Dear Cathy,
Plants of the genus Hydrangea contain substances known as cyanogenic glycosides, which when fresh or dried have the potential to cause vomiting, diarrhea, a lack of oxygen in the blood (cyanosis), decreased heart rate, respiratory congestion, seizures, coma, and can ultimately lead to respiratory failure and death if ingested in large amounts. Because of its toxic potential, hydrangea should be kept out of the reach of all pets.