Slugs are in every garden, and cause more damage than most garden invaders. Commercial slug killers are available, but they can be toxic to birds and other wildlife, and are less effective after rain, when slugs are most active.
We have found that non-toxic, food grade Diatomaceous Earth (Insect Dust) is effective for slugs, but there are also many other methods available for little to no cost.
Before reaching for the pesticides, here are a few alternative natural, non-toxic methods of slug control:
Watering Schedule - Far and away the best course of action against slugs in your garden is a simple adjustment in the watering schedule. Slugs are most active at night and are most efficient in damp conditions. Avoid watering your garden in the evening if you have a slug problem. Water in the morning - the surface soil will be dry by evening. Studies show this can reduce slug damage by 80%.
Seaweed - If you have access to seaweed, it's well worth the effort to gather. Seaweed is not only a good soil amendment for the garden, it's a natural repellent for slugs. Mulch with seaweed around the base of plants or perimeter of bed. Pile it on 3" to 4" thick - when it dries it will shrink to just an inch or so deep. Seaweed is salty and slugs avoid salt. Push the seaweed away from plant stems so it's not in direct contact. During hot weather, seaweed will dry and become very rough which also deters the slugs.
Copper - Small strips of copper can be placed around flower pots or raised beds as obstructions for slugs to crawl over. Cut 2" strips of thin copper and wrap around the lower part of flower pots, like a ribbon. Or set the strips in the soil on edge, making a "fence" for the slugs to climb. Check to make sure no vegetation hangs over the copper which might provide a 'bridge' for the slugs. Copper barriers also work well around wood barrels used as planters. A non-toxic copper-based metallic mesh Slug Shield is available which can be wrapped around the stem of plants and acts as a barrier to slugs. When slugs come in contact with the mesh they receive an electric-like shock. The mesh also serves as a physical barrier. These slug shields are reusable, long-lasting and weather-proof.
Electronic "slug fence" - An electronic slug fence is a non-toxic, safe method for keeping slugs out of garden or flower beds. The Slugs Away fence is a 24-foot long, 5" ribbon-like barrier that runs off a 9 volt battery. When a slug or snail comes in contact with the fence, it receives a mild static sensation that is undetectable to animals and humans. This does not kill the slug, it cause it to look elsewhere for forage. The battery will power the fence for about 8 months before needing to be replaced. Extension kits are availabe for increased coverage. The electronic fence will repel slugs and snails, but is harmless to people and pets.
Lava Rock - Like diatomaceous earth, the abrasive surface of lava rock will be avoided by slugs. Lava rock can be used as a barrier around plantings, but should be left mostly above soil level, otherwise dirt or vegetation soon forms a bridge for slugs to cross.
Salt - If all else fails, go out at night with the salt shaker and a flashlight. Look at the plants which have been getting the most damage and inspect the leaves, including the undersides. Sprinkle a bit of salt on the slug and it will kill it quickly. Not particularly pleasant, but use as a last resort. (Note: some sources caution the use of salt, as it adds a toxic element to the soil. This has not been our experience, especially as very little salt is used.) Beer - Slugs are attracted to beer. Set a small amount of beer in a shallow wide jar buried in the soil up to its neck. Slugs will crawl in and drown. Take the jar lid and prop it up with a small stick so rain won't dilute the beer. Leave space for slugs to enter the trap.
Overturned Flowerpots, Grapefruit Halves, Board on Ground - Overturned flowerpots, with a stone placed under the rim to tilt it up a bit, will attract slugs. Leave overnight, and you'll find the slugs inside in the morning. Grapefruit halves work the same way, with the added advantage of the scent of the fruit as bait. Another trap method, perhaps the simplest of all, is to set a wide board on the ground by the affected area. Slugs will hide under the board by day. Simply flip the board over during the day to reveal the culprits. Black plastic sheeting also works the same way.
Garlic-based slug repellents - Laboratory tests at the University of Newcastle-Upon-Tyne (UK) revealed that a highly refined garlic product (ECOguard produced by ECOspray Ltd, a British company that makes organic pesticides) was an effective slug killer. Look for garlic-based slug deterrents which will be emerging under various brand names, as well as ECOguard.
Coffee grounds; new caffeine-based slug/snail poisons - Coffee grounds scattered on top of the soil will deter slugs. The horticultural side effects of using strong grounds such as espresso on the garden, however, are less certain. When using coffee grouds, moderation is advised. A study in June 2002 reported in the journal Nature found that slugs and snails are killed when sprayed with a caffeine solution, and that spraying plants with this solution prevents slugs from eating them. The percentage of caffeine required in a spray (1 - 2%) is greater than what is found in a cup of coffee (.05 - 07%), so homemade sprays are not as effective. Look for new commercial sprays which are caffeine-based.
Commercial Slug Baits - You can buy slug bait at garden centers marketed under several name brands. These baits commonly use one of two active ingredients in their formulations - Ferric Phosphate and Ferric Sodium. Although both formulations are effective, there are significant differences in their chemical toxicities. Ferric Phosphate, is on the OMRI products list for certified organic growers. It is non-toxic to people, pets, birds, insects, earthworms and other wildlife. Ferric Sodium, however, is highly toxic to wildlife. The most common brand, of slug bait, Safers, sells both kinds but they are packaged in similar looking plastic boxes. If you are buying commercial slug bait, check to be sure it uses the Ferric Phosphate formulation. When applying commercial slug bait, apply thinly over a wide area and not adjacent to your plants. This is because these baits attract slugs but do not kill them instantly, so the slug can still damage your plants.