How to keep away slugs and snails
In a healthy and living garden slugs and snails feel comfortable, too. They are useful creatures that eat all sorts of waste in the garden. Their dung enriches the soil and their slime stimulates natural processes in the ground. But certainly you don't want them to eat your dahlias. It is possible to trap them with a shallow bowl filled with beer or keep them out by scattering some broken glass and shells around your plants. This will probably help a little, but there are videos that show slugs bravely crossing the glass border.
Copper however really keeps them away. Slugs get a slight electric shock from copper and which makes them turn around. There are several ways to use copper. Copper tape is easy to use, but does not look very nice in your garden. A copper grid is more subtle. If you have any copper wire lying around, just try it out. Also try to make your garden a welcome place for the natural enemies of slugs, like hedgehogs and birds, and create a natural balance.