Slugs and snails are the most common pest enquiry the Royal Horticultural Society (RHS) receives, according to its research project page. This article summarises the RHS advice page “Slugs and snails” and the RHS science page describing its “Gastropod barriers” experiment. It concerns gardens in the UK and reports a small trial as the RHS describes it. It is general gardening information, not a product recommendation. The related legal question of slug pellet products is covered in the site’s article on the metaldehyde slug pellet ban in Great Britain.
Why the RHS urges a lighter touch
The RHS says gardeners have long thought of slugs and snails as the bad guys, but they are valuable in the garden ecosystem. It says many species feed on decomposing organic matter such as dead leaves, dung and dead slugs and snails, helping the composting process, and that they are food for birds, frogs, toads, hedgehogs, slow-worms and ground beetles. It says only a few species feed on live plants, and that only young seedlings and particularly tasty plants such as hosta are severely eaten. The netted field slug, brown soil slug and common garden snail are named as the species most likely to be found feeding on garden plants. The RHS says there are over 44 species of slugs and 90 species of snails in the UK.
The RHS says slugs and snails are so abundant that some feeding is normal, that they cannot and should not be eradicated, and that targeted management to protect vulnerable plants such as seedlings and soft young shoots gives the best results.
Practical steps from the RHS
- Choose less palatable plants, such as woody plants, those with thick or waxy leaves and some herbaceous plants; the page lists examples such as lady’s mantle, geranium, foxglove and fuchsia.
- Transplant sturdier plantlets grown in pots rather than vulnerable seedlings, with cloches for protection.
- Encourage predators.
- Search by torchlight on mild, damp evenings and hand-pick into a container, moving them elsewhere in the garden, though the RHS warns that some species have homing instincts and may return.
- Water in the early morning rather than the evening, because evening watering provides ideal conditions for movement at night.
What the RHS barrier trial found
The RHS project page says barriers and repellents are often suggested in gardening publications, that copper had shown effects in laboratory studies, and that egg shells and bark mulch had not been scientifically studied. The trial ran from February to September 2018 at the Field Research Facility at RHS Wisley, using lettuce, a favourite of slugs and snails. It compared no treatment, copper tape around pots and collars in the ground, sharp horticultural grit, pine bark mulch, wool pellets and washed, dried and roughly crushed eggshells. Treatments were tested in pots on a small patio and in a raised vegetable bed, with nine replicated blocks, each with a paired patio and bed and randomised treatment order. Damage was scored weekly for six weeks, then measured with a leaf area meter and dry mass at harvest.
The RHS advice page reports that a recent RHS study in a garden-realistic scenario found no reduction in slug damage from barriers made of copper tape, bark mulch, eggshells, sharp grit or wool pellets. It adds that copper-based barriers have repelled slugs in some studies, and that many substances thought to repel slugs, including rough or strong-smelling ones, lack scientific evidence. The trial used one crop and one site over one season, so it shows these barriers did not work under those conditions and does not prove that no barrier works anywhere.
Traps and biological control
The RHS says gardeners who want to protect vulnerable borders or vegetable beds can manage numbers with traps or biological control. Traps include scooped-out half orange, grapefruit or melon skins laid cut side down, or jars part-filled with beer and sunk into the soil; they should be checked and emptied regularly, preferably every morning. It describes the nematode biological control sold as Nemaslug as specific to molluscs, with no adverse effect on other types of animals, and says the nematodes enter slugs’ bodies and cause a fatal disease. It warns that it should be used in a targeted way because it will affect all species of slug, including beneficial ones, and is unlikely to affect snails because they rarely contact soil-dwelling nematodes. For effectiveness the soil must be moist and warm, 5 to 20°C, so spring to early autumn is the best time; evening application to moist but well-drained soil is recommended, and heavy soils such as clay may give poorer results.
Slug pellets
To the question whether to use slug pellets, the RHS answers no: it says slug pellets, even organic ones, have been shown to have negative effects on wildlife in the garden, and that it is better to encourage natural predators and use cultural and biological controls. That is the RHS’s position as a gardening charity, not a legal statement.
Common questions
Do eggshells or copper tape keep slugs off?
In the RHS Wisley trial as summarised on its advice page, they did not reduce slug damage to lettuce in pots or a raised bed.
The bottom line
The RHS advises living with slugs and snails, protecting seedlings and vulnerable plants, choosing tougher plants, hand-picking and trapping, and using slug nematodes only where targeted. Its Wisley trial found no reduction in damage from several popular barriers, and it advises against slug pellets because of effects on wildlife.
Sources
- Royal Horticultural Society, “Slugs and snails” (rhs.org.uk)
- Royal Horticultural Society, “Gastropod barriers” (RHS science project) (rhs.org.uk)