Yes, Dock is toxic to dogs!
Dogs accidentally consuming the Dock 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).
Dock consumption is also toxic to Cats and Horses.
Toxic Principles Of Dock: Soluble calcium oxalates.
More Info About The Dock Plant
Scientific Name: Rumex sp.
Family: Polygonaceae.
Also known as: Sorrel.
The answer is based on information found on The American Society for the Prevention of Cruelty to Animals (ASPCA) website.