Pests

Slugs and Snails in the Greenhouse: Find Them, Trap Them, Keep Them Out

A greenhouse should be the one place slugs cannot get at your seedlings. Instead, it often becomes their favorite: damp, sheltered, full of tender plants and hiding places under staging and pots. One slug in a seed tray can clear a row overnight. The fix is a mix of evening hunting, removing their hiding places, simple traps, and barriers on benches, with baits used carefully if at all.

Signs you have slugs or snails

• Ragged holes in leaves and missing seedlings, often with stems chewed off at the base.
• Silvery slime trails on benches, pots, glazing and paths.
• Damage that appears overnight with no insects visible by day.
• Small slugs in pots when you knock out a root ball, or snails tucked under bench rims.

Other pests that chew leaves include caterpillars and woodlice. Slime trails and night-time damage point strongly to slugs and snails. A quick torch check after dark confirms it.

Where they hide in a greenhouse

Evening hunting: the free fix

Slugs and snails are most active after dark on damp, mild nights. A torch and a container is the most effective control for a small greenhouse.

1. Go out an hour or two after dark, ideally after watering or on a damp evening.
2. Check seedlings, benches, pot rims and the glass, plus under the edges of benches.
3. Collect them into a lidded container and dispose of them as you see fit, well away from the garden.
4. Repeat for several nights. Numbers drop quickly with consistent effort.

Traps that work

Board and tile traps

Lay a few boards, tiles or upturned pots on the floor near crops. Slugs hide under them by day. Lift them each morning and remove what you find. It is simple and reliable.

Beer and yeast traps

Sink a container into border soil with the rim slightly above the surface, and fill partly with beer or a yeast mixture. Slugs are attracted and drown. Empty and refill regularly. Leaving the rim raised helps reduce the chance of catching beetles that prey on slugs.

Barriers and bench protection

Getting seedlings off the floor helps a lot. Then make it harder to climb up:

• Raised benches with smooth legs are harder to climb than wooden staging covered in moss.
• Copper tape around bench legs or pot rims is widely used. Some growers find it helps; others see little effect. It is not a reliable stand-alone defense.
• Petroleum jelly or a band of grease on bench legs can deter climbing, but it collects debris and needs refreshing.
• Keep foliage off walls and glass so slugs cannot use leaves as a bridge onto benches.
• Grit and wool pellet barriers around plants are popular, but evidence for their effectiveness is mixed.

Biological control: nematodes

Nematodes sold for slugs (Phasmarhabditis species) are applied as a drench to moist soil, and target slugs in the soil. They are mainly effective against slugs rather than snails and need moist soil within a suitable temperature range. Follow the supplier's instructions for storage and application. Availability varies by country.

Slug pellets: use carefully

Pellets are effective but come with trade-offs. Rules and available products differ by country. In some places, certain older active ingredients have been restricted or banned because of risks to wildlife and pets.

• Choose a product approved for use where you live. Ferric phosphate is a commonly sold active ingredient regarded as lower-risk than some older chemicals, but it is still a pesticide and must be used as labeled.
• Use the lowest rate on the label. Scatter thinly; do not heap pellets.
• Keep pets, children and wildlife away from treated areas and stored products.
• Consider pellets a backup to hunting, traps and hygiene, not a first move.

Protecting seedlings specifically

• Keep seed trays on raised benches, not the floor.
• Check the undersides of trays and modules when you move them.
• Use clean, sterile compost and new or washed trays.
• Water in the morning so surfaces are drier at night.
• Do an evening check for a week after sowing and pricking out.
• Move hardening-off plants outside on benches or tables, not on the ground.

Encourage natural predators outside

Outside the greenhouse, birds, frogs, toads, hedgehogs and ground beetles all eat slugs. A wildlife-friendly garden with some cover and a small pond helps keep numbers down in the long run, which means fewer slugs wandering in through the door. Avoid using broad-spectrum pesticides that harm those predators.

Watering habits that help

Slugs need moisture. You cannot keep a greenhouse bone dry, but you can avoid giving them a damp highway every night:

• Water in the morning so the floor and benches dry before dark.
• Avoid soaking paths and floors in the evening when damping down.
• Fix leaking hoses, taps and drippers that keep corners wet.
• Stand pots on mesh or slatted staging rather than wet capillary matting on the floor.

Snails vs slugs: any difference?

For control purposes they behave similarly, but snails are more likely to climb glazing and frames and hide in high places, such as under bench rims and in roof corners. Slugs tend to stay lower and hide in soil and under pots. Slug nematodes are aimed at slugs, so if snails dominate, focus on hunting, traps and exclusion.

Check what you bring in

Slugs and their eggs often arrive in the pots of plants from nurseries, plant swaps and friends' gardens, and in bags of compost or soil left standing on damp ground. Knock plants out of their pots and check the root ball and pot base before moving them onto your benches, and store compost bags on a pallet or shelf rather than directly on the soil. Small clusters of round, pearly eggs in compost or under pots are worth removing whenever you see them.

Seasonal timing

The bottom line

Slugs win in greenhouses because they have shelter and tender food close together. Take away the shelter, hunt on damp evenings, set board and beer traps, raise seedlings on smooth benches, and keep pellets as a careful backup. A week of consistent effort usually turns overnight losses into the occasional nibble.

Looking for more on protecting young plants? See our guide to common seedling problems.