Yes, Rhododendron is toxic to dogs!
Dogs accidentally consuming the Rhododendron 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).
Rhododendron consumption is also toxic to Cats and Horses.
Toxic Principles Of Rhododendron: Grayantoxin.
More Info About The Rhododendron Plant
Scientific Name: Rhododendron spp
Family: Ericaceae.
Also known as: Rosebay, Azalea.
The answer is based on information found on The American Society for the Prevention of Cruelty to Animals (ASPCA) website.