Tsuga heterophylla

Western hemlock is a large evergreen conifer growing to 50–70 metres (165–230 feet) tall, exceptionally 83 m (273 ft),^[7] and with a trunk diameter of up to 2.7 m (9 ft). It is the largest species of hemlock, with the next largest (mountain hemlock) reaching a maximum height of 59 m (194 ft). The bark is brown, thin, and furrowed (outwardly appearing similar to that of Douglas-fir).^[6] The crown is a very neat broad conic shape in young trees with a strongly drooping lead shoot, becoming cylindrical in older trees, which may have no branches in the lowest 30–40 m (100–130 ft). At all ages, it is readily distinguished by the pendulous branchlet tips. The shoots are very pale buff-brown, almost white, with pale pubescence about 1 millimetre (1⁄32 in) long.

The leaves are needle-like, 5–25 mm (3⁄16–1 in) long^[8] and 1.5–2 mm (1⁄16–3⁄32 in) broad, strongly flattened in cross-section, with a finely serrated margin and a bluntly acute apex. They are mid to dark green above; the underside has two distinctive white bands of stomata with only a narrow green midrib between the bands. They are arranged spirally on the shoots but are twisted at the base to lie in two ranks on either side of the shoot. The cones appear on trees over about 25 years old;^[6] they are small, pendulous, slenderly cylindrical, 14–30 mm (1⁄2–1+1⁄8 in) long and 7–8 mm (1⁄4–5⁄16 in) broad when closed, opening to 18–25 mm (3⁄4–1 in) broad. They have 15–25 thin, flexible scales 7–13 mm (1⁄4–1⁄2 in) long. The immature cones are green, maturing gray-brown 5–7 months after pollination. They are usually plentiful enough to cover the ground beneath the tree.^[6] The abundant seeds are brown, 2–3 mm (1⁄16–1⁄8 in) long, with a slender, pale-brown wing measuring 7–9 mm (1⁄4–3⁄8 in) long.^[4][5][6]

Initial growth is slow; one-year-old seedlings are commonly only 3–5 centimetres (1+1⁄8–2 in) tall, and two-year-old seedlings 10–20 cm (4–8 in) tall. Once established, saplings in full light may have an average growth rate of 50–120 cm (20–47 in) (rarely 140 cm, 55 in) annually until they are 20–30 m (65–100 ft) tall, and in good conditions still 30–40 cm (12–16 in) annually when 40–50 m (130–165 ft) tall. The tallest specimen, 82.83 m (271 ft 9 in) tall, is in Prairie Creek Redwoods State Park, California (United States). The species is long-lived, especially at higher elevations,^[6] with trees over 1,200 years old known.^[5]

T. heterophylla is an integral component of Pacific Northwest forests west of the Coast Ranges, where it is a climax species. It is also an important timber tree throughout the region, along with many of its large coniferous associates.^[9] The species is closely associated with temperate rainforests, and most of its range is less than 100 kilometres (62 miles) from the Pacific Ocean. Valleys it can be found in usually receive at least 80 cm (31 in) of rain annually.^[6] It mostly grows at low altitudes, from sea level to 600 m (2,000 ft). In western Washington, it can be found up to elevations of 1,070 m (3,510 ft).^[6] The species can also be found in humid areas of mountains further inland, where western white pine is normally dominant.^[6] For example, in the Columbia Mountains in and around southeastern British Columbia and northern Idaho, it grows up to 1,500 m (4,900 ft), particularly on north-facing slopes.^[6] In the interior part of its range in Idaho, it can be found up to 1,800 m (5,900 ft).^[4][5]

Western hemlock is a very shade-tolerant tree; among associated species in the Pacific Northwest, it is matched or exceeded in shade tolerance only by Pacific yew and Pacific silver fir.^[9] Young plants typically grow up under the canopy of other conifers such as Sitka spruce or Douglas-fir, where they can persist for decades waiting to exploit a gap in the canopy. They eventually replace these conifers, which are relatively shade-intolerant, in climax forest. However, storms and wildfires will create larger openings in the forest where these other species can then regenerate. Its thin bark and shallow roots makes it susceptible to fire.^[6] At higher elevations, the species can be found mingling with T. mertensiana (mountain hemlock), seeming to take on some of its characteristics although there is no hard evidence of hybridization.^[6]

Western hemlock forms ectomycorrhizal associations with some well-known edible fungi such as chanterelles (Cantharellus formosus, C. subalbidus, and Craterellus tubaeformis).^[10][11] It is capable of associating with wood-decay fungi in addition to soil fungi; this enables its seedlings to survive on rotting stumps and logs.^[12] Older forests are damaged by rot-causing fungi, dwarf mistletoe, and leaf-consuming insects such as Acleris gloverana and Lambdina fiscellaria.^[6]

The bark has long served as a source of tannin for tanning leather.^[6]

Western hemlock is the state tree of Washington.^[17]