I bet that at least once you heard that coffee ground can be beneficial for your soil. Should you use it in your soil? Should you put it dry or wet? Is coffee ground really beneficial for potted plants? Here a detailed analysis on what science and other expert gardeners say on the topic. Let’s dive in!
Coffee ground is beneficial for plants and herb for many scientifically proven reasons such as 1) provide nutrients 2) improve soil structure 3) repel pets 4) repress harmful fungi, and 5) reduce waste. Coffee ground is not a fertilizer despite being useful for other reasons beyond its nutrient content.
However, coffee ground should be used properly, otherwise, it can cause drainage problems and/or creating a nutrient imbalance.
Coffee Ground On Your Potted Soil? 5 Good Reasons To Use It
Here you can find 5 critical facts supported by gardeners and researchers for using spent coffee ground (spent means already used by your coffee machine, hence the waste product that the majority of us through away) in your potting soil.
The spent coffee ground contains on average around 1-2% of its volume in nitrogen. Some studies claim 2%, while others around 1.5%, probably depend on the quality of the coffee. (source)
It is not officially a fertilizer becase the precise amount of plant nutrients is not known. Indeed, depending on the way it has been processed (french press, Moka) its nutrient content, as well as other substances, can vary significantly.
Nitrogen, the main plant nutrient in spent coffee ground, is one of the 3 macronutrients (with phosphorus (P) and potassium (K)) that your herb needs to thrive.
Nitrogen is also, among the three, the most famous among the nutrients, probably because it can accelerate the speed of growth and even the green color sign of healthy growth. More info on nutrients in this article.
Nitrogen, although in larger quantity, it is not the only nutrient.
The spent coffee ground also contains around 0.2-0.3% of potassium (responsible for plant respiration), 0.04-0.5% of phosphorous (accountable for root development), and approximately 30-40% of organic matter.
Organic matter is fundamental in healthy soil for indoor herbs. That’s why you might also use compost (that is decomposed organic matter) in your soil.
You also have to know that coffee ground is even better than that! It has more than the classic three macronutrients. Indeed, it provides crucial minerals (like 0.4% Calcium and Magnesium, here) that combined with the primary nutrients, are critical for the full development of your herbs.
If you do not trust me, you can always read some of the latest University scientific studies as this one. One of the conclusions of such a study is:
The use of spent coffee ground as organic amendment increases the concentrations of multiple essential elements such as V, Fe, Co, Mn and Zn (Vanadium, Iron, Copper, Magnesium, and Zinc), some of them of great nutritional importance, therefore increasing the nutritional value of lettuce.Scientify Study- University of Granada
Similar conclusions were found in another study published in an international conference:
With this work was possible to conclude that fresh and composted SCG can be used as fertilizer in agriculture. Both forms of SCG (fresh and composted) have showed to improve significantly lettuces plant growth when compared to the control.Scientific Study-Polytechnic Institute of Braganca
The study was performed for lettuce. However, every plant, herbs included, need the same type of minerals to produce large and tasty leaves.
Another study, from a different University, arrived at a similar conclusion for tomato and basil plants. In such a study, peat moss (widely used medium as you can read in this article) was replaced with spent coffee ground in the soil mix. The authors claimed:
Tomato and basil recorded good growth rates on SCGC (spent coffee ground) amended media, showing quality indices similar to those obtained under fertilization, evidencing compost plant nutrition provision.Scientific Study-Research Centre for Animal Production and Aquaculture – Italy
Improve Soil Structure
Spent coffee ground improves soil structure allowing a better development of your herb roots and, ultimately, better growth. How is this possible?
This is a consequence of the coffee ground decomposition due to bacteria found in the soil. This process originates so-called humus substances as detailed in this scientific study. Without getting into many scientific details, I would say that such chemicals act as glue, creating clumps in the soil (cementation). This makes the soil more irregular, improving aeration and drainage.
Again, if you do not believe me, here a study proving the beneficial effect of such substances in the soil as also reported clearly by the same authors.
The positive influence of humus substances (HS) on the quality of soil and soil aggregates is evident.Scientific Study – University of Agricultre in Nitra – Slovakia
Although this does not apply to typical potting soil, it is essential to mention that spent coffee ground is tasty food for worms. Hence, in the unlikely case you have worms on your potted herbs (worms are great but difficult to maintain in pots, more Why you should not have worms in potted herb) this will make them thrive. Moreover, they will better bury the coffee ground more in the soil, beneficial for roots (closer to the nitrogen sources).
This is a great plus for outside gardeners as their plants are exposed to any type of pests that can be found in nature. For use, indoor gardener life is a bit easier for this aspect.
However, if you have your potted herb on a windowsill and you (of course!) open the window from time to time, it is not unlikely to have some nasty outdoor visitors such as snails. The coffee ground can also help here!
Coffee spent ground can help in that. Indeed, its fragrance (loved by the majority of us) is hated by some insects like ants, hornworms, milkweed bugs, and also slug, and snails as also stated by the authoritative USA Environmental Protection Agency here.
Regarding slug and snail, someone says that it is the coarse nature of the coffee ground that makes them away. In any case, they are kept away. The capability to repel ants is a great plus. Indeed, ants are a curse for plants as they can magnify existing aphid problems (farming aphids, one of the worst pests for herbs) leading to a quick death of your herbs if not through difficult eradication actions.
Another side advantage: your cat (if you have any) hates the smell of coffee, as discussed here. Why is this good? Cats in general love playing (and munching) herbs that might found around your house. Hence, if you do not apply any strategy to protect your potted friends, you might found them half munched or, even worse, lying on the floor due to a too playful cat. That’s why the simple action to spread a bit of coffee ground on your potting soil can avoid such situations.
Repress Harmful Fungi
On this aspect, there is not a broad consensus at the moment if not for a handful of studies. Essentially, it was found that coffee ground promotes the development of a specific category of (health-promoter) fungi. This, in turn, affects the population of a harmful variety (like the Fusarium oxysporum) that see their population decrease when the coffee ground was added.
More research is definitely needed as the results are at an early stage on which and I hope to update you soon on the matter. However, these are signs in the right direction. For an interesting read, I suggest this scientific article.
Unfortunately we, humans, since the industrial revolution, are producing a massive amount of waste of every type (from organic to inorganic) that is polluting and damaging the ecosystem. This ultimately will harm ourselves.
Coffee ground, although not among the most significant problems at the moment, represents a significant amount of waste each year. Indeed, with coffee being among the top 5 most popular drinks in the world, around 20 million of spent coffee ground is produced every year! This is a crazy amount of material that goes on the landfill contributing to the production of methane (many times more dangerous than the well-known CO2).
Doing your part in recycling a waste product is a massive plus that will make you (at least it works for me) proud in helping the environment. Every little help!
How Much Coffee Ground For Potted Plants?
The amount of coffee ground to be applied to a potted plants is roughly 5% of the soil volume. Anything above such limit can cause damange to the herb or plant in the pot.
If a larger amount of coffee ground is placed can cause an excessive dose of caffeine (a toxic for them) that can reduce the plant growth rate due to difficulties in absorbing nutrients. This is caused by nutrient knockout triggered by excessive nitrogen.
Indeed, a plant can be seen as a voracious food eater. If overeats of nitrogen will not be able to pick up other nutrients. Think of you eating a massive pizza (nitrogen). Would you also be able to eat dessert (phosphorus and potassium)? Probably not. This will cause an imbalance in the plant damaging its growth.
How To Use Coffee Ground In Your Potted Soil And What Not To Do
I found quite a few people around struggling with this question. Indeed, as simple as it sounds, the answer to this question requires the knowledge of a few fundamental facts on spent coffee ground that the majority of people ignore. Let’s dive in.
Wet Or Dry Coffee Ground On Plants?
The spent coffee ground, produced by your coffee machine comes out quite warm and wet/moist. This is because of the high-pressure hot water that extracted the caffeine content to leave the precious oils in your favorite daily drink.
Hence, you might wonder if you need to apply the coffee ground as it is, or you need to dry it.
The coffee ground should be applied to the soil only when cold and only slightly moist. Hence, once produced the coffee ground should be let cool down and dried a little by squeezing it with your hands. Avoid applying dry spent coffee ground. This is because dry coffee ground is a decent water-repellant.
If you do it wrong: Hence, if you apply a compact layer of coffee ground on top of your plant soil, you make create a water-repellent barrier that will prevent the water to reach your herb roots, leading to a (mysterious if you do not check the soil) dead of your plant.
Lots Of Coffee Ground is Bad
Given the great benefits of coffee ground in your potting soil (for the best potting soil you can check this article) you might be tempted to think that the more, the better. This is not the solution. Indeed, as also scientifically demonstrated in this (and others) scientific studies:
The application of fresh SCG (spent coffee ground) at concentrations above 10% (Referred to the soil weight) showed to induce plant stress, probably due to the increase of phytotoxic compounds (such as caffeine).Scientific Study – Polythecnic Institute of Braganca
This is because caffeine, although it is a water-lover chemical will “almost entirely” end up in your cup, a small percentage remains in the spent coffee grounds.
This is not a problem as many claims. Indeed, if you apply the right amount of coffee ground in your potting soil, the caffeine in it can start affecting your plant.
Spent Or Fresh Coffee Ground?
This is a question I found a lot around. Indeed, someone might think that if spent coffee ground is so beneficial for your plant, the fresh version of it (unused coffee grounds) can be even better. Bad news. This not only will nullify one of the main advantages to recycle waste (as the fresh coffee ground is not a waste) but also will kill your plant.
Indeed, as discussed above, we do not want dry coffee ground as this is a water repellent. However, more importantly, the high caffeine content and acidity (on this later) of fresh coffee grounds is something that your herb will not tolerate.
If you do it wrong: The excess caffeine and acidity will simply kill your plant. For more read this article on the sever effect of pH alteration on herb growth.
How To Use Coffee Ground In Plants?
Now you know that you need around 5% of cold, moist, spent coffee ground. You have it. How to use it?
The question is not trivial. Many suggest to place it on top of your potting soil. This is, in general, a good idea, however, has a drawback. If the coffee ground gets dry (you forget to water your plants), it might become a wall of water-repellant that you might need to replace.
Tip: what I usually do, especially for potted plants (and the pot often are not massive) I use a small spoon or tooth stick to move the soil and allow the coffee ground to go a bit deeper in the soil (avoiding damaging the roots). Having the coffee ground surrounded by soil particle has a great plus of:
- Speeding up the decomposition process and so producing precious nitrogen slightly faster
- Avoid water-wall effect as the deeper layer of the soil stay wet for longer compared to the surface, and the coffee ground particle are not all in the same place;
Moreover, a few gardeners also suggest not to place the coffee ground close to the plant stem. This is to avoid to burn them due to the nitrogen content.
Coffee Ground As Fertilizer?
Many stated that coffee ground can be used as fertilizer. This is true, but only partially.
Indeed, coffee ground contains a healthy amount of nitrogen, potassium, and phosphorus. However, these are available to your potting herbs (if you apply coffee ground directly to the soil) only after the slow decomposing actions of bacteria and fungi are over. These generally take 2-3 months.
Hence, first, you need to check if the soil for your herbs is deficient in nitrogen. You can use a simple and inexpensive test as the one discussed in this article. If so, you need first a fast-acting fertilizer now, as this releases the nutrients straight away to your plant without even passing in the soil.
Tip: In this case, I would apply coffee ground and commercial liquid fertilizer at the same time (like the ones with the features suggested in this article). This is an ideal solution as, in the meantime, your herbs are fed by the liquid fertilizer, the coffee ground has all time to decompose and release its nutrients.
Coffee Ground Killing Your Herbs With Its Acidity?
Coffee Ground has been accused of damage herbs due to its high acidity. This is the results of misleading information. Indeed, coffee beans are definitely acidic. However, the acidic it is almost washed away when the coffee is produced. Indeed, the acidity “goes” in your coffee, leaving the coffee ground.
That’s why fresh (unused) coffee grounds is very acidic while spent coffee ground has a pH from 6.1 to 6.9 (7 is neutral). Hence, it is only slightly acidic (if used in adequate quantity as discussed before) doing no damage at all to your potted herbs.
Tip: If you want and even more neutral pH, you can wash with tap water your spent coffee ground. This will wash away a bit of the acid (water-lover) chemicals making your coffee ground pH-proofed also for the most sensitive herb.
Remember that herbs are pretty flexible as they will not suffer if the pH is only “around” the neutral level and not exactly 7. Many herbs can easily strive with anything between 6 and 7.5. A table is reported here.
Coffee Ground: Toxic To Dogs And Cats
You need to know that for cats and dogs coffee is a natural toxin. Hence, the ingestion of coffee ground can even be life-threatening for your four-legged friends. Symptoms go from restlessness, loss of liquid through vomiting, heart problems, spasm and even collapse.
Hopefully, the possibility of accidental ingestion is rare (but not impossible) as pets usually dislike the smell of coffee. Some dogs are seriously scared by the smell as they start jumping around like they were threatened.
However, I would be careful concerning where I place the plant to avoid any possible accident. A few suggestions on how to manage an indoor garden with a pet around can be found here.
Effects On Seed Growing
Given the beneficial property of coffee ground, you might be tempted in using coffee ground as a medium where let some seeds germinate. Although studies on the subject have not been found, from an expert gardener that tried to use such soil for seedling, the results were quite poor. Indeed, the seeds either did not germinate, or they were slower than average.
Moreover, a medium entirely of coffee ground is not good due to the high moist level, the lack of structure, and the high nitrogen level. Indeed, seeds are equipped with all the nutrients needed for a plant to germinate. They just need the right temperature and humidity condition that in general, a 100% spent coffee ground medium do not provide.
Coffee Ground And Compost?
This is an excellent alternative to better use your coffee grounds. Indeed, if you have only indoor plants (as, like me, you live in an apartment), you might be producing way more coffee ground that you can use for your herbs and plants. Indeed, as stated before, around 5% of the soil weight can be used as coffee ground, I would avoid more than that.
However, if you have a composter (you can get even one for indoor use) can be a fantastic idea to add to it coffee ground. However, as a rule of thumb, I would not add more than a quarter of the compost pile weight in coffee ground.
If your pile also has worms is even better. Indeed, worms love coffee and, by eating it, they fasten the decomposing process.
Finally, small advice. Remember that the nitrogen in coffee ground will be available in 2-3 months to your herbs as it must be broken down first. However, once the compost-based coffee is ready, its nitrogen content is immediately available to the plant. This because the decomposition process already happened (not in the pot, but in the composter).
What herbs can thrive in acidic soil? Some indoor herbs like garlic chives, horehound, lemongrass, marjoram, oregano can strive in acidic soil with a pH as low as 5.
Does coffee ground expire? No, coffee ground does not expire although, after time they might develop mold. This is totally fine for composting avoid its use in plants or compost if it produces mold on top of it.
Which herbs can be grown in soil using coffee ground? Given that coffee ground is only slightly acidic (lowest pH of around 6) and cannot be used in large amount (due to its caffeine content) it is not expected to alter the original pH balance of the soil significantly. Hence, any herb can be grown in it.