Your dog is at risk from many plants. One example is the cyanide found in stone fruit pits and seeds. Tobacco potatoes and peppers are members of the nightshade family. For more information it would be wise to visit the AKA poisonous plants page.
Slugs and snails are another potential threat in your garden. Some of them carry parasites that can harm your dogs digestive tract. Some of them carry tapeworms and roundworms. You are at risk of contracting these infections as well.
Its also possible for your pet to get into trouble with mulch and compost. It contains cacao remnants which makes cacao mulch a particularly problematic material. It contains the same amount of theobromine as regular chocolate and is just as unhealthy. You should be very careful if you use homemade compost as it might contain mold and possibly food poisoning.