Half-eaten leaves and slimy, silver trails on the earth are signs that you have one of the worst of all garden pests. Snails and slugs abound in lush, closely planted vegetables and are especially difficult to control in cool, damp climates. They feed mostly at night and sleep during the day in sheltered spots. They are voracious and can decimate a planting in just days. If you do not get rid of them, you will quickly have a population explosion.
You can pick them off by hand in the evening or early morning and toss them into a wide-mouthed can filled with soapy water. Or you can put out saucers filled with beer for them to climb into and drown. You can also spread a barrier of diatomaceous earth around plants you want to protect; snails will not cross it. This mined material consists of shells of minute sea creatures called diatoms. It loses its effectiveness when wet, however, and must be renewed.
A very effective barrier can be made by tacking a copper-plated strip all around the garden or around each bed. Snails will not cross the copper. It gives them a galvanic shock, a little like biting on a piece of aluminum foil with a filling.
This method only works if you have something to tack the copper to, such as a raised bed. Be sure to clear out any snails already in the bed, and keep plants from reaching over the barrier to touch the ground outside. Snails will use this bridge as an entry to the bed. Copper strips made for this purpose can be purchased in most garden centers.
Bait containing metaldehyde is very effective. It keeps the snail population under control if applied every couple of weeks. Scatter it where the snails live, often a ground cover near the garden, scatter a band as a barrier between their cover and the garden, or scatter it throughout the garden.