Two species are commonly known as hemlock: Conium maculatum and Tsuga canadensis. The first is a weedy plant, and all parts of it are poisonous. In ancient times, minimal doses of the plant were used to relieve pain, although with a great risk of poisoning from this form of treatment. This hemlock was also used to carry out death sentences in ancient times; for example, the Greek Athenian philosopher Socrates (469 B.C.E.–399 B.C.E.) was condemned to death and sentenced to drink a potion made from hemlock. The poisonous species should not be confused with Tsuga canadensis, a member of the evergreen family. Its leaves are not poisonous and are often used to make tea.

Deciduous trees called dawn redwoods (Metasequoia) have bright green leaves in the summer that turn coppery red in the fall before they drop. Previously known only as a fossil, a living tree was found in China in 1941 and has been grown in the United States since the 1940s. The U.S. Department of Agriculture eventually distributed seeds to experimental growers in the United States—and the dawn redwood tree now grows all over the country.