The first thing to realize is that snails and slugs cannot be vanquished. Pressure from these slimy scourges may abate for a while in a drought, but with the rains they will be back, as inevitable as death, taxes, and telemarketers at dinnertime. But although they will be with you always, populations can be kept to tolerable levels through a variety of controls.
Not counting predators, of which there are mercifully many (wild birds, ducks, geese, chickens, toads, turtles, and snakes among others), control methods include poisons, traps, hand-picking, and barriers.
The poisons are metaldehyde, a chemical that is also toxic to mammals and birds, and iron phosphate, a natural mineral that its marketers claim harms nothing except slugs. The traps — fancy plastic “snail hotels” or unfancy empty tuna fish cans — are filled with an attractant, most commonly beer, and are buried just below ground level. Slugs and snails come to drink, fall in, and drown. Traps can often be effective, but they sometimes attract more pests than they kill, thus compounding rather than solving the problem.
Hand-picking is self-decriptive: you wander around at night, when slugs and snails are most active, finding them with a flashlight and plucking them off the plants. Then, to kill them, you either sprinkle them with salt or drop them in a can of soapy water. Hand-picking is time consuming, messy, and in the case of the salt, somewhat grisly.
There are two main barriers. The most commonly recommended one is diatomaceous earth, the powdery remains of prehistoric shellfish. It feels smooth to the touch but is unbearably gritty to slugs and snails, which must exude so much slime to crawl over it, they dehydrate themselves and die. Unfortunately, once it becomes thoroughly wet, they can slide over it on the water layer, so it’s least effective when most needed.
Diatomacious earth is environmentally benign, no danger to fish or friendly insects, and harmless to birds and mammals — assuming they don’t inhale it. Remember to wear a mask if you’re sprinkling out large amounts.
If you have raised beds or other confined garden areas around which to use it, the longest-lasting and most effective barrier is copper, which reacts electrochemically with the beasts’ slime so they will not cross it. Strips of copper at least 2 inches wide around a raised bed will keep lettuce safe in a rainstorm, which is really saying something. Be sure to install the strips in dry weather and to keep checking from time to time for stragglers and newly hatched babies. The pests can’t get in, but they can’t get out either.