Western hemlock (Tsuga heterophylla) is a hemlock tree species native to North America. Western hemlock is planted throughout California and other regions for ornamental purposes as well as forestry.
I. Appearance and Characteristics
Tsuga heterophylla, the western hemlock or western hemlock-spruce, is a species of hemlock native to the west coast of North America, with its northwestern limit on the Kenai Peninsula, Alaska, and its southeastern limit in northern Sonoma County, California. The Latin species name means ‘variable leaves’.
Western hemlock is a large evergreen conifer growing to 50–70 metres (165–230 feet) tall, exceptionally 83 m (273 ft), 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). 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, 5–23 mm (3⁄16–29⁄32 in) long 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; they are small, pendulous, slenderly cylindrical, 14–30 mm (9⁄16–1+3⁄16 in) long and 7–8 mm (9⁄32–5⁄16 in) broad when closed, opening to 18–25 mm (23⁄32–31⁄32 in) broad. They have 15–25 thin, flexible scales 7–13 mm (9⁄32–1⁄2 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. The abundant seeds are brown, 2–3 mm (3⁄32–1⁄8 in) long, with a slender, pale-brown wing measuring 7–9 mm (9⁄32–11⁄32 in) long.
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, with trees over 1,200 years old known.
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. 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. 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.
Western hemlock forms ectomycorrhizal associations with some well-known edible fungi such as chanterelles (Cantharellus formosus, C. subalbidus, and Craterellus tubaeformis). 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. Older forests are damaged by rot-causing fungi, dwarf mistletoe, and leaf-consuming insects such as Acleris gloverana and Lambdina fiscellaria.
II. How to Grow and Care
Sunlight
Western hemlock flourishes optimally under partial sunlight, which supports vigorous growth and maintains robust health. While western hemlock can adapt to both full sun and full shade, these extremes can lead to suboptimal conditions. In full sun, its growth might slow as it could struggle with excessive light and heat exposure, potentially leading to leaf burn.
Conversely, in heavy shade, western hemlock may exhibit less dense foliage and a more columnar growth form, diminishing its ornamental value. Outdoors, western hemlock performs best in dappled sunlight, mimicking its natural understory habitat in mixed evergreen forests. By planting western hemlock in an environment that replicates its native light exposure, the plant’s dynamic light adaptation traits, such as foliage density and chlorophyll concentration, contribute to its survival and aesthetic appeal.
Watering
Originating from the moist, temperate rainforests of the Pacific Northwest, western hemlock thrives in environments with consistent moisture. This species exhibits a preference for evenly damp soil and is moderately drought-tolerant once established. For optimal health, watering should occur once every 1-2 weeks, aligning with its natural need for steady hydration without waterlogging. As an evergreen plant commonly grown outdoors, western hemlock benefits from mulching to retain soil moisture, which is particularly important during its active growth periods to support its lush foliage.
Fertilizing
For western hemlock, fertilize with a balanced, slow-release formula in spring as growth resumes; this encourages vibrant health and dense foliage. Apply according to package instructions, typically 1-2 lbs per 100 sq. ft., avoiding excess to prevent root burn. Seasonal adjustments: lighter dose in fall to prepare for dormancy. A consistent routine optimizes western hemlock’s growth without over-fertilizing, maintaining ecosystem harmony.
Propagation
Western hemlock is best propagated in autumn or winter through methods such as layering, cutting, or sowing. It has moderate difficulty in propagation, with successful growth indicated by root development and new foliage. Ensure proper humidity and temperature for optimal results.
Transplanting
The optimal time to transplant western hemlock is during the late summer to early fall (S3-S4), as it can establish roots before winter. Western hemlock prefers locations with good drainage, indirect light, and somewhat acidic soil. When transplanting, ensure roots remain moist to avoid shock to the plant.
Repotting
Repotting western hemlock, a large conical evergreen, should be done every 2-3 years to accommodate root growth. Best repotted in late winter before new spring growth. Choose deep containers mimicking its natural wooded habitat. Ensure pots have good drainage. After repotting, place western hemlock in a shaded area, keep the soil moist, and avoid fertilizing for several months to prevent shock and encourage strong root development.
III. Uses and Benefits
- Ornamental uses
Depending on how you prune your western hemlock, it can work as either an ornamental shrub or a large specimen tree in informal or cottage gardens. Smaller versions of this plant make for perfect ground cover for these same types of gardens, or as evergreen borders in rock gardens. It also drapes gracefully over garden trellises with some shaping.
- Forestry
When planted on the banks of a river, western hemlock can help reduce erosion. Outside of its natural range, the tree is of importance in forestry.
Until the early 1920s, the tree was largely ignored for use as lumber due to its presumed similarity to the poor-quality eastern hemlock. Since then, it has been greatly utilized for timber (as a softwood) and paper production; it is used for making doors, joinery, and furniture. Its fiber is used to make rayon and various plastics.
It has naturalized in some parts of Great Britain and New Zealand—not so extensively as to be considered an invasive species, but as an introduced one.
- Food and medicine
The edible cambium can be collected by scraping slabs of removed bark. The resulting shavings can be eaten immediately, or can be dried and pressed into bread, as was done by the natives of Southeast Alaska. The inner bark was eaten by some Native American tribes as an emergency food, and the bark was cooked to make medicinal extracts for tuberculosis, rheumatic fever, and hemorrhage. The bark could also be boiled to make dark red dyes to make fishing nets and lines less visible to fish.
Western hemlocks have been submerged to collect herring eggs during the spring spawn in southeast Alaska. The boughs provide an easily collectible surface for the eggs to attach to as well as providing a distinctive taste. This practice originates from traditional gathering methods used by Native Alaskans from southeast Alaska, specifically the Tlingit people.
Tender new-growth needles can be chewed directly or made into a bitter tea, rich in vitamin C (similar to some other hemlock and pine species).