to the amount of carbon. Ratios higher than 30:1 are browns. Coffee grounds have a C:N = 20:1, which is similar to kitchen scraps and grass clippings. So you are correct – they are green. Your compost heap just needed more nitrogen to get the microbes growing. Any nitrogen source would do the same including commercial fertilizer with a high first number.
See: https://www.gardenmyths.com/how-to-compost-browns-greens/
I do recall spreading coffee grounds on the large lumpy fungus that occasionally appears in my lawn, they were gone in a matter of hours and never reappeared.
The above part of fungus that we see can have very short lives – above ground. I have not heard of any chemical that would make a fungus disappear in hours that was not already planning to disappear – coffee grounds certainly would not do it.
This is another example of why anecdotal observations are not of much help in understanding the garden. Two events – adding coffee grounds, and fungus disappearing – happen at the same time. Without a control there is no way to assign cause and effect to the situation. See my post Anecdotal Evidence – Not Worth The Screen It’s Displayed On for more details.
I read with interest whenever I see your authorship… especially on Garden Professors BLOG… I am intrigued by your almost endless curiosity in areas I’m interested in but I obviously see the forest and you see the forest and the trees… I consider myself a plantaholic and in my circles maybe I am, but you’ve got me beat by a long, long, long shot… and I consider that a well deserved compliment. .
Thanks for all your comments, research and ideas…
CHEERS
Steve Morse
California
Thank you for the kind words. It makes writing this blog worthwhile.
I enjoy reading your blog. Highly educational and sprinkled with humor. Love it!!
Thanks a lot.