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. I 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:
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%.
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.
Diatomaceous earth (Also known as "Insect Dust") is the sharp, jagged skeletal remains of microscopic creatures. It lacerates soft-bodied pests, causing them to dehydrate. A powdery granular material, it can be sprinkled around garden beds or individual plants, and can be mixed with water to make a foliar spray.
Diatomaceous earth is less effective when wet, so use during dry weather. Wear protective gear when applying, as it can irritate eyes and lungs. Be sure to buy natural or agricultural grade diatomaceous earth, not pool grade which has smoother edges and is far less effective. Click for more information or to purchase Food Grade Diatomaceous Earth.
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.
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.)
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.