It is well-known that leftover coffee or spent coffee grounds makes a great organic fertilizer for the garden.
But, is coffee really good for your orchids as well, especially if you grow them indoors?
Coffee is not good for epiphytic orchids because they don’t grow in soil with organisms to break down the nitrogen in coffee; coffee is too acidic and doesn’t have enough caffeine to kill snails; Mulching using coffee grounds is impractical for orchids and can clog up the medium to cause root rot.
Don’t get me wrong. I am not against using organic fertilizers for orchids. I have actually successfully used a natural homemade fertilizer on my orchids with great results.
But, not everything organic and natural works, especially for orchids. A similar problem goes with using rice water to fertilize orchids.
Some of the reasons have to do with how epiphytic orchids are grown, and the others are related to the problem of coffee for orchids.
In this article, I will take an in-depth look at the common claims of why coffee is a good fertilizer and present 6 reasons why it is not for orchids.
Myth of coffee as orchid ferrtilizer
Before we start, let’s first take a look at how people fertilize orchids using coffee.
There are different ways of preparing a coffee fertilizer for orchids. According to one popular recipe, it is made by mixing ½ lb. of used coffee grounds in 5 gallons of water, let sit overnight, and drained to discard the grounds. The coffee grounds tea is then mixed with ground eggshells, 1 tsp of molasses and diluted before using it to water orchids monthly.
Other recommendations include using coffee grounds as mulch to put on top of the orchid medium, or incorporating coffee grounds into the orchid medium.
The general rationale behind using coffee to feed orchids is that:
- Coffee is rich in nitrogen, a macronutrient crucial for plant growth
- Coffee improves soil structure and soil aeration
- The caffeine in the coffee kills snails
- Coffee lowers the pH of the growing medium which is beneficial to acid-loving plants like orchids
I will refute these claims one by one, present 6 facts below to prove why you should not feed your orchids with coffee.
Fact 1: Orchid medium has no organisms to break down nitrogen
It is true that coffee or coffee grounds have nitrogen, a macronutrient that is crucial for plant growth.
For coffee to release its nitrogen, it needs to be first broken down into ammonia and then nitrate which is a form that can be absorbed by plants. This decomposition process is long and requires nitrifying bacteria which only exist in soil and a moist medium.
But, epiphytic orchids do not grow in soil. They also do not stay in a moist medium all the time as they like to go through wet and dry cycles. Once the orchid medium dries up, nitrifying bacteria, if any, would die. Even though coffee is rich in nitrogen, it cannot be absorbed by orchid roots.
Nevertheless, used coffee grounds can be used with terrestrial orchids because they grow in soil.
Fact 2: Mulching with coffee grounds is impractical for orchids
The idea of sprinkling coffee grounds above an orchid substrate as mulch is not a good idea.
First, it is impractical for orchids that are generally flushed every few weeks. Flushing is done to prevent root burn due to any buildup of minerals and salts from the use of fertilizers. (Here’s an article about fertilizer burn for orchids’ roots) During flushing, any coffee mulch would likely be washed away.
Second, coffee grounds can clog a moss substrate, preventing airflow to the roots. This can easily lead to root rot.
Also, when coffee grounds are sprinkled on top, the process of decomposition would attract gnats and molding.
Fact 3: Coffee doesn’t contain enough caffeine to kill snails & slugs
It is true that caffeine can kill snails and slugs. According to a study by Hollingsworth, Armstrong & Campbell (2002), a 2% caffeine solution can kill 95% of snails and slugs.
But, dosage is important. The study found that a 1% caffeine solution could only kill 50% of snails, and a 0.1% caffeine solution could kill much less, around 20% of snails.
The coffee we drink generally contains only 40mg (or 0.04g) of caffeine for every 100g of coffee, which is only a 0.04% caffeine concentration. Such a weak caffeine solution would not be strong enough to kill snails. What’s more, used coffee grounds contain even less caffeine.
Fact 4: Coffee accelerates decomposition of orchid medium
The acidity of coffee lowers the pH of a growing medium which will not be beneficial to orchids even though they are acid-loving plants. Depending on the coffee, some types have a pH as low as below 5, but orchids prefer to be in an environment that is only slightly acidic.
The acidity from coffee can also accelerate the decomposition of a medium of bark or moss. A decomposed medium is often compacted and stays moist for too much time for orchid roots, making them easy to rot.
Fact 5: Coffee grounds reduce airflow to orchid roots
Instead of improving soil aeration, incorporating coffee grounds into the orchid substrate would actually do the opposite: clogging up the medium, making it easy to develop root rot.
Epiphytic orchids do not grow in soil and are adapted to growing on trees with their roots exposed to the air.
Fact 6: Benefits of tannic acid in coffee are not proven
Coffee, just like tea, has a high content of tannic acid. Some people claimed that the tannic acid in coffee have stimulated their orchids to grow roots.
But, their claims are not backed up by any studies. Nobody knows if roots grow and if flowers bloom because of tannic acid. A controlled study comparing orchids fertilized with tea and without tea is needed.
The only fact about tannic acid we can be certain is that it gives a low pH to coffee. And a low pH can be a problem for accelerating the decomposition of medium, as previously mentioned.
To sum up, coffee may be good for your garden for its nitrogen content, for mulching, for retaining moisture and for improving soil texture and aeration.
But, do not use it for your epiphytic orchids, simply because they do not live in the soil. They are used to growing in well-ventilated environments.
There are no benefits of using coffee as orchid fertilizer. Orchids are not able to absorb the nitrogen in coffee because of a lack of organisms in the orchid medium to convert it into a useable form. In fact, using coffee as orchid fertilizer may even cause problems, such as issues of molding, pests, root rot due to the high water retention nature of coffee grounds.
There are natural fertilizers that do work for orchids. But coffee is definitely not one of them.
Hollingsworth, R., Armstrong, J. & Campbell, E. (2002). Caffeine as a repellent for slugs and snails. USDA National Wildlife Research Center – Staff Publications. 470.