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. A hemlock that is a hybrid of the Carolina hemlock and the Chinese hemlock, T. chinensis, and is resistant to the woolly adelgid has been developed for the horticultural trade. 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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