Gardenias can be poisonous to pets depending on how much the pet eats and how small the pet is. Generally, the smaller the pet, the faster they can become sickened by a gardenia. For example, a large animal like a human can eat a gardenia and survive. If the gardenia has been treated with pesticides, it will be even more toxic.
According to the ASPCA Poison Control Center, a pet sickened by gardenia will have hives, vomiting and diarrhea. If the animal is a horse, which cannot vomit, it can develop hives and diarrhea. Guinea pigs have been known to eat a few gardenia petals and survive, but they often suffer diarrhea, which can kill them since they are so small. However, rabbits usually suffer no ill effects if they eat a gardenia.
Gardenias are particularly toxic to cats. If a cat has eaten a gardenia and has not already begun to vomit, the cat needs to be induced to vomit. Ideally, one person should call a vet while another induces vomiting in the cat. To induce vomiting, weigh the cat. For every pound the cat weighs, give one tablespoon of 3% hydrogen peroxide every 10 minutes until the cat vomits.
Horses need to eat a lot of gardenias in order to suffer ill effects. However, it's not worth taking the chance. All gardenias should be pulled by hand from the pasture or anywhere on a property that a horse mouth can reach. Using chemical herbicides is not recommended as they may be far more toxic to the horse than a gardenia.
Gardenias are not one of the most toxic plants a pet could eat, although they shouldn't eat them. Plants that are even more toxic than gardenias include jimsonweed, marijuana, foxglove, buttercup, black locust, yew, oleander, oak, wisteria, sweet pea, jasmine, horse chestnut, holly, cherry trees and castor oil plants.
It is a common misconception that animals instinctively know what food is good for them and what is bad. This is not true, especially in mammals. Pets will stick anything in their mouths, usually out of boredom. Very young animals like puppies, kittens and foals stick anything in their mouths as part of learning about the world.