Zyklon B was the trade name of a cyanide-based pesticide invented in Germany in the early 1920s. It consists of hydrogen cyanide (prussic acid)
No coincidence they have Hydrangea plants
Sadly, the entire plant, including the flower buds, is quite toxic to pets and children. All parts of Hydrangea contain traces of cyanide. Accidental ingestion could cause shortness of breath, dizziness, fainting, and a rapid pulse. Severe cases of poisoning could also lead to convulsions. Cyanide kills muscular cells permanently.