When snails and slugs come into contact with copper a chemical reaction is triggered, same principal as a battery. The snails or slugs get a slight non fatal electrical shock and back off. It's a deterrent only, and useful in protecting plants you are particularly fond of.
Copper barriers are practical for keeping snails and slugs out of Cold Frames or raised beds, containers, or as a border around prized smaller trees .
Conductive adhesive copper foil tape is the most cost effective item to use in this scheme. The copper strip should be wide enough so that snails or slugs can't raise their bodies over and past it. Most copper stripping is not wide enough to create an effective barrier, so you would have to use multiple strips. Leaving a minute space between strips is fine as snails and slugs are pretty much guaranteed not to be smart enough to figure this out and their slimy bodies will most certainly come into contact with enough surface area to generate the desired shock.
The copper should be cleaned periodically to minimize oxidation and normal dust and residue, vinegar will suffice.
Copper screening or flashing will work even better if you happen to have some around, personally I wouldn't spend the money on it - but I'm a frugal [polite word for cheap] kinda gardener.
Diatomaceous earth also works well as a barrier. This method won't kill the snails and slugs, only deter them. Fine sandpaper also works but I've personally found it to be a nuisance in the outdoor garden as it needs to be anchored down.
Controlling snails and slugs with Dog and Cat food
Some gardeners use Dog food or Cat food to attract and trap snail and slugs, I don't, and I don't recommend it. If you don't have a rodent , raccoon or varmint problem when you begin this practice you probably will once it gets going. It's a definite no-no.
Snails and slugs have many natural enemies, including ground beetles, , snakes, toads, turtles, and birds. Poultry such as chickens and ducks will generally snack on any passing snail or slug they see. This is generally only feasible in rural setting.
Decollate snails are somewhat effective in controlling common garden slugs . Decollate snails will feed on young snails , but they are omnivorous and will also eat your plants just like any other snail. These snails are semi-tropical and don't thrive in low temperatures , and are banned as potentially invasive species in some places. If you are going to use Decollate snails, do not use any kind of snail bait, organic or otherwise.It is applied to the soil as a pellet or powder which also contains a bait that attracts snails and slugs. When the slime balls ingest iron phosphate it interferes with their metabolism, causing the snails and slugs to immediately loose their voracious appetites, they stop eating get anorexic and die within a week.
The best organic snail and slug bait products is Sluggo which contains iron phosphate. It is safe to use around pets, humans, birds, beneficial insects, and other mammals. You can safely use sluggo around edible food crops, greenhouses, and landscape plants. Iron phosphate used in Sluggo is an organic compound found naturally in the soil, and if the bait is not consumed it breaks down in the soil and becomes fertilizer.
Metaldehyde is a similar product, but is not as effective as it stops working once it gets wet - rain or watering the garden, whereas iron phosphate remains active after repeated soakings, for up to 2 weeks. Metaldehyde baits can be dangerous around pets as it looks like food to them . Metaldehyde based products however are generally less expensive than Sluggo.
The best time to apply Sluggo for long-term control is in the autumn, preferably in damper weather. Come spring, there will be few adult slugs and snails laying eggs. Another application in very early spring when the hatchlings are emerging, and then again when the leaves are all greening and you shouldn't have any slug or snail problem for the rest of the year.
Don't use Sluggo if your are using predatory, snail-eating, decollate snails.
Other Methods of Controlling Snails and slugs
A vinegar water mix or just straight vinegar will kill snails and slugs but must be sprayed directly on them. It works in the same way as salt does. Vinegar is an acid and dissolves the mucus soaked slime blobs we call snails and slugs. Put salt on a slug and the same thing happens.
Diluted household ammonia will work in the same fashion as the vinegar mix mentioned above. Ammonia in moderate quantities won't hurt the plants and may actually help them as it is almost pure nitrogen which plants love.