Hydrangea (Hydrangea macrophylla)
Why we grow it: The large, pink, blue, or white flower clusters on these shade-loving shrubs perk up any landscape, blooming at the start of summer and into the fall.
Deadly parts: The entire plant, especially the flower buds.
Toxic toll: Swallowing hydrangea is like popping a cyanide pill. The present poison, hydragin, is a cyanogenic glycoside, meaning it will cause shortness of breath, dizziness, fainting, and a rapid pulse, along with a drop in blood pressure that can cause convulsions and death.