Yes, Gardenia is toxic to dogs!
Dogs accidentally consuming the Gardenia plants can show the following clinical symptoms: Vomiting, Prolonged Depression, Incoordination, Hypersalivaton, Sleepiness Or Excitation, Dilated Pupils, Low Body Temperature, Low Blood Pressure, Coma, Seizure And Death (In Rare Cases).
Gardenia consumption is also toxic to Cats and Horses.
Toxic Principles Of Gardenia: Genioposide, gardenoside.
More Info About The Gardenia Plant
Scientific Name: Gardenia jasminoides
Family: Rubiaceae.
Also known as: Cape Jasmine.
The answer is based on information found on The American Society for the Prevention of Cruelty to Animals (ASPCA) website.