Eggshells won't stop slugs, study finds — but here's what you can do this October to keep them away

It is one of the oldest garden tips: save the eggshells, crush them, and sprinkle a ring around the lettuce so slugs won't cross the sharp edges. Britain's Royal Horticultural Society put it to the test in a garden-realistic trial, together with copper tape, bark mulch, sharp grit and wool pellets. None of them reduced slug damage.

Slugs can produce protective slime to cross rough surfaces, and damp conditions make these barriers particularly unreliable. But that doesn't leave gardeners empty-handed. Autumn is one of the seasons when slugs lay clusters of eggs beneath pots and stones. Finding them now is one way to tackle the next generation before spring.

What happened when experts tested eggshells?

In a 2018 experiment, RHS researchers grew 108 lettuce plants in patio pots and raised beds over six weeks. Some were surrounded by crushed eggshells, copper tape, bark, grit or wool pellets. Others received no protection.

The result was disappointing for anyone who regularly saves eggshells for the garden. Protected lettuces suffered just as much slug damage as untreated plants.

That doesn't mean every barrier fails in every situation. Copper, for example, has shown some effectiveness in laboratory experiments. But the RHS results suggest gardeners shouldn't rely on these popular remedies to protect their vegetables outdoors.

Why don't slugs mind the sharp edges?

A slug's soft body looks vulnerable, but its slime provides remarkable protection.

According to the RHS explanation, slugs can change their slime to protect themselves while travelling over sharp material, including broken eggshells.

Those rough pieces that seem so uncomfortable to us may therefore be little more than another surface for a slug to cross.

Rather than throwing eggshells away, you can add them to your compost heap. Just don't expect them to form a dependable protective fence around your lettuce.

October is the time to look under flowerpots

There is another garden job worth doing this autumn, and it starts with lifting a few pots.

According to BBC Gardeners' World Magazine, slugs lay eggs in both spring and autumn, sometimes in batches of 50.

The eggs resemble tiny, translucent white pearls, usually only a few millimetres across.

Look beneath flowerpots, saucers, loose stones and boards. Check damp pockets of soil and sheltered corners where the ground stays cool.

If you discover a cluster, expose it for birds and other predators to eat, or crush the eggs. Be careful not to disturb other wildlife sheltering nearby.

Checking these hiding places won't eliminate slugs, but it may help reduce the number hatching around vulnerable plants.

What else can gardeners do?

The RHS recommends several practical approaches for gardeners experiencing serious damage.

One is collecting slugs after dark, when they're feeding. Take a flashlight and inspect lettuce, hostas and new shoots. Gardeners' World suggests going out about two hours after dusk.

Traps offer another option. Place hollowed-out orange or grapefruit halves cut-side down, or use beer traps with their rims slightly above ground level. Check them regularly and protect other wildlife from falling in.

For persistent problems, biological controls containing nematodes can help. These microscopic organisms attack slugs in suitably moist, warm soil, although they can also affect beneficial slug species.

A garden doesn't need to be slug-free

Not every slug is a plant-eating nuisance. Many feed on decaying material and help recycle nutrients, while birds, frogs, hedgehogs and beetles rely on them for food.

The sensible approach is to protect the most vulnerable plants rather than trying to remove every slug.

Grow seedlings until they're stronger before planting them outside, and consider plants with tougher leaves in areas where damage repeatedly occurs.

And next time you're clearing up the garden on a damp October afternoon, lift a few flowerpots. What lies underneath may be more useful to your slug-control efforts than another ring of crushed eggshells.