Water hemlock is the most violently toxic plant that grows in North America. Only a small amount of the toxic substance in the plant is needed to produce poisoning in livestock or in humans. The toxin cicutoxin, acting directly on the central nervous system, is a violent convulsant. Clinical signs of poisoning occur when a throshold dose is reached after which grand mal seizures and death occur.
Water hemlock has small, white flowers that grow in umbrella like clusters.
Side veins of the leaves lead to notches, not to tips at the outer margin. The thick rootstalk of water hemlock contains a number of small chambers. These hold a highly poisonous brown or straw-colored liquid that is released when the stem is broken or cut. Thick, fleshy tubers and slender individual roots grow from the bottom of the rootstalk. Water hemlock grows in wet seepage areas of meadows, pastures, and in streams.