Tsuga

Tsuga is a genus of conifers in the family Pinaceae. The common name hemlock is derived from the perceived similarity in the smell of the crushed foliage to that of the unrelated herb Poison hemlock; see hemlock for other senses of the word. Unlike the herb, the species of Tsuga are not poisonous. The name Tsuga is the Japanese name for Tsuga sieboldii.

The genus includes nine species, four in North America and five in Asia. They are medium-size to large evergreen trees, 20-65 m tall, with conical to irregular crown shape and drooping branch tips. The leaves are needle-like, 8-40 mm long and 1.5-3 mm wide, arranged spirally on the stem but twisted at the base to lie flat on either side of the shoot; they are green above without stomata (except T. mertensiana, see below), and with two white stomatal bands below. The cones are pendulous, small (15-35 mm long; 35-70 mm in T. mertensiana), ovoid to cylindrical, and mature in autumn, 6-8 months after pollination. The seeds are very small, 2-4 mm long, with an 8-12 mm wing.

Western Hemlock Tsuga heterophylla is the largest species, reaching heights of 70 m. It is a particularly common timber tree in the Pacific Northwest of North America and is the state tree of Washington. It is also planted for timber in northwest Europe and other temperate areas with high rainfall and cool summers.

The two species in eastern North America, Eastern Hemlock T. canadensis and Carolina Hemlock T. caroliniana, are threatened by a sap-sucking insect, the Hemlock Woolly Adelgid (Adelges tsugae). This aphid was introduced accidentally from eastern Asia. Extensive mortality has occurred, particularly east of the Appalachian Mountains. The Asian species, and also the two west American hemlocks, are relatively resistant to this pest. Tsuga species are used as food plants by the larvae of some Lepidoptera species including Autumnal Moth and The Engrailed.

Mountain Hemlock T. mertensiana is unusual in the genus in several respects. The leaves are less flattened and arranged all round the shoot, and have stomata above as well as below, giving the foliage a glaucous colour; and the cones are the longest in the genus, 35-70 mm long and cylindrical rather than ovoid. Some botanists treat it in a distinct genus as Hesperopeuce mertensiana, though it is more generally only considered distinct at the rank of subgenus.

Another species, Bristlecone Hemlock, first described as Tsuga longibracteata, is now treated in a distinct genus Nothotsuga; it differs from Tsuga in the erect (not pendulous) cones with exserted bracts, and male cones clustered in umbels, in these features more closely allied to the genus Keteleeria.

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