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
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.
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 🙂
1) The Truth About Coffee Grounds in Your Garden: http://www.myearthgarden.com/2012/02/the-truth-about-coffee-grounds-in-your-garden/
2) Coffee Grounds and Composting: http://extension.oregonstate.edu/lane/sites/default/files/documents/cffee07.pdf
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