Hydrangeas are a plant that do well indoors and outdoors making them a popular household flower. They are colorful and give off a wonderful smell. For dogs and cats, that smell can be inviting and sometimes hard to resist. These plants do have a secret weapon though. When they are damaged, they give off the chemical compound cyanide (found in many types of poisions). Luckily, when exposed to small amounts of these plants, dogs and cats usually only end up with minor GI concerns such as vomiting and diarrhea. If large amounts of this plant have been eaten, please consult with your veterinarian right away for treatment.