Yes, Hills of Snow is toxic to dogs!
Dogs accidentally consuming the Hills of Snow 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).
Hills of Snow consumption is also toxic to Cats and Horses.
Toxic Principles Of Hills of Snow: Cyanogenic glycoside.
More Info About The Hills of Snow Plant
Scientific Name: Hydrangea arborescens
Family: Hydrangeaceae.
Also known as: Hydrangea, Hortensia, Seven Bark.
The answer is based on information found on The American Society for the Prevention of Cruelty to Animals (ASPCA) website.