Water hemlock. Cicuta, commonly known as water hemlock, is a small genus of four species of highly poisonous plants in the family Apiaceae.Water hemlock is considered one of North America's most toxic plants, being highly poisonous to humans.en most deadly plants in the world. Jimson Weed. Datura stramonium, known by the common names Jimson weed, Devil's snare or datura, is a plant in the Solanaceae family. It is believed to have originated in the Americas, but is now found around the world. Rhododendron.