According to the American Society for the Prevention of Cruelty to Animals (ASPCA), certain parts of hydrangea plants are particularly toxic to dogs. The buds, flowers and leaves are highly toxic and, in extreme cases, can prove fatal.
Hydrangeas contain cyanogenic glucosides, which, according to the ASPCA and the Pet Poison Helpline, are toxic to dogs, cats and horses. The highest concentration of these toxic principles are in the leaves, buds and flowers of the hydrangea plant. If a dog ingests hydrangeas, symptoms include lethargy or depression, vomiting and diarrhea. If the dog has a particularly severe reaction, the toxicity goes untreated, or the dog eats a particularly large amount, it can be fatal. Therefore, if owners suspect their dog has eaten any part of the hydrangea plant, the ASPCA advises owners to seek veterinary advice immediately.