Some types of hibiscus are toxic to dogs while others are not. The American Society for the Prevention of Cruelty to Animals specifically lists the hardy hibiscus, Rose of Sharon (Hibiscus syriacus), as toxic to your canine companions if ingested, although the flowers are considered generally safe for humans to eat (But, depends on the individual), according to the University of Arkansas, Division of Agriculture Research & Extension. Rose of Sharon grows in USDA zones 5 through 8. The reason why this and other hibiscus are toxic to dogs is unknown. While the Rose of Sharon is specifically named by the ASPCA as being toxic to dogs, other species may cause toxic reactions as well, especially the flowers, although poisoning is usually mild. For safety's sake, keep your hibiscus plants, both inside and outside, away from the reach of your pooch. Eating plant matter, even nontoxic flowers, can cause gastrointestinal upset, diarrhea and vomiting if your dog ingests a large amount of it. Prevent issues by monitoring your dog around any plants, especially the hibiscus.