Mountain hemlock (Tsuga mertensiana) is an evergreen, coniferous tree whose small seed cones are light to dark purple. It prefers partial shade and moist, well-drained soil. Prefers colder climates where the growing season is short and the winter is long.
I. Appearance and Characteristics
Tsuga mertensiana, known as mountain hemlock, is a species of hemlock.Mertensiana refers to Karl Heinrich Mertens (1796–1830), a German botanist who collected the first specimens as a member of a Russian expedition in 1826–1829.
The distribution of T. mertensiana stretches from Kenai Peninsula, Alaska, to northern Tulare County, California. Its range fairly closely matches that of T. heterophylla (western hemlock), found less than 100 kilometres (62 miles) from the Pacific Ocean, apart from an inland population in the Rocky Mountains in southeast British Columbia, northern Idaho, and western Montana. The inland populations most likely established after deglaciation by remarkable long-distance dispersal of more than 200 km (120 mi) from the coast populations.
Their ranges differ in California, where western hemlock is restricted to the Coast Ranges and mountain hemlock is found in the Klamath Mountains and Sierra Nevada. Unlike western hemlock, mountain hemlock mostly grows at high altitudes except in the far north, from sea level to 1,000 m (3,300 ft) in Alaska, 1,600 to 2,300 m (5,200 to 7,500 ft) in the Cascades in Oregon, and 2,500 to 3,050 m (8,200 to 10,010 ft) in the Sierra Nevada.
Tsuga mertensiana is a large evergreen conifer growing up to 20 to 40 meters (66 to 131 feet) tall, with exceptional specimens as tall as 59 m (194 ft) tall. They have a trunk diameter of up to 2 m (6+1⁄2 ft). The bark is about 3 centimeters (1+1⁄4 inches) thick and square-cracked or furrowed, and purplish-brown to gray in color. The crown is a neat, slender, conic shape in young trees with a tilted or drooping lead shoot, becoming cylindric in older trees.
At all ages, it is distinguished by the slightly pendulous branchlet tips. The shoots are orange–brown, with dense pubescence about 1 millimeter (1⁄16 in) long. The leaves are needle-like, 7 to 25 mm (1⁄4 to 1 in) long and 1 to 1.5 mm (1⁄32 to 1⁄16 in) broad, soft, blunt-tipped, only slightly flattened in cross-section, pale glaucous blue-green above, and with two broad bands of bluish-white stomata below with only a narrow green midrib between the bands; they differ from those of any other species of hemlock in also having stomata on the upper surface, and are arranged spirally all around the shoot.
The cones are small (but much longer than those of any other species of hemlock), pendulous, cylindrical, 3 to 8 cm (1 to 3 in) long and 8 to 10 mm (5⁄16 to 3⁄8 in) broad when closed, opening to 12 to 35 mm (1⁄2 to 1+3⁄8 in) broad, superficially somewhat like a small spruce cone. They have thin, flexible scales 8 to 18 mm (5⁄16 to 11⁄16 in) long. The immature cones are dark purple (rarely green), maturing red–brown 5 to 7 months after pollination. The seeds are red–brown, 2 to 3 mm (1⁄16 to 1⁄8 in) long, with a slender, 7 to 12 mm (1⁄4 to 1⁄2 in)-long pale pink–brown wing.
II. How to Grow and Care
Mountain hemlock thrives in cool, humid environments and requires consistent moisture for optimal growth. Notably sensitive to drought, ensuring adequate water supply during dry spells is vital. Mountain hemlock prefers well-drained, acidic soil and partial to full sunlight. Protection from harsh winds and extreme heat is crucial for its health.
Sunlight
Mountain hemlock thrives in full sun conditions, where it can receive unfiltered light throughout the day. This exposure maximizes mountain hemlock’s growth potential and enhances its overall health, including richer needle color and dense growth patterns. However, mountain hemlock can tolerate partial sun, allowing it to adapt to environments with some afternoon shade or dappled light. Deviations from ideal light can lead to reduced vigor and sparse foliage.
Mountain hemlock demonstrates resilience to varying light through flexible adaptations, such as needle arrangement that maximizes light capture. In outdoor settings, mountain hemlock should be planted in locations with clear access to the sky, away from taller structures or trees that could obstruct significant sunlight.
Watering
Originating from high-altitude regions, mountain hemlock is accustomed to moderate moisture and well-draining soils. This species thrives with a balance of hydration, showing a preference for consistent moisture without waterlogging. Its watering regimen aligns with its natural alpine environment, necessitating watering once every 1-2 weeks. As an evergreen plant typically grown outdoors, mountain hemlock’s needle-like leaves are adapted to conserve water, reflecting its efficient use of precipitation in its native habitat.
Fertilizing
For mountain hemlock, applying balanced nutrition fertilizers promotes robust growth and vitality. Fertilize in spring and fall, adjusting quantity according to plant size and soil fertility. Limit fertilization in late summer to avoid fresh growth susceptible to frost. Use slow-release formulations for steady nutrition. Over-fertilizing can damage mountain hemlock; always follow the manufacturer’s recommended dosage. Novices should note that less is often more when fertilizing.
Propagation
Mountain hemlock is effectively propagated via the sowing method. Successful cultivation starts with stratified seeds that ensure higher germination rates. Cold stratification mimics winter conditions and should be done prior to planting. Sow seeds in well-draining soil and maintain consistent moisture but avoid waterlogging as it can lead to fungal diseases. The use of a mist system can aid in keeping the humidity at an optimal level for seedling growth. Gradually acclimate young plants to outdoor conditions before transplanting to their permanent location.
Transplanting
The vernal season is when mountain hemlock settles in best, owing to mild temperatures and damp soil. Ensuring a cool, shady locale mimics its native highland habitat, and substantial mulch post-transplant can help ease the transition.
III. Uses and Benefits
- Ornamental uses
Outside of its native range, mountain hemlock is grown as an ornamental tree. It is planted as a specimen tree in native plant landscapes in California, and particularly in gardens in northern Great Britain and Scandinavia, where it is appreciated for its blue–green color and tolerance of severe weather.
- Other uses
Native Americans used the species less often than western hemlock, but specifically used mountain hemlock boughs as beds and bathing brushes.
Since the mid-20th century, the species has been used for lumber, often mixed with western hemlock.
