The effect of caffeine on the human organism is something many of us have already experienced for ourselves. But did you know that snails also react to this nerve agent? Can this effect be used, and can coffee grounds help against slugs in the garden?
Coffee grounds, consisting of ground coffee beans, are a by-product of coffee preparation. It contains about 2% nitrogen, 0.8% potassium and 0.4% phosphorus, important plant nutrients, and also tannic acids, antioxidants and caffeine. Therefore, one or the other has certainly already encountered it as a fertiliser. In addition, coffee grounds in the soil attract earthworms, which loosen the soil. Ants and cats, on the other hand, are kept away from the bed. Whether this also applies to snails, you will now learn in the following.
Does coffee grounds help against snails?
First of all, a distinction should be made between different applications. On the one hand, you can scatter coffee grounds as a control method against slugs, on the other hand, one can spray plants with cold coffee or coffee grounds extracts. When used correctly, coffee and coffee grounds can help against snails – but they also have side effects. We will discuss these in the next chapter. The caffeine contained in coffee literally gets on the nerves of snails; after all, it is a neurotoxin.
Dry coffee grounds against snails
As for pure coffee grounds, it is only a deterrent and does not work quite reliably, because the concentration of caffeine in coffee grounds is not sufficiently high. If slugs feel a particularly large appetite, the coffee grounds can hardly stop them.
Cold coffee or coffee extracts against snails
As part of the University of Nebraska research, substrate or leaves of plants were sprayed with caffeine solutions of varying concentrations. It was found that snails are deterred from a caffeine content of more than 0.1% and are even killed in some cases from 1%. The caffeine content varies in different coffee beans. Arabica coffee has a content of 0.8 to 1.4%, Robusta coffee – 1.7 to 4%. However, boiled coffee has a lower caffeine content of approximately 0.06 to 0.1%. So if you want to make a higher concentration caffeine solution like in the experiment above, you have to make a very strong coffee. However, the high concentration of caffeine can harm other creatures in the garden, as you will learn below.
Tip: What you should know with both methods: Caffeine, which provides the effect, is water soluble. That is, when it rains, it is washed away.
What effect does coffee have against snails?
Physiological studies on mollusk neurons suggest that caffeine releases calcium from their internal stores, thereby prolonging the duration of the action potential. The action potential serves to transmit stimuli. If it is elevated for longer than usual, in addition to increased oxygen and energy consumption, other reactions occur in the body that can eventually lead to collapse. However, a snail crawling over soil mixed with coffee grounds is unlikely to ingest enough caffeine to die from it.
Other studies provide evidence that certain ingredients in coffee grounds stimulate mucus production and deter slugs, as they would thus fear increased water loss.
It is also suspected that the smell of coffee alone deters snails, as they have a sensitive sense of smell and coffee scent does not appeal to them much.
Important: Caffeine has a highly concentrated non-directional harmful effect on most living organisms.
First, the neurotoxin caffeine also has a lethal or at least deterrent effect on beneficial insects such as housing snails, earthworms and insects. On the other hand, high concentrations can cause damage to the plant, according to one study.
Use coffee grounds against snails in the garden
In the garden, dried coffee grounds are used against slugs. Coffee powder can be used as well, this even has a higher caffeine content. However, it is questionable whether an expensive and resource-intensive stimulant like coffee should really be scattered in the garden.
We recommend spreading coffee grounds around susceptible plants. The intense odor inhibits the snails to overcome such a wide barrier. The ring should be renewed only once during the year to avoid damaging the soil life with caffeine.
Undiluted cold coffee can be poured on the soil near the plant. Most plants can probably tolerate being sprayed directly with cold coffee. However, flowering plants should be excluded, as the coffee deters pollinators as well as slugs.
So that the coffee is not washed away again directly by the rain, it is advisable to keep an eye on the weather. Therefore, do not spray the plants if a shower is already raining down on them afterwards. Since the application is still relatively new and unexplored, there is unfortunately no precise information yet on which plants might be affected by leaf damage from coffee and in what form. The motto here is therefore: try it out for yourself.
Conclusion: Unfortunately, coffee grounds against slugs does not work reliably and can affect beneficial insects in your garden. If the snail population in the garden is too large, this is not a solution.
How to effectively prevent and get rid of the pests, you can learn from our article on combating slugs. There you will find many tips, such as the use of slug pellets.
If you are not a coffee drinker, do not want to harm beneficial insects with caffeine, or have more than one or two slugs in your flower bed, you’ll naturally look around for alternatives – you can learn more about slug pellets in our dedicated article. This remedy is usually very effective in eliminating slugs in the garden.
In addition to various means that are ready against slugs, you can also rely on the support of certain plants. You can find out what these are in our articles on slug-resistant plants and plants against slugs.