Slugs and snails

What you are looking at

Slug and snail damage is a hole in the middle of a leaf with a smooth, rounded edge, and the animal is rarely there when you find it. Look for the second sign. A dried slime trail catches the light on a leaf or a paving slab in the morning, silvery and irregular, and nothing else in a garden leaves one.

Both animals rasp rather than bite. A tongue covered in thousands of tiny teeth scrapes the tissue away, which is why the edge of the hole is smooth instead of chewed. On a seedling there may be nothing left to inspect. A row that had forty lettuce plants in it at dusk can have eleven at dawn, with a few short stems still standing.

The four groups that do the damage

Britain has around forty-four species of slug and rather more snails, and the great majority never touch a living plant. They eat fallen leaves, dung and dead wood, and a garden runs better with them in it.

Four groups are worth knowing. The grey field slug, Deroceras reticulatum, is small, pale and mottled, and it does more damage than everything else combined. The garden slug group, Arion hortensis and its relatives, is dark with an orange sole and works at and below the surface. Keeled slugs, Tandonia and Milax, are the ones that hollow out potatoes without ever appearing above ground. The garden snail, Cornu aspersum, climbs, and it is why a barrier around the base of a plant so often achieves nothing.

The big black slug, Arion ater, is the one people photograph and the one that does least harm. It is largely a scavenger. The Spanish slug, Arion vulgaris, arrived more recently, grows large, breeds heavily and does eat living plants, so an unusually big brown slug in numbers is worth taking seriously in a way the native black one is not.

When they feed, and why you never see them

They come out after dark and after rain, and they need a moist surface to move on. On a dry night in June a garden that lost a row of seedlings last week can look completely clear.

Egg batches of ten to fifty sit in soil crevices, under stones and in the crowns of plants, pearly white and about the size of a peppercorn. The grey field slug can produce eggs in any month mild enough, which is why a warm wet winter is followed by a bad spring rather than a good one.

The practical use of the timing is that everything on this page works better at night. A torch at ten o’clock finds the breeding adults; the same border at noon looks fine.

What it does to a plant

An established hosta or a mature cabbage loses leaf area and keeps going. The plants that die are the young ones, and the window is short. From the moment a seedling goes out until it has hardened and made half a dozen leaves, one night can end it, and after that the same feeding is disfigurement rather than a threat.

Below ground the arithmetic is different. Keeled slugs feed on potato tubers through late summer and autumn, leaving clean-walled cavities that only turn up at lifting. There is no leaf damage to warn you, and the crop looks perfect from above until you dig it.

Grazed patches on strawberries and courgettes are usually where the fruit rests on the soil. Lifting the fruit onto straw, a tile or a mat solves that one outright.

What to do about them

Take the young plants out of reach first. Sowing into modules and planting out larger, later and hardened off removes most of the risk before any control is needed, and it is free. Water in the morning rather than the evening so the surface is dry when they are travelling.

Then go out with a torch. An hour after dusk on a damp night, two or three nights running, will take out most of the breeding adults in a town garden. Nothing else on this page comes close, and everyone would rather buy something than do it.

Where a bed keeps losing plants, water on nematodes in spring once the soil has reached about 5 degrees C. They reach the slugs that live below the surface, which is the group no barrier and no torch will find, and they leave snails alone entirely because snails are not down there.

Barriers earn a mixed report and it is worth being honest about which. A cloche or a cut plastic bottle over each seedling works, because it is a physical enclosure. Copper tape works while it stays bright and stops working when it tarnishes, so a pot rim needs cleaning to keep its edge. Crushed eggshell, sharp grit, coffee grounds and bran have all been tested and none of them held up.

The predators

Ground beetles matter more than anything else and are the ones a gardener is most likely to kill by accident. They eat slug eggs and young slugs, they are fast, black and shiny, and they fall into a beer trap sunk level with the soil. Standing the rim two centimetres proud fixes that. They overwinter in rough grass and leaf litter, so a strip of the garden left undisturbed pays back across the whole plot.

Hedgehogs, frogs, toads and slow-worms all take slugs in quantity. A pond with one shallow sloping side is the strongest single thing you can add, and a gap under the fence lets a hedgehog work a range far larger than one garden.

Song thrushes take snails and break them open on a stone they use for years. If you find a flat rock ringed with broken shells, leave it where it is.

Where the advice changed

Most of what still ranks for this search recommends a product that has been illegal to use outdoors in Great Britain since 2022. Metaldehyde was withdrawn for outdoor amateur and professional use because of what it does to birds, hedgehogs and water, and the blue pellets that remain in sheds across the country cannot legally be scattered. Take unused stock to a household waste site rather than emptying it onto a border.

What replaced it is ferric phosphate, which is approved for organic growing and much safer around pets and wildlife. It is not a licence to scatter, though. Slugs that eat it stop feeding and die out of sight, so nothing appears on the soil to tell you it has worked, and the temptation is to keep adding more. A few pellets to the square metre is the rate, and it should be the last thing tried rather than the first.

What it gets mistaken for

Vine weevil adults cut neat semicircular notches in from the edge of a leaf and never make a hole in the middle. Caterpillars leave dark pellets of frass on the foliage below, and they chew rather than rasp, so the edges of the damage are ragged. Neither leaves a slime trail.

Holes with a yellow halo around them are a leaf spot disease rather than anything eating the plant, and the tissue falls out of the centre after the infection has killed it. Slug holes have living green tissue right up to the edge.

Seedlings that vanish leaving no stem, no slime and a small hole in the soil are more likely to be mice, which take newly sown peas and beans in quantity and get blamed on slugs every spring.