Slugs and Snails
Slugs and are often considered a perennial nuisance for many UK gardeners, munching through young plants, vegetables, and flowers, leaving frustrated gardeners in their wake.
While slugs can be frustrating for gardeners, they are an integral part of the ecosystem. From recycling nutrients to serving as a food source for wildlife, slugs contribute in ways that benefit soil health, biodiversity, and scientific research. Rather than viewing slugs as purely destructive, it’s important to recognise their ecological value and the role they play in sustaining healthy environments. Understanding and respecting their place in nature can help us create more balanced approaches to managing them in our gardens.
Despite the wealth of advice available on how to deter these slimy visitors, not all methods stand up to scientific scrutiny. In this blog post, we explore the effectiveness of various slug repellents and deterrents, based on UK studies and science-backed insights.
At Wellies and Waxjackets, we don't endorse, recommend or suggest using any of the methods described in this blog. This is simply an informational point for anyone considering the use of any slug or snail repellents, deterrents or killers.
About Slugs: Tiny Teeth and Big Appetites
Before delving into slug repellents, it's important to understand the slugs themselves. The UK is home to around 30 to 40 species of slugs, but only about 10 species commonly cause problems in gardens. Slugs belong to the mollusc family, and unlike snails, they lack the protective shell. They have an incredible number of tiny, serrated teeth — some species have up to 27,000, which are constantly replenished, allowing them to grind down plants like sandpaper.
While many species of slugs primarily consume decaying organic matter, certain species, such as the deroceras reticulatum (grey field slug), are notorious for devouring live plants. Another garden villain is the brown garden snail (Cornu aspersum), which can cause extensive damage in both gardens and agricultural crops.
Slug Pellets: Harmful or Helpful?
Slug pellets are a common go-to for many gardeners, but how effective and safe are they? In the UK, slug pellets containing iron phosphate have largely replaced the previously available metaldehyde pellets, which were withdrawn due to their negative impacts on wildlife, soil fauna, and pets. Metaldehyde was fast-acting but indiscriminate, often harming not just slugs but other beneficial animals such as birds and hedgehogs.
Iron phosphate works by disrupting the digestive systems of slugs, causing them to stop feeding and eventually die. However, this is not an instantaneous process. Slugs often retreat underground to perish, making it less obvious that the treatment has worked. While iron phosphate is considered a safer option, it’s worth noting that "organic" does not always equal "safe." Naturally occurring iron compounds are indeed organic but can still affect the balance of soil life over time.
Nematodes: Nature’s Tiny Assassin
For gardeners seeking a more natural, eco-friendly approach, nematodes offer an intriguing solution. These microscopic parasitic worms specifically target slugs. When applied to the soil, nematodes seek out slugs, penetrating their bodies and releasing bacteria that are fatal to the slug. An organic method of dispatching. This method is highly effective and safe for pets, humans, and wildlife, as it targets only slugs and snails.
However, nematodes do not differentiate between "good" and "bad" slugs. Some slugs, such as those that consume dead or decaying plant material, are beneficial for the garden ecosystem. Therefore, nematodes should only be used in a targeted manner, avoiding over-application that could harm non-pest species.
Common Slug Repellents: What the Science Says
Now, let’s explore some of the popular slug repellents often recommended by gardeners and evaluate their efficacy based on available research.
1. Crushed Eggshells: A Potential Attractant?
Crushed eggshells are often touted as a barrier for slugs due to their sharp edges. The theory is that slugs, with their soft bodies, will be deterred by the sharp fragments. However, studies show little to no effectiveness. Slugs are capable of altering the consistency of their mucus to navigate rough or sharp surfaces without harm.
Interestingly, there’s some evidence to suggest that eggshells may even attract slugs, as they could be viewed as a food source. Tiny bits of egg residue or the calcium content of the shells might entice rather than repel slugs. So, while crushed eggshells might add nutrients to your soil, they are unlikely to be a reliable slug deterrent.
2. Wet Wool Pellets: An Unproven Approach
Wet wool pellets are another commonly suggested remedy, claimed to deter slugs by creating a barrier that slugs find difficult to cross. Unfortunately, little scientific research exists to validate this method. While wool might have some drying effect on the surface, making it less hospitable to slugs, there is no concrete evidence that it provides significant protection.
3. Copper Tape: A Tarnished Solution
Copper tape is frequently recommended as a way to keep slugs out of plant pots and garden beds. The idea is that when slugs come into contact with copper, it creates a mild electric shock due to their moist bodies reacting with the metal. However, research shows mixed results. One issue is that slugs can easily bypass copper barriers by burrowing under the tape or simply waiting for it to tarnish, at which point it loses its repellent effect.
In addition, many copper products marketed as slug repellents contain very little actual copper, meaning they are unlikely to work as advertised. Copper collars and other devices have a limited role, but they are not foolproof solutions.
4. Bark Mulch: Drying Out the Surface
Mulch, especially bark mulch, can serve as a deterrent by keeping the soil surface drier, which slugs generally dislike. Bark mulch doesn’t directly repel slugs, but it creates less favourable conditions for them to thrive. Additionally, it can offer some protection by making it more difficult for slugs to move across the surface, especially in very dry conditions.
It’s important to note, however, that mulch needs to be applied thickly to be effective and that slugs may still find their way through if the mulch becomes damp.
5. Horticultural Grit: A Sharp Deterrent?
Horticultural grit is often suggested as a physical barrier to slugs. Similar to the reasoning behind crushed eggshells, the theory is that slugs won’t want to cross sharp, jagged surfaces. While this sounds plausible, slugs can often secrete enough mucus to slide across these barriers without much trouble. Like eggshells, grit may help improve soil drainage, but it won’t stop a determined slug.
6. Beer Traps: Poison-Free, But How Effective?
Beer traps are a well-known method for dealing with slugs. Slugs are attracted to the yeast in beer, and once they fall into the trap, they drown. Some studies have shown that beer traps can be moderately effective, especially in small areas. However, they need to be maintained regularly, as the traps fill with rainwater and debris.
Alternative Methods: Natural and Organic Repellents
Several other natural and organic repellents are frequently suggested in gardening forums and blogs. Here’s what science says about them:
1. Coffee Grounds: Possible Repellent and Pesticide
There is some evidence to suggest that coffee grounds might act as a repellent, possibly due to their sharp texture and strong odour. In addition, caffeine in high concentrations has been shown to have a pesticidal effect on slugs. However, sprinkling coffee grounds around plants is unlikely to deliver a high enough concentration of caffeine to make a significant impact.
2. Calcium Chloride Sprays: No Strong Evidence
Some gardeners use calcium chloride sprays, which coat the leaves and stems of plants in the hopes of deterring slugs. However, there’s little evidence that calcium chloride is an effective slug deterrent. In fact, it may do more harm than good by altering the soil's chemical balance.
3. Plant Oils: Garlic, Lavender, and Others
Plant oils, particularly garlic oil, are sometimes suggested as natural slug repellents. Garlic is known for its strong smell and, in high concentrations, can have a pesticidal effect on pests. However, achieving a concentration high enough to deter slugs may require more than the average home gardener is willing to apply.
Slug Control Strategies: Prevention and Protection
Aside from repellents, there are several strategies you can adopt to make your garden less attractive to slugs:
- Mulching: Mulch can help keep the soil surface dry, creating less favourable conditions for slugs. However, avoid organic mulches that might attract them, such as composted materials.
- Sacrificial Plants: Planting certain “sacrificial” companion plants that slugs are more likely to consume can help protect your more valuable crops. These decoy plants act as a distraction, drawing slugs away from your prized flowers and vegetables.
- Handpicking: For small gardens, handpicking slugs at dawn or dusk can be an effective way to keep their numbers down, especially when combined with traps or barriers.
Conclusion: What Works and What Doesn’t?
When it comes to slug repellents, not all methods are created equal. Based on scientific studies, nematodes and iron phosphate pellets are among the most effective ways to control slug populations but they are non-discriminant and will kill all slugs and snails that they contact. These methods are not ideal if you wish to keep biodiversity in your garden. Other methods like copper tape, crushed eggshells, and horticultural grit, While completely harmless to biodiversity in your garden, all have little scientific backing with regards to their effectiveness.
If you would like to find out more information, the RHS website can be found here: https://www.rhs.org.uk/biodiversity/slugs-and-snails
