Slugs can wreak havoc on gardens, voraciously devouring young seedlings and tender plants with alarming efficiency. While conventional methods of slug control often involve harmful chemicals that pose risks to beneficial insects, pets, and children, there’s a safer, environmentally friendly alternative, which is using a coffee soil drench, which can be made from a common home or cafe waste product, spent coffee grounds.
In this article, we’ll explore how to effectively eliminate slugs using a coffee soil drench.
Understanding Slug Behaviour
Before diving into the solution, it’s essential to understand the pest’s behaviour first. Slugs thrive in moist environments, emerging at night to feast on a variety of plant material. They leave behind a telltale silvery trail of slime, and can quickly decimate a garden if left unchecked.
Both snails and slugs feed on a living plants and decaying plant matter. They cut holes in the middle of plant leaves, eat young seedlings completely to the ground, damage fruit located close to the soil surface such as strawberries, attack low-hanging fruit such as such as tomatoes.
These pests have a rasp-like tongue known as a radula, which is covered in rows of teeth which they use to scrape off particles of food to eat. It can also be used to cut holes in the middle of a leaf, rather than just having to eat from the leaf edges only. As such, large holes in the middle of leaves usually indicates snail or slug damage.
Unlike their shell-bearing counterparts, snails, slugs can burrow underground to escape the dry conditions on the surface, especially during hot or dry weather. Even though snails can also burrow, their shell restricts their ability to burrow as deeply or as easily as slugs do. Slugs also often burrow into the soil to hibernate during the colder months. This behaviour allows them to survive freezing temperatures and emerge to feed again in the spring. Pests which hide underground can be more easily targeted by using soil drenches, which are liquid pest controls that are poured onto the soil.
Traditional slug control methods often involve toxic chemicals, such as methiocarb or metaldehyde, which can harm beneficial insects and contaminate the soil. However, by harnessing the natural compounds in coffee grounds, we can combat slugs without resorting to harmful chemicals.
The Power of Coffee Grounds
Coffee grounds are more than just the leftovers of a morning pick-me-up beverage!
We can use coffee grounds as a soil amendment and fertiliser in the garden as a sustainable way to improve soil structure and promote healthy plant growth.
Additionally, coffee grounds can also can be used as a natural and effective pest control measure due to the caffeine they contain. Since caffeine is toxic to slugs and snails, it can be used as a barrier can deter them from crossing treated areas, and as a means of eliminating them after they come in contact with it.
What Effects Does Caffeine Have on Slugs (and Snails)
Caffeine (also known as 1,3,7-trimethylxanthine) is a plant alkaloid belonging to the class of compounds known as the methylxanthines. The primary effect of caffeine is that it’s neurotoxic to snails and slugs. As with humans, caffeine acts as a stimulant, but its effects are significantly more harmful to these creatures, potentially leading to their death.
Caffeine specifically disrupts the neurotransmitters within their nervous systems, chemicals crucial for transmitting nerve signals. By interfering with these signals, caffeine can induce paralysis in snails and slugs, which can lead to their death if the exposure (dose) is sufficient.
Interestingly, the presence of caffeine in plants is to serve as a natural defense against insect predation, that works by reducing insect feeding. Despite snails and slugs not being insects but molluscs, caffeine can also cause adverse changes in their behaviour, making them less likely to feed or reproduce. This indirect effect can also contribute to their decline in gardens treated with caffeine.
How to Make a Coffee Soil Drench
Materials Required
- Used coffee grounds
- Water
- Large container or bucket
- Watering can (optional)
- Strainer or cheesecloth (optional)
Instructions
- Collect used coffee grounds from your daily brews. If you don’t drink coffee, many local coffee shops often give away used grounds for free if you ask them.
- Combine the coffee grounds with water in a large container, using about one part coffee grounds to three parts water. The exact measurements don’t need to be precise, the aim is to create a concentrated coffee solution.
- Stir the mixture thoroughly to ensure the coffee grounds are fully soaked.
- Allow the mixture to steep for at least 24 hours. This will extract the maximum amount of caffeine and other compounds from the coffee grounds.
- Optionally, after steeping, strain the mixture through a fine-mesh strainer or cheesecloth to remove any solid particles. This can help prevent clogs if you plan to apply the drench using a sprayer for foliar application.
Application Method
Once the coffee soil drench is made, here’s how to apply it to the garden beds:
- Apply the drench on a dry, sunny day, as this allows the solution to dry quickly, so that it remains more potent and effective.
- Pour the coffee soil drench directly onto the soil around vulnerable plants, creating a barrier that slugs will be reluctant to cross. Focus on the soil around the perimeter (edges) of the garden beds, near areas where slugs are most prevalent, and around plants prone to slug damage.
- Pouring the coffee soil drench solution into a watering can first (without all the solids) can make application a bit easier than pouring the solution straight out from a bucket, especially around young seedlings.
- Reapply the coffee soil drench after heavy rain or watering, as its effectiveness can decrease over time.
Alternative Method: Strong Brewed Coffee Solution
Research by Linda Brewer, a soil scientist with the Oregon State University Extension Service, showed that using a 1% to 2% caffeine solution mixed with water as a soil drench caused 100% of slugs to leave the treated soil and then die of caffeine poisoning. She also found that a 2% solution of caffeine applied to the growing medium of orchids killed 95% of orchid snails and worked better than a commonly used (and toxic) liquid metaldehyde slug bait product.
The 1%-2% caffeine solution soil drench used in this study was prepared using strong brewed coffee rather than spent coffee grounds. Here’s how it was done, it’s another method we can use to prepare a coffee soil drench:
- For a soil drench, mix 1 part water with 2 parts strong brewed coffee. For example, use 1 cup water to 2 cups of coffee.
- For a foliar spray to protect foliage and deter slugs from feeding on leaves, dilute 1 part brewed coffee with 9 parts water and apply as a spray.
Note: When using a strongly brewed coffee solution as a foliar spray, it’s a wise precaution to test this solution on a few leaves first, and then wait for a few hot and sunny days, watching for leaf burn or any other damage. If all looks well, then proceeding with further spraying over a whole plant.
Benefits of Using a Coffee Soil Drench
Natural pest control methods are always a better choice than using toxic synthetic chemical controls that harm the environment.
There are various benefits to using coffee grounds as a pest control for slugs in the garden:
- Environmentally friendly – Coffee grounds are a natural byproduct that would otherwise end up in the trash and landfill. By repurposing them as a slug deterrent, we can reduce waste and minimise our environmental footprint.
- Safe for plants, pets, and people – Unlike chemical slug control methods, coffee grounds pose no harm to plants, animals, or humans. They’re safe to use around children and pets, providing peace of mind for gardeners.
- Improves soil health – In addition to deterring slugs, coffee grounds enrich the soil with organic matter, improving its structure and fertility over time. Once the soil drench is made, add the solid coffee ground material that remains into the compost or soil.
Controlling slugs in the garden doesn’t have to involve harmful chemicals or expensive treatments. By using of coffee grounds, we can prepare a simple coffee soil drench which can effectively deter and eliminate slugs while nourishing the soil and minimising environmental impact.
For more information on natural and environmentally safe snail and slug control methods, see the following articles:
- Garden Pests – How To Control Snails And Slugs Without Toxic Chemicals
- Do Copper Strips Really Work As An Effective Barrier To Snails And Snails?
References
- Used appropriately, coffee grounds improve soil and kill slugs. (2023, June 23). Oregon State University. https://today.oregonstate.edu/news/used-appropriately-coffee-grounds-improve-soil-and-kill-slugs
- Hollingsworth, Robert & Armstrong, John & Campbell, Earl. (2002). Pest Control: Caffeine as a repellent for slugs and snails. Nature. 417. 915-916. 10.1038/417915a.
- Nathanson JA. Caffeine and related methylxanthines: possible naturally occurring pesticides. Science. 1984 Oct 12;226(4671):184-7. doi: 10.1126/science.6207592. PMID: 6207592.
ADULTS: Slugs are hermaphrodites—every slug is born with both male and female reproductive parts and any slug is capable of laying eggs, though self-fertilization can occur.
Yes, slugs and snails are quite unusual in that respect. Here’s another interesting fact. Unlike our red coloured, iron-based blood pigment hemoglobin that carries oxygen, some slugs and snails have blood that is copper-based, and uses the oxygen-carrying pigment called hemocyanin. Hemocyanin contains copper atoms, which give it a blue color when oxygenated. When oxygenated, hemocyanin is blue-green, but when deoxygenated, it can appear colorless. Also, they don’t technically have blood, it’s known as hemolymph, the fluid equivalent of blood in invertebrates!