hemlock, any tree of the genus Tsuga, coniferous evergreens of the family Pinaceae ( pine family) native to North America and Asia. The common hemlock of E North America is the eastern hemlock, T. canadensis, an ornamental tree (sometimes cultivated as a hedge) with small cones and short, dark green leaves so arranged as to give the branchlets a flattened appearance. Found from the S Appalachians to N Wisconsin and SE Canada, the tree has been highly valued as a source of tanbark but is now seriously reduced in number. Its wood is soft and light. The Carolina hemlock, T. caroliniana, found on the slopes of the S Appalachians, is grown as an ornamental. Both trees are threatened by the hemlock woolly adelgid, an insect that is native to E Asia and was first found in their range in the 1960s. The western hemlock, T. heterophylla, is the tallest tree of the genus. Found along the coast from S Alaska to N California, it has more valuable wood, which is used in construction work. The mountain hemlock, T. mertensiana, has a similar range but is found inland S of Canada. Both western species are grown as ornamentals. The ground hemlock is a species of yew . The poison hemlock and water hemlock are herbs of the family Umbelliferae ( carrot family) of the division Magnoliophyta . True hemlock is classified in the division Pinophyta , class Pinopsida, order Coniferales, family Pinaceae.

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