Copper does discourage snails. In my garden I have some nearly snail-proof stands for potted plants. They consist of a piece of wood with long nails driven through for legs. On the protruding end of the nail-leg, I slipped a short piece of copper tubing from the hardware store. Snails will climb it in an emergency, but they generally avoid it. The arrangement has allowed only one molluscan trespasser this season.
No barrier is always effective. I have seen young snails slide happily across copper strips once or twice. Older snails flinch and try to avoid it. You still have to keep checking your plants.
Bordeaux mix
Use with care. This commercially sold compound, sold as a soil fungicide, is made from hydrated lime and copper sulphate. Snails will not cross a barrier of this when it is dry or fresh. Harms earthworms.
Home-made barriers
Salt, lime, dry wood ash, and diatomaceous earth are all somewhat effective snail barriers. Salt may damage plants by inhibiting water uptake. Too much lime will cause alkaline soil to become even more alkaline. Ash and diatomaceous earth are effective as long as they are applied to the ground in strips that are dry, deep and wide. Two to three inches deep for ash, and at least a foot wide for diatomaceous earth.
Irrigation control
Snails thrive in wet gardens. his reduces the snail food supply without reducing garden productivity. Selective watering also means less moist soil for snails to lay eggs in. Dry, semi-wild areas near a garden also harbor lizards and other predators that keep snail populations down.
Natural predators
Snails have many natural predators. Birds, lizards, toads, raccoons and opossums are among the large ones. Overwintering ants, beetles and other ground bugs eat snail eggs whenever they find them. Moral: keep a healthy garden with lots of bugs, and you'll have fewer snails.
The small, carnivorous decollate snail has been imported to California citrus orchards to kill populations of brown snails. In orchards where decollate snails have been introduced, brown snail damage was controlled within three years. The imported snails themselves have population explosions however, and do minor crop damage of their own. In most cases, resident predators are enough.
