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Orchids and Coffee Grounds: Myths That Are Half True

Coffee beans have had the fame of being an excellent source for supplementing your household plant fertilizer, but when it comes to orchids, are the benefits the same? Do they help?

In this article, I’m going to analyze what is in coffee beans (as in main chemicals elements and minerals), how that interacts with your plant, and why it is (or isn’t) beneficial for your orchids.

Even as a mulch, adding leftover, brewed coffee beans to your orchid pot doesn’t aid your orchid because 1) coffee in its natural form cannot be absorbed by orchid roots without the presence of nitrifying bacteria, and 2) the nitrogen that is released is caused by the presence of bacteria and earthworms and not the coffee grounds.

The only benefit of adding coffee to your orchid would be to serve as a pest repellent, although the high caffeine concentration stunts plant growth, instead of aiding it.

Although high in nitrogen, coffee beans are not an adequate source of fertilizer for orchids because, unlike potted houseplants, certain microbes (called nitrifying bacteria) need to be present in the soil to degrade the coffee beans. Since orchids are epiphytes, the microbes around their roots are not the same ones present in the soil. The coffee grounds will sit on the pot and not be useful in a form that the plant can absorb.

Let’s look at those, one by one.
Orchids and Coffee
Image Credit: © 2021 Orchideria.  All Rights Reserved. 

Orchids and Coffee Beans: Ask for Sources

If you look superficially on the internet, there are several articles—most all to be honest—that state that orchids will benefit from having the ground-up coffee placed around the pot and go on to cite the benefits of coffee grounds for your orchids. It’s too easy to be mistaken about this if you don’t take the information and apply it to the knowledge you already have about orchids.

This is quite confusing because if you take a quick look at the internet, tons of articles will say that coffee grounds are beneficial to plants, even orchids. But the problem here (and I have done the same thing—so I unfortunately speak from experience) is that when you are reading a good article about fertilizer supplements, it usually applies to household plants that live in the soil.

The person writing the article may have some knowledge of orchids, but not to the point that it is a hobby or that they even own an orchid. For example, I can write about boats without ever owning a boat. All I must do is look at what others wrote, copy and paste the information, and make an article.

The result: so much confusing and conflicting information on the internet about orchid care.

That is why it’s important to present the scientific studies, the research, name the authors, and look a little deeper into the chemistry behind the plant cells. Even though I’m not an orchid expert and don’t claim to be, I like to read. I like to figure stuff out. Curiosity is what makes my wheels turn.

For household plants, coffee beans have their purpose and are great. For orchids, coffee beans are not beneficial and can present some very harmful side effects.

Coffee Beans Need to Be Reduced into A Form the Orchid Can Absorb

In the soil, several beneficial bacteria break down elements and present them in a form that is absorbed by the plants. The goal in breaking down coffee is to present it in a form that plants can absorb, which essentially is ammonia and nitrate. This is easiest performed by nitrifying bacteria, present in the soil.

Yet...Orchids don’t live in soil.

According to Lawrence Belser and his study called “Population ecology of nitrifying bacteria”, published in New Zealand, “Nitrifying bacteria are present in distinct taxonomical groups and are found in highest numbers where considerable amounts of ammonia are present (areas with extensive protein decomposition, and sewage treatment plants). Nitrifying bacteria thrive in lakes and rivers streams with high inputs and outputs of sewage and wastewater and freshwater because of the high ammonia content.” (Source)

These same bacteria are not present on the tree bark and do not interact the same way with epiphytic orchids as they would terrestrial orchids. I won’t go into plant bacteria, but I found the article cited below extremely eye-opening. I had to read it a couple of times to grasp it—it’s not your relaxing read before bed type article.

To summarize: the bacteria necessary to change the coffee grounds into something the orchid can absorb just aren’t present in orchid bark (or at least not in enough quantity to make any difference). In the ground soil, this is a different matter. (Source)

This is the main principle of why coffee beans (either fresh seeds or ground and brewed) will not affect your orchid. The orchid roots just can’t absorb the coffee in the form that it is in and need bacteria—earthworms also work—to help them. Once that coffee is digested and expelled into ammonia and nitrate, then the orchid can absorb the nitrogen, potassium, calcium, and all the other minerals that coffee contains.

In its natural form, the coffee just tans the orchid roots and makes them brown. Coffee doesn’t make it across the velamen and even if it did, the roots wouldn’t know what to do with it.

The best comparison I have (please forgive this example upfront) is an adolescent trying to make lunch. Time and time again my kids complain that there is nothing to eat but both the pantry and fridge are full. What they want is for me to cook it for them, into a meal they can eat.
Coffee and Orchids
Image Credit: © 2021 Orchideria.  All Rights Reserved. 

Ground Coffee Beans Serves as an Orchid Pest Repellent

Since I don’t want this article to be all negative, I will add this section to say the benefits of coffee grounds and orchids. The main one is that it works as a pest repellent. If you scatter the beans around your orchid display, the strong fragrance of coffee will repel most insects and pests that attack orchids.

It has been proven that coffee beans repel slugs and snails. In this article called “Caffeine as a repellent for slugs and snails: At high concentrations, this stimulant becomes a lethal neurotoxin to garden pests”, published by Nature’s Publishing Group, the authors proved that because of the toxicity of caffeine, 60% of the slugs died when sprayed with 1% solution, and 95% died when sprayed with a 2% solution.
(Source)

Yet 5% of snails have been known to slide right across the coffee barrier if they were determined to get to the other side. So, it’s not 100% slug repellent and that is a solution prepared by scientists. It’s questionable how much caffeine is still left in the coffee after I’ve brewed it and left the stained filter out to dry.

Other sources have mentioned that coffee grounds repel and deter mosquitoes, fruit flies, beetles, but not ants. If you have ants on your orchids, you might want to read this article (link) I wrote that talks about a few methods to get rid of them.

On one side of the coin, coffee beans repel most insects, but the other side is that they also attract tiny critters. Coffee beans to earthworms are the best smell ever, and where you spread them around in your garden outside, it can be an excellent way to attract more earthworms. For the outside garden and terrestrial orchids, attracting earthworms is the only benefit of coffee and orchid care.

Coffee Beans as Mulch in Your Orchid Pot

Since coffee beans attract earthworms, the benefits of using them as mulch are beyond doubt beneficial in your garden. The problem arises when you bring that pot that was planted outside and try to imitate that same condition with your epiphytic orchids.

The microbes that are necessary to break down the coffee beans into elements that are plant accessible are just not present. This is what took me the longest time to understand. I keep thinking that there are bacteria on the tree trunks, and beneficial bacteria are present even when the orchid is attached to the tree.

Beneficial bacteria are present with epiphytic orchids, but they aren’t the same bacteria. That’s where I was always making my mistake. This is why eggshells aren’t the best either, since the bacteria that break down the eggshells aren’t present in the orchid bark. So, the eggshells sit on the top of the orchid pot and never degrade.

By the way, I wrote a whole article about eggshells and their benefits for your orchid, which you can read here.

I need to rewrite that article since I know more now than I did when I wrote it. In that article, I cite 3 methods—so just keep the last one. The other 2 methods don’t work.

Yet let’s imagine you have an outside garden and you have coffee beans that have been used. You’re going to discard them anyway, so you might as well use them as mulch. They will attract earthworms, which make your garden healthier. Many articles cite the benefits of coffee grounds as a mulch.

If you are going to use the coffee beans as a mulch, add some source of carbon to aid in the decomposition process. Since the coffee beans have such a high source of nitrogen (earthworm or not), they must also have carbon to aid in the process.

The ratio is 20 carbon (C) to 1 nitrogen (N). So, add the coffee filter to the compost pile, or shred newspaper or any other source of carbon. Without it, the decomposition will take longer and not be fully complete. (Source)

There are no nitrifying bacteria in the orchid pot, neither are earthworms present. This is why it’s not a good idea to use coffee in your orchid pot even as a mulch.

Coffee Grounds Raise Nitrogen
An Element Orchids Need

There are so many places that state that coffee beans will raise nitrogen in the soil, with freshly ground coffee beans or the leftover brewed coffee grounds. This isn’t exactly true.

What happens is that the coffee attracts the earthworms and they will degrade the coffee. Those residual “leftovers” are rich in nitrogen. So, the credit is all with the earthworms, and not the coffee.

Because earthworms dig trails and burrows underground, they aerate the soil, not the coffee, nor the nitrogen in the coffee. The soil becomes better off because of the earthworms, and coffee is just the dangling carrot in front of them.

What exactly is in the coffee bean? That’s what I’m getting into next.

Grinding the Coffee Bean:
What Elements Does It Contain?

You’ve probably heard that coffee contains a high percentage of nitrogen, right? But what else is in coffee? Coffee contains caffeine, and honestly, that and prayer is what keeps me going.

I practically live off macchiato mugs and all things related to coffee. It’s what keeps me up writing articles when I should be sleeping… Being raised in a coffee-loving country, Brazil, had a little to do with that. Anyway, besides caffeine, which plants don’t have any practical use for, let’s look at what is in coffee.

In an article from Eleven Coffees, the author states that “on average, a single arabica coffee bean contains 1.9 milligrams of caffeine (1.2 – 1.5g of caffeine per 100g). A single robusta coffee bean has 2.9 milligrams of caffeine (2.2 – 2.7g of caffeine per 100g). A 250ml (8.5fl oz) cup of arabica filter coffee contains 100 milligrams of caffeine.” (Source)

In essence, that means 2% of caffeine is present in the ground coffee. Yet I haven’t found one place that grinds pure coffee beans and uses that as a fertilizer, so I looked into how much caffeine was present after brewing.

In an article by Coffee Chemistry, the author states that 90% of the caffeine is extracted in the first minute of brewing. (Source)  That means there’s not much caffeine after brewing. That’s a real disappointment because I always heard otherwise.

Caffeine doesn’t do much besides serve as an insect repellent since it is toxic to most orchid predators. Since caffeine was ruled out of the picture, let’s look at the other elements.

According to Belitz HD, Grosch W, and Schieberle P., in their book, “Coffee, tea, cocoa. Food Chemistry”, Potassium is the major mineral in coffee ash (1.1%), followed by equal portions of calcium (0.2%), and magnesium (0.2%). The major anions include phosphate (0.2%) and sulfate (0.1%), along with traces of other elements.
Nitrogen
 2.0
Potassium
 1.1
Calcium
 0.2
Magnesium
0.2
Phosphate
 0.2
Sulfate
0.2
According to “A Detail Chemistry of Coffee and Its Analysis” as far as the chemical formation and main constituents that are present in coffee that would be of any concern to orchid care, caffeine is the first. Tannin, fixed oil, carbohydrates, and protein follow. Unbrewed, fresh coffee beans (or seeds) contain 2–3% caffeine, 3–5% tannins, 13% proteins, and 10–15% fixed oils. (Source)

Tannins are excellent for orchids, and I wrote an article here about how tea benefits orchid care (read it here). Yet tea is very different than coffee.

In essence, if the orchid could absorb the coffee in its natural form, it would be a source of mainly potassium and nitrogen…still it can’t. So none of those are absorbed, no matter how good they are.
Coffee and Orchids
Image Credit: © 2021 Orchideria.  All Rights Reserved. 

Coffee Interferes with the pH
of Acid-Loving Orchids

Since the orchid can’t absorb these minerals, what about the benefits of lowering the pH of the soil? Since coffee is extremely acidic, then it would benefit the orchid pH…right?

Here is another example of how a truth (coffee is acidic) can turn bad (coffee makes the soil acidic, lowering the pH, therefore, orchids love it). This is where a lot of the articles that support the use of coffee stake their truth.

Coffee is indeed acidic, and the coffee we drink can take us for a loop acidity-wise. The problem is that none of us are pouring pure coffee on our orchids, but we are putting the leftover coffee grounds on the orchids. (Well, I hope we’re not doing this, but for argument's sake, I’m using this as an example.)

In this article by Well and Good, the author states that “coffee grounds lose their acidity through the coffee-making process”. The main benefit of using coffee as a fertilizer supplement was not the chemical elements of nitrogen and potassium that coffee releases, but that coffee will raise the humidity in the soil.

In this sense, moisture-loving plants would aid from the sprinkling of coffee around the pot. These plants include Bugbane, Calla, Crinum, Elephant Ear, Forget-Me-Not, Hibiscus, Iris, Lily of the valley, Marigold, Meadowsweet, and Sedge. Orchids are not on that list. (Source)

In another article, the pH of brewed coffee beans was measured and found to be in between 6.5 to 6.8. This is a very good range for most orchids that we grow indoors or in our homes. Most common orchids sold on the market today (Phalaenopsis, Cattleya, Dendrobium, Epidendrum, etc…) have a pH preference of 5.5 to 6.5, so using coffee would not lower the pH in the potting medium to critical acidity levels.

If anything, leftover brewed coffee beans are a tad bit more alkaline than orchids like.

Coffee Grounds Do not serve as a Substitute for Nitrogen Fertilizers

Even though the high amount of nitrogen that is present around coffee beans, it is not considered suitable for a replacement if your orchid has nitrogen deficiency. By the way, I made a video about nitrogen deficiency specific to orchids that you can watch on my YouTube channel. (LINK)

According to a germination test performed by GrassRoots Gardens in Eugene, Oregon, the plantations that were fertilized with coffee didn’t show great results. In fact, the pests were reduced by 25% but also were the crops. When lettuce showed signs of growth, it was stunted growth, if any at all.

In some cases, coffee has been used not as a nitrogen fertilizer supplement, but to remove the high content metals in the soil. A study published by the National Center of Biotechnological Information stated that “spent coffee grounds (SCG) and charred spent coffee grounds (SCG-char) have been widely used to adsorb or to amend heavy metals that contaminate water or soil and their success is usually assessed by chemical analysis.”

That study proved that the coffee wasn’t responsible for such evidence, but the microorganisms in the soil which were attracted by the coffee. (Source)
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By now, there’s not much else to say… Tea for orchid care is fine, coffee is not. I hope that this article explained why to stay away from using leftover brewed coffee grounds. If you have a question, please ask below. It’s always a pleasure to research questions, as I sip on my cup of coffee.

In all, I wish you the best in orchid care, and happy cultivating!
Signature Amanda Matthews
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This article was published on:
September 13, 2021
Written by:
Amanda Matthews
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I write all the tutorials on Orchideria so unfortunately, I can't blame anyone else for all the spelling mistakes.   :)

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