A tree 70 to over 100 ft high, the trunk 12 ft or more in girth; bark red-brown; young shoots downy. Leaves shortly stalked, set all round the branchlet, although more crowded on the upper side, linear, curved, 1⁄2 to 1 in. long, 1⁄20 to 1⁄16 in. wide, rounded at the apex, margins not toothed; sometimes grey-green, sometimes conspicuously blue-green. There are inconspicuous lines of stomata on both surfaces. Cones without stalks, rich purple when young, becoming red-brown, oval-cylindric, 11⁄2 to 3 in. long, 1⁄2 to 3⁄4 in. thick.
Native of western N. America from S. Alaska south through the coastal ranges to N. California, where it is confined to the Sierra Nevada; there is also a less extensive area in the northern Rocky Mountains. The western hemlock, T. heterophylla, has a similar distribution but reaches its maximum development in the coastal ranges at altitudes of up to 2,000 ft, while the mountain hemlock does not descend below 3,000 ft except in Alaska and ascends into the subalpine zone, where it is sometimes a stunted shrub. It was discovered in 1827 on Baranof Island, Alaska, by the German botanist K. Mertens during the Russian circumnavigation of the globe led by Capt. Luetke. In 1851 John Jeffrey sent seeds from Mt Baker in Washington to the Oregon Association but they germinated poorly and the only recorded tree from this sending was a hybrid (see T. × jeffreyi below); he sent further seed in the following year from the Cascades around 42°, and it was from a coning specimen in this consignment that Andrew Murray described Abies pattoniana, naming it in honour of George Patton of The Cairnies (later Lord Glenalmond), who was the originator of the Oregon Association – a group ‘of Gentlemen interested in the promotion of the Arboriculture and Horticulture of Scotland’ (see Notes Roy. Bot. Gard. Edin., Vol. 20 (1939), pp. 1-53). But apparently no trees from the 1852 seed have survived and the introduction of the species is usually attributed to William Murray, who sent seed from Mt Scott in Oregon in 1854, while collecting for Messrs Lawson of Edinburgh. Confusingly, his brother Andrew Murray made a new species of this collection, calling it Abies hookeriana.
T. mertensiana is remarkable for its radially arranged leaves with stomata on both sides and its large cones. Engelmann gave it generic rank and more recently some French investigators have advanced the theory that it is the result of hybridisation between T. heterophylla and Picea sitchensis, and that the minor variant named T. crassifolia by Mme Flous is a hybrid between the mountain hemlock and Picea engelmannii, thus having the western hemlock and two spruces in its genetic make-up.
The colour of the foliage of T. mertensiana is usually of a glaucous hue, but of varying shades, which it is pointless to distinguish varietally. But plants in which the leaves are silvery glaucous could take the name f. argentea (Beissn.) Rehd.
In all its forms the mountain hemlock is remarkably beautiful. It needs a moist climate and a pure atmosphere. At Murthly Castle, near Perth, there is a group of several trees (one with pendulous branchlets) which makes one of the most beautiful garden pictures one can imagine, produced by foliage alone. The largest of these trees are: pl. 1862, 94 × 81⁄4 ft; pl. 1863, 85 × 81⁄2 ft and, date unrecorded, 84 × 103⁄4 ft (1970). Among other Scottish trees are: Blair Atholl, Perths., pl. 1872, 102 × 71⁄2 ft (1970); Fairburn, Ross and Cromarty, 102 × 71⁄2 ft (1970). At Patterdale Hall, Westmorland, there is a broad, silvery grey bush measuring 42 × 73⁄4 ft (1976). Examples in eastern and southern England are: Fulmodestone, Norfolk, 72 × 51⁄4 ft (1969); Tilgate, Sussex, pl. 1905, 45 × 61⁄2 ft (1964); National Pinetum, Bedgebury, Kent, an example of f. argentea, pl. 1925, 24 × 21⁄4 ft (1969).
T. × jeffreyi (Henry) Henry T. pattoniana var. jeffreyi Henry; × Tsugo-Piceo-Tsuga jeffreyi (Henry) Campo-Duplan & Gaussen – As noted above, Jeffrey sent seed of T. mertensiana in 1851 from Mt Baker, where T. heterophylla also occurs. A tree in the Edinburgh Botanic Garden, raised from this batch was anomalous in having flatter, shorter, relatively broader, finely serrated leaves, more pectinately arranged, the exposed side lighter green, with fewer stomata. Henry at first gave this tsuga varietal rank but later concluded that it was a hybrid between T. mertensiana and T. heterophylla. Trees under the name T. × jeffreyi are in cultivation, but their provenance is unknown. They are very near to T. mertensiana but with greener leaves more pectinately arranged.
Intermediates or hybrids between the western and mountain hemlocks have been observed in the wild where the two are in contact, and are said to be common in parts of Washington, east of the Cascades (Hitchcock et al., Vasc. Pl. Pacif. Northwest, Part I (1969), p. 133). They have also been observed on Mt Baker, whence came the type of T. × jeffreyi.