Many plants are poisonous to varying different degrees, though perhaps water hemlock’s root would be the most likely to kill a human, a small mouthful being sufficient. Many plants of the genus Aconitum sp. are common garden plants – such as Monkshood and Wolfsbane – and prolonged handling can cause death.
The family Solanaceae contains belladonna, much loved by Cleopatra to dilate her pupils but nonetheless deadly; also Datura a popular bedding plant. The castor oil plant carries the Guinness Book Of World Records title as the world’s most poisonous plant, used recently to assassinate a Bulgarian dissident. Alpha-Amanitin is found in several members of the Amanita genus of mushrooms, most notably the Death cap and the Destroying Angel. They will cause rapid liver failure and death, after a day with no symptoms.