Tsuga is a genus of conifers in the family Pinaceae, comprising of 8-10 species. Four species occurs in North America, and the rest in eastern Asia. Hemlock is the common name for Tsuga. The genus got its name from the similarity in the smell of crushed foliage to that of the unrelated herb poison hemlock. Unlike the herb, Tsuga species are not poisonous.
Tsugas are evergreen conifers, growing to about 10-70 m tall. These graceful trees have drooping or arching branches, and they make fine ornamental plants. Tsugas are among the few conifers that can be safely clipped and pruned, and for this reason, they can be used for hedges, when they should be clipped from early to late summer.
Tsuga mertensiana (Mountain Hemlock) is native to the west coast of North America. It is a large evergreen coniferous tree growing to 20-40 m tall. It is a columnar, densely branched tree, with blue-green foliage and oblong cones. The cones are small and somewhat like a small spruce cone, with thin, felxible scales of 8-18 mm long. The immature cones are dark purple, maturing red brown 5-7 months after pollination. The seeds are red brown, 2-3 mm long, with a slender, long pale pink-brown wing. It is grown as an ornamental tree in gardens, particularly in northern Great Britain and Scandinavia, where it is appreciated for its blue-green color and tolerance of severe weather.
Tsuga heterophylla (Western Hemlock) is an elegant tree, with attractively cracked, purplish bark. The needles are dark green but silvery beneath. Its cones are pale green at first, ripening to dark brown color. This makes a good ornamental plant and also as a fine hedge.
Tsuga species are used as food plant for larvae of some Lepidoptera species such as Autumnal Moth and the Engrailed. The foliage of young trees is often browsed by deer, and the seeds are eaten by finches and small rodents.
T. canadensis and T. caroliniana, two eastern North American species, are under threat by the sap-sucking insect Adelges tsugae ( Hemlock Woolly Adelgid). Older trees are commonly attacked by various fungal disease and decay species, notably Heterobasidion annosom and Armillaria species, which rot the heartwood and eventually leave the tree liable to windthrow.
Plant Tsuga in sun or partial shade, in moisture-retentive but well-drained, slightly acid soil. Propagation is by seed in spring and semi-ripe cuttings in summer.
Tsuga mertensiana http://commons.wikimedia.org/wiki/File:Tsuga_mertensiana_cones_Skyscraper_Mountain.jpg Peter Stevens
http://commons.wikimedia.org/wiki/File:Tsuga_mertensiana_cones_Skyscraper_Mountain.jpg
Peter Stevens
