Who hasn't heard of coffee grounds and their use for garden plants?
This simple, free product, accessible to all, is one of the most popular tips on the internet… that wonderful world where, every couple of days, a new revolutionary practice or a grandmother's secret (that will yield a five-kilo courgette in just seventeen minutes or exterminate all slugs within a three-kilometre radius) pops up. All of this is quickly relayed by the entire internet community with the catchphrase "everything on the internet is true!" and the few rare individuals with a critical and scientific mindset are swiftly silenced by the spectre of conspiracy theories.
So let's dare to ask the uncomfortable question: is coffee grounds in the garden fact or fiction?
After all, its benefits in the garden have been discussed for quite some time, and it would be strange if it turned out that all of this was just nonsense...
Coffee grounds and their many uses in the garden
Coffee grounds: a good fertiliser, rich in nitrogen, phosphate... and everything else...
This is the first thing that comes up when discussing the benefits of coffee grounds in the garden: their nitrogen content.
Coffee grounds indeed contain between 2 and 3% nitrogen, which isn't too bad, but this nitrogen is not directly assimilable by plants. Coffee grounds must be composted to release their nitrogen.
The potassium (0.6%) and magnesium (0.3%) levels are not negligible, as well as phosphorus (0.05%) and copper (0.03%). These various elements are, on the other hand, immediately assimilable by plants and in amounts that can prevent deficiency risks.
However, the levels of manganese, zinc, calcium, and iron are far too low to be significant.
Also note that coffee grounds contain a good amount of organic matter but a Carbon/Nitrogen ratio of 24/1. This means there isn't enough nitrogen to allow for the decomposition of carbon. In practice, applying coffee grounds directly to the soil will result in nitrogen being drawn from the soil to initiate mineralisation. This nitrogen will therefore no longer be available to plants: they will be deficient, which is known as nitrogen hunger. The decomposition of this organic matter will also occur very slowly.
In summary, coffee grounds are rich in several elements necessary for plant growth, but not more, and often less, than another amendment like well-matured compost, for example. They should also be used sparingly to avoid nitrogen hunger.
Coffee grounds: an ally in the fight against "pests"?
The caffeine that persists in coffee grounds does indeed have a repellent, even biocidal effect on pest insects but also on other beneficial arthropods in the garden, especially earthworms.
It is quite an effective repellent against ants, and thus, by extension, can help prevent an aphid invasion later on. However, placing coffee grounds at the base of plants infested with aphids will be of no use, as they do not descend to the ground.
Molluscs, including slugs and snails, are not directly affected by coffee grounds. They may be physically disturbed by dry conditions if you surround your delicate plants with a "physical" barrier of coffee grounds. Like ash, this will not work in wet conditions.
In short, the effectiveness of coffee grounds in combating pests is weak and can be risky. An excessive application can have disastrous consequences for the fauna in your garden, as well as in the compost heap. So, caution is advised...
Coffee grounds: a solution against fungal diseases?
No scientific publication has been able to separate fact from fiction regarding this claim. For now, the results from field trials are inconclusive. But if this is indeed the case, one can assume that coffee grounds "will make no difference" between fungal contaminants and the beneficial soil flora. Once again, caution is warranted.
Coffee grounds: a boon for acid-loving plants?
Coffee grounds have a pH of around 6-6.5. One might assume that once on the soil, it will acidify for the benefit of our so-called "heather soil" plants.
In reality, like any organic amendment, there will be a slight acidification during decomposition and mineralisation. But the pH will rise again to approach neutrality.
So, a "drop in the ocean," coffee grounds will not acidify the soil.
Coffee grounds: a growth inhibitor?
Research is ongoing to determine whether a derivative of coffee grounds could one day serve as a non-selective herbicide (yes, again...). For indeed, coffee grounds can significantly slow down plant growth, even kill certain plants such as solanaceous plants like tomatoes, aubergines, and even potatoes.
For now, researchers have not been able to demonstrate which compound might have this inhibitory power but have already concluded that it is not caffeine.
Coffee grounds: a compost accelerator?
As mentioned earlier, in high doses, coffee grounds, or rather the caffeine it still contains, can kill worms, particularly compost worms that are so useful.
What happens is the opposite of what we hoped. We thought we were helping the composting worms by "boosting" them a bit, and instead, they die. It's all about dosage... If you really want to activate or accelerate compost, it's better to use an addition of comfrey leaves, nettles, or ferns, or even a bit of well-matured compost. This will be safer...
In conclusion
Some benefits of coffee grounds can be highlighted: a nutrient supply, a slight repellent effect, but they are unfortunately accompanied by far too many drawbacks to be used thoughtlessly. Like all amendments, in the end.
Coffee grounds are not a miracle product but just... coffee grounds. So don't expect to see your garden transform overnight into a plant paradise just because you've sprinkled coffee grounds everywhere. No, that’s nature's job and... the gardener's.
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