The common name “hemlock” is used to refer to several different trees and plants with a number of different properties. Hemlocks can be found growing naturally in many regions of the world and some are deliberately cultivated as ornamental plants. Because of the fact that this common name is applied to a range of plants, botanists tend to use scientific names when talking about hemlocks to avoid confusion.
The parsley family, Apiaceae, contains several genera which are referred to as hemlocks including Conium, Cicuta,, and Oenanthe. All of these genera contain plants which are toxic, and the hemlocks look very similar to each other. Their foliage is similar to that of carrot greens, and their flowers grow in white umbrels. Many of the hemlocks are very open and branching, and can grow quite large. Some have stems speckled with purple.
Conium is probably the most famous genus because plants from this genus were once used for executions, perhaps most notably in Ancient Greece, where hemlock was provided to the philosopher Socrates when he was condemned to death. This genus is also sometimes referred to as poison parsley, spotted hemlock, or spotted corobane. The Cicuta genus or water hemlocks, also known by names such as cowbane or poison parsnip, is also highly toxic, and several of the Oenanthe species, known as water dropworts, are also poisonous. However, some are also used for food, making it especially important to be able to distinguish between different species in this genus to avoid gathering the wrong plant for the dinner table.
Poison hemlock is usually not ingested by humans because in regions where it grows wild, people who eat wild plants are familiar with the appearance and sharp scent of hemlock and know to avoid it. However, animals sometimes eat hemlock and they can become very sick. Repeated exposure to hemlock can lead to death in grazing animals like cattle and horses. For this reason, farmers inspect their fields for hemlock and other toxic plants before setting animals loose.
A completely unrelated genus, Tsuga, is also known as hemlock. The genus contains a number of species which are grown as ornamental trees. The “hemlock” name is used for these trees because they smell somewhat like the poisonous species discussed above. However, they are not toxic, and are perfectly safe to grow as ornamentals although they can tend to steal water from neighboring plants, so they should be positioned with care in the garden.