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Nine species are recognised in the genus Tsuga, found in North America and Asia. Most are exclusively monopodial, including the largest, the American T. heterophylla, but others may be multistemmed or forked, and this is commonly the case with the Asian species. Young trees look more alike, with their conical form and drooping terminal shoots. The bark is generally scaly, and develops longitudinal cracks in later life. The branchlets are initially soft and bright green, in stark contrast with the dark mature foliage. The buds are not resinous, or only slightly so. The needle-like leaves are spirally arranged, on small pegs (pulvini), and twisted at the base; they are short, linear or ovate, entire or denticulate, with considerable variation in apex shape. The male strobili are solitary and grow from axillary buds on second-year shoots in the outer crown. They are very small, 0.3–0.6 cm long, subglobular, and change from reddish to yellow to reddish purple. The cones are sessile or with a short peduncle and are distributed across the entire crown; they are erect at pollination, but become pendulous; cones are green to greenish purple, and mature in one year. The seed scales are peltate or orbicular, petiolate, more or less persistent, and open to release the seeds. There are two seeds on each seed scale, and the bract scales are small, rhombic and included. The seeds are partially enclosed in a membranous cup, which extends to form a persistent wing (Farjon 1990).
The hemlocks are an important small genus of beautiful conifers, whose elegant outlines and dark foliage give them great value for the garden and landscape where their requirements of well-drained but moist acid soil can be met. Unfortunately, in the northeastern United States at least, Tsuga are ravaged by the Hemlock Woolly Adelgid (Adelges tsugae), which has decimated the native and horticultural population of T. canadensis, and also affects T. caroliniana. A resistant replacement has been sought, and may have been found in T. chinensis (see below). Much valuable information on the Hemlock Woolly Adelgid is available on the internet, with the latest findings on its control, including biological methods. Several other pests can cause problems with these species – in particular, the Elongate Hemlock Scale (Fiorinia externa), often found together with the adelgid, and causing further debilitation and death of infested trees.
A group of eight or ten evergreen trees of great beauty and elegance, represented on both sides of N. America, in China, Japan, and the Himalaya. They have very slender twigs, and short linear leaves, arranged, except in one species (T. mertensiana), mainly in two opposite ranks, each leaf seated on a cushion-like projection (as in Picea), and closely set on the twigs – twelve to twenty-four to the inch. They differ, however, from those of Picea in being always borne on a short but distinct petiole; in Picea they are sessile on the cushions. Cones solitary, rarely more than 1 in. long, and usually pendulous at the end of the twigs. Seeds winged. In places where they thrive, which is where the rainfall is abundant and the soil is deep and well-drained, they are not exceeded in beauty of form by any other evergreen trees. They are best propagated by means of seed; but the Japanese and Chinese species, perhaps the others also, can be propagated by cuttings.
A recent work on this genus, by an American authority, is: John C. Swartley, The Cultivated Hemlocks (1984). It is a revision, by Humphrey Welch, of a thesis on T. canadensis and its variations originally submitted by Dr Swartley to Cornell University in 1939, and is still largely devoted to that species.