Slugs and Snails

You go out in the morning and a plant that was fine yesterday has large irregular holes chewed through its leaves, and there is nothing on it. Nothing all day. The next morning there is more.

That is slugs or snails, and the reason you cannot find the culprit is that they feed at night and hide during the day.

What you are seeing#

Large, irregular holes with smooth edges, often in the middle of a leaf rather than at the margin. Slug damage looks eaten rather than torn, with a rounded ragged outline.

Slime trails. The confirmation. Dried mucus leaves a silvery, faintly iridescent track on leaves, stems, the pot rim or the ground nearby. Catch it in low morning light and it is unmistakable.

Damage concentrated low on the plant, because they climb from the ground. Seedlings and young plants are most at risk, and a slug can take an entire seedling in a night.

Nothing visible during the day. They shelter under pots, in mulch, under the rim of a container, beneath leaf litter and in any damp dark gap.

Damage appears after rain or heavy dew and stops during dry spells, which is a strong seasonal tell.

How to tell it apart#

From caterpillars: caterpillars leave droppings, small dark pellets, on leaves and in bud sites. Slugs leave slime and no droppings you will notice. Caterpillars also bore into buds, which slugs rarely do.

From thrips and aphids: both scar and discolor leaf surfaces rather than removing whole sections. Holes you can see through are chewing damage, not sucking damage.

From physical damage: wind, hail and handling tear leaves with rough edges and often damage the surrounding tissue. Chewing leaves a clean smooth margin.

To confirm beyond doubt, go out with a flashlight two hours after dark. They will be on the plant.

Why it happens#

Moisture and cover. Slugs and snails need damp conditions and a dark place to spend the day, and an outdoor grow provides both: mulch, a saucer under a pot, long grass, a stack of empty containers.

They are worst in spring and autumn, after rain, and in shaded beds that stay damp. A container grow on a patio is much less exposed than a plant in a mulched bed.

How to fix it#

1. Hand pick at night, for a few nights running, after rain. Most effective single action.
2. Remove the daytime shelter. Lift pots onto feet or a rack, clear leaf litter and empty containers from around the plants, pull mulch back from the stems, cut long grass nearby. This is the fix that lasts.
3. Copper tape around a pot rim or raised bed. They avoid crossing it. Works well for containers, needs to stay clean to keep working.
4. A physical barrier for seedlings. A cut plastic bottle around a young plant protects it through the stage where a single night can kill it.
5. Iron phosphate pellets are the pet and wildlife safe option and work well. Scatter thinly around the plants rather than in a pile. Avoid metaldehyde baits, which are genuinely dangerous to pets and wildlife.
6. Water in the morning, not the evening. A bed that is damp at dusk is an invitation. This one change does a surprising amount.
7. Beer traps work, modestly, and catch fewer than people expect. Fine as a supplement.

What it costs you#

Mostly leaf area, which a vegetative plant recovers from easily. A mature plant losing some lower leaves to slugs is a cosmetic problem.

The real risk is at two points. A seedling or young transplant can be destroyed outright in one night, which is why the barrier matters in spring. And late in flower, holes chewed into dense buds open the tissue and create an entry point for rot, which matters far more than the damage itself.

If you are finding slug damage on a flowering plant in a wet autumn, deal with it promptly for that reason rather than for the leaves.