Are Hydrangeas Poisonous to Pets?

Hydrangeas (Hydrangea spp.), with their attractive foliage and large, showy clusters of blossoms, are classic garden shrubs. Growing in U.S. Department of Agriculture plant hardiness zones 3 through 9, depending on the variety and cultivar, these plants are attractive to look at, but dangerous for pets. The American Society for the Prevention of Cruelty to Animals notes that hydrangeas are toxic to dogs, cats and horses.

Hydrangeas contain amino acid-derived compounds, called cyanogenic glycosides. These compounds are part of a plant self-defense mechanism designed to discourage animals from eating the plant. When the animal chews the leaves, the cyanogenic glycosides interact with enzymes in the plant to give the hydrangea an unpleasant flavor. The chemical process creates hydrogen cyanide, a deadly poison.

Hydrangeas taste awful, so it’s unlikely your pet will eat enough to get cyanide poisoning. It’s more likely your pet will get an upset stomach. Don’t plant hydrangeas in areas where your pets regularly visit, and monitor your animals if they go in that area.

Dogs, cats or horses that eat hydrangea leaves may develop diarrhea, begin vomiting or act depressed. If they eat enough of the toxin, they might develop breathing problems or begin to hyperventilate. Weakness, loss of coordination, seizures, tremors and coma are also symptoms of cyanide poisoning. Some animals that have been poisoned have a smell similar to that of bitter almonds. Cyanide is a fast-acting poison, and if the pet has eaten enough of the plant, it's possible the pet could die before you're aware or before you're able to get help.

If you suspect cyanide poisoning, take your pet to a veterinarian or emergency pet clinic immediately so the doctor can administer a cyanide antidote. Do not attempt to force the animal to vomit, as this can cause secondary contamination and make the situation worse. The veterinarian can administer treatment even if the animal is not breathing, as long as its heart is still beating and it is still alive. The antidote is usually highly effective and can save the animal’s life if given quickly enough.