According to "Sunset" magazine, slugs, which are snails that don't have shells, damage flowers and other garden plants by chewing up the foliage. You can tell if you have slugs in your garden by the trails of slime they leave in their wake. Slugs are serious garden pests in many parts of the United States.
Pick the slugs off the plants by hand.
Set a board onto the garden soil, raising it a bit off the ground by putting a small stone under each of its four corners. Early the next day, go out into the garden, turn the board over and dispose of the slugs that you find hiding underneath it.
Dig out a shallow circular space in your garden soil just large enough to hold a disposable aluminum pie plate. Fill the pie plate about a quarter full with stale beer. The next day, pour the beer and any slugs that have gone into it into the trash.
Dig out an area all around your flower bed that is about 5 inches wide. Fill it with fine sawdust. Unless the sawdust gets very wet, slugs won't try to cross it to get to your flowers.
If despite these efforts slugs continue to be a problem, apply slug bait containing metaldehyde or methiocarb. You can buy slug bait at garden nurseries and many home improvement warehouses.
Both these chemicals are strong and potentially toxic, so use them carefully and follow the directions on the packaging to the letter. Keep kids and pets away from slug bait that contains these chemicals.