Prevention. Slugs like a dark, damp environment, so anything you can do to create a light, dry environment will discourage them from hanging around. If you have slug problems, forgo mulch, space plants farther apart, and cultivate the soil frequently to keep it dry on the surface.
Barriers.Being soft-bodied creatures, slugs don't like anything sharp. If cut, they often will dry up and die. Sprinkle sharp sand, diatomaceous earth, or dried, crushed egg- or oyster shells around favored plants. Refresh the materials after heavy rains. If you have a raised bed, permanent border, or container, fasten strips of copper flashing around the bed. When slugs come in contact with the copper, they get an electric shock. (Think of it as an electric fence for slugs.)
Traps. Using slugs' propensity for hiding in the dark, place boards on garden pathways. Early in the morning, turn over the boards and knock the slugs into a bucket of soapy water. Slugs also like beer (actually, the yeast in beer). Create a slug trap by sinking an empty coffee-can-sized container into the soil to the soil line and filling it with beer. At night the slugs will dive in to feast and eventually drown. Clean out the trap in the morning.
Baits. Although the commonest slug control baits contain the slightly toxic chemical metaldehyde, some new baits, such as iron phosphate, are less toxic to animals and humans. Any bait will have to be reapplied after a rain, and you should be careful not to sprinkle baits where pets and young children can find them.
Decollate snails prey upon other species of snail and slug. You can purchase them from biological control suppliers.
Slugs cause the most damage on young plants, so make extra efforts to protect them.
Photography by Suzanne DeJohn/National Gardening Association