Coffee… Good for Your Plants?

In North America, we consume an average of three cups of coffee per day. Imagine what that means for the coffee residue, commonly known as grounds! Not surprisingly, many scientists have been asking: How can we reuse what is left in our filters? How can we integrate these millions of tons of waste into a more circular economy?

Coffee is attributed several properties. Many urban legends are widely circulated about it. However, as you will see, most of the claims about coffee are false!

What Is Found in Coffee Grounds?

Most of the myths about coffee grounds probably start with the fact that coffee is particularly rich in nutrients. In fact, coffee grounds are particularly rich in nitrogen. The ratio of carbon to nitrogen is generally estimated to be 10:1. Small amounts of potassium, phosphorus, magnesium, and calcium are also found in coffee grounds. Then, on a smaller scale, there are lipids, triglycerides, fatty acids, lignin and phenols, and some traces of aluminum, iron, manganese and copper. And, of course, coffee contains caffeine!

Reading this long list, you might think this is a miracle ingredient. Unfortunately, that’s not always the case. Let’s take a moment to debunk some persistent myths and discover how coffee grounds can be useful in the garden.

Repel Cats?

Not proven! There are no studies to prove the true effectiveness of coffee grounds in attracting or repelling animals and insects. It is not known whether cats flee from places where coffee grounds have been scattered on the ground.

Kill Slugs?

Not proven! Many gardeners looking for eco-friendly solutions have suggested sprinkling coffee grounds around hostas to kill slugs. While coffee may not be effective in killing these gluttonous gastropods, a recent study has shown that coffee grounds do indeed have a repellent effect! But don’t go bonkers yet. The same study reveals that coffee grounds, which are effective against slugs, are also… toxic to plants! And when you get to a compost mix that is safe enough for plants… it loses all its effectiveness against slugs! In other words, you can’t successfully repel slugs with coffee grounds.

Fertilize Plants?

Oh dear! There are several nuances to consider. First, coffee grounds contain caffeine and other toxins that inhibit plant growth. In other words, despite the interesting amounts of nitrogen, phosphorus and potassium, at first glance, pure coffee grounds is generally harmful to plants. It has been proven that a thin layer of coffee grounds incorporated into the surface soil is detrimental to the growth of cabbage, radishes, sunflowers, leeks and violets!

I can personally attest to this! Every year when I teach botany, I do an experiment with the students. One of them repots a plant in a substrate made entirely of coffee grounds. And every year it is the first plant to die!

Limit Weed Growth?

Possibly! As mentioned earlier, coffee grounds used directly on the ground have a detrimental effect on plant growth. We can therefore assume that they would prevent the growth of weeds. Unfortunately, it is quite rare that our weeds do not coexist with “good plants”… A bit like slugs: we would have to kill our plants to get the benefits of coffee as a natural herbicide!

To Use as Mulch?

No! All studies confirm that thick layers of coffee residue deposited on the surface of the soil can become problematic. First, the coffee particles on the surface form an impenetrable crust. Then the plants will start screaming in agony, begging you to end their lives! Everyone agrees that coffee mulch is detrimental to healthy plant growth and sometimes even survival!

However, little effect is seen when coffee grounds are applied in thin layers and incorporated into the top few centimetres of soil.

Good for Acidifying Soil?

Yes and no! In fact, the big challenge here is that not all coffee beans are created equal and not all coffee extraction techniques have the same effect. For example, we can find coffee grounds with an acidic, neutral or even alkaline pH! So we cannot say that coffee acts as an acidifier. Moreover, even if the coffee grounds are acidic, this acidity does not last very long and the microbial activity of the soil quickly neutralizes this property.

Coffee in the Compost?

Yes! This is where you really get the most out of your coffee grounds. To take full advantage of coffee’s fertilizing properties, it must be diluted and composted over a long period of time. Most studies suggest a maximum of 10 to 20% coffee grounds in the compost. Beyond this percentage, the toxic effects of coffee become apparent.

Also, a recent study confirms that compost aged for a long time, more than 14 months, becomes very interesting for plant growth. Without really being able to explain it in detail, the researchers come to the conclusion that the effects of caffeine and other toxins are neutralized by a prolonged period of composting.

Furthermore, more and more research confirms that the larvae of the black soldier fly (Hermetia illucens) can live and feed directly in pure coffee grounds. It would therefore be realistic to produce coffee-based frass!

Another interesting fact if you are into indoor mushroom growing is that incorporating coffee grounds into growing substrates has become a very good way to recover these residues. This is especially true for growing oyster mushrooms.

Fun fact: It has been discovered that caffeine is transferred into the fruiting bodies of mushrooms, but you would have to eat 250 kilos of oyster mushrooms to get the effect of one cup of espresso! So I will continue to grind my morning coffee, while continuing to look for the best ways to reuse my coffee grounds.