Plants that are poisonous to dogs include American holly, amaryllis, castor bean plants, garlic and other members of the allium family, and tomato plants, says the American Society for the Prevention of Cruelty to Animals. The American holly contains saponins, which if eaten by the dog causes diarrhea, vomiting and depression. Fortunately, the leaves and berries of the plant have low toxicity.
Amaryllis is somewhat more toxic, states the ASPCA. A dog that eats this beautiful flower experiences vomiting, diarrhea, depression and abdominal pain. Trembling is another a sign that a dog has ingested amaryllis.
Castor bean plants contain the deadly poison ricin, and eating even a few of the beans can be fatal, claims the ASPCA. Eating the plant burns the mucous membranes, causes great thirst, vomiting, bloody diarrhea, clumsiness, sweating and trembling. If the dog is not treated, castor bean plant poisoning leads to kidney failure, convulsions and death.
Though good for humans, garlic and onions are quite bad for dogs, says the ASPCA. Eating garlic destroys a dog's red blood cells. Dogs that eat garlic become weak, develop bloody urine, pant a great deal and suffer tachycardia. Chewing on a tomato plant depresses a dog's central nervous system and causes gastrointestinal distress.