Hydrangea is not toxic and even consumed in large amounts will not cause serious effects. The main toxic principle in Hydrangea is an alkaloid called Hydrangeine. However, it is not present in the leaves but in the roots. The roots making up a small portion of the whole plant contain Hydrangeine. Hydrazine is actually more toxic to people than to dogs. Since leaves have very little Hydrangeine, dogs are unlikely to get poisoned by ingesting a large number of leaves.