Tsuga mertensiana, mountain hemlock
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Coniferous Trees

Tsuga mertensiana

mountain hemlock

Pinaceae

Alaska south through BC, Washington, Oregon to California; subalpine to alpine mountain forests 3,000–7,500 ft in Cascades, Olympics, Coast Ranges

At a Glance

TypeTree
HabitUpright
FoliageEvergreen
Height50-100 feet (15-30 m)
Width15-30 feet (4.6-9 m)
Maturity50 years

Growing Zones

USDA Hardiness Zones

4 - 8
Zone 4
Zone 5
Zone 6
Zone 7
Zone 8
These zones indicate the coldest temperatures this plant can typically survive.
Frost Tolerancehardy

Key Features

Deer Resistant
Native to North America
Maintenancevery low

Overview

A large, long-lived, evergreen coniferous tree in the family Pinaceae, native to subalpine and alpine forests from Alaska south through British Columbia, Washington, Oregon, and into California, primarily in the Cascade Range, Olympic Mountains, and Coast Ranges at elevations of 3,000–7,500 feet (900–2,300 m) — the characteristic conifer of Pacific Northwest high-mountain forests and the timberline zone. Named for Karl Heinrich Mertens (1796–1830), a German botanist who participated in a Russian expedition to Alaska and the Pacific Coast. Mature trees grow 50–100 feet (15–30 m) tall with a narrow to broadly conical crown. Mountain hemlock is immediately distinguished from western hemlock (Tsuga heterophylla) by three characters: (1) NEEDLES ARRANGED RADIALLY all around the stem in a bottle-brush pattern — not in two flat ranks as in western hemlock; (2) BLUE-GREEN, GLAUCOUS foliage with a frosted appearance — not dark glossy green; and (3) LARGER CONES 1–3 inches (2.5–8 cm) long — noticeably larger than the 0.6–1 inch cones of western hemlock. Both species share the characteristic NODDING/DROOPING LEADER. Mountain hemlock's radially arranged foliage enables heavy snow loads to slide off the branches — an adaptation to its high-elevation snowpack environment. Non-toxic.

Native Range

Native to subalpine and alpine forests from Alaska south through British Columbia, Washington, Oregon, and into California, primarily in mountain ranges at 3,000–7,500 feet (900–2,300 m) elevation.

Suggested Uses

Planted as a specimen or screening conifer in cool, moist Pacific Northwest garden settings where a blue-green color and high-mountain character are desired. The blue-green glaucous needles, graceful drooping leader, and narrow spire-like form in youth distinguish it from other native PNW conifers. Suitable for large garden or landscape settings. Of educational value for comparing with western hemlock — the radial vs. two-ranked needle arrangement and cone size differences are immediately visible and memorable diagnostic characters.

How to Identify

Distinguished from western hemlock (T. heterophylla) by three characters: (1) NEEDLES RADIALLY ARRANGED all around stem in bottle-brush pattern — not in two flat ranks (mertensiana = named for Karl Mertens); (2) BLUE-GREEN GLAUCOUS frosted appearance — not dark glossy green; (3) LARGER CONES 1–3 inches (2.5–8 cm) — noticeably bigger than T. heterophylla cones (0.6–1 inch). Both hemlocks share NODDING/DROOPING LEADER. High-elevation specialist — subalpine and alpine forests 3,000–7,500 ft. Radially arranged needles = snow-shedding adaptation.

Appearance

Size & Dimensions

Height50' - 100'
Width/Spread15' - 30'

Reaches mature size in approximately 50 years

Colors

Flower Colors

brown

Foliage Colors

green
blue

Fall Foliage Colors

no change

Bloom Information

Bloom Period

~3 weeks
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Summer
Pollen released from small male strobili at branch tips in June–July (later than lowland conifers due to high elevation). Female cones 1–3 inches (2.5–8 cm) long, oval to cylindrical with thin rounded scales, ripen September–October, releasing small winged seeds. Wind-pollinated. Cones often persist on branches through the following year. Seeds provide food for red crossbills, Clark's nutcrackers, and other high-elevation seed-eating birds.

Detailed Descriptions

Flower Description

LARGER CONES 1–3 inches (vs. 0.6–1 inch for T. heterophylla) — most visible difference from western hemlock; ripen Sept–Oct; wind-pollinated; crossbill/nutcracker seed food; DATA CORRECTIONS: species null; rewrote curly-brace marketing prose

Foliage Description

BLUE-GREEN GLAUCOUS — frosted appearance (vs. dark glossy green of T. heterophylla); RADIALLY ARRANGED in bottle-brush pattern around stem (vs. two flat ranks); mertensiana = named for Karl Mertens; nodding/drooping leader shared with T. heterophylla; subalpine specialist 3,000–7,500 ft; radial arrangement = snow-shedding adaptation

Growing Conditions

Sun Requirements

Full Sun
Partial Shade
Requires 3-10 hours of direct sunlight daily
• Full Sun: 6+ hours of direct sunlight
• Partial Shade: 3-6 hours of direct sunlight
• Full Shade: Less than 3 hours of direct sunlight

Soil Requirements

pH Range4.5 - 6.5(Acidic)
357912
Soil Types
loamsandrocky
Drainage
well drained

Water & Climate

Water Needs

Medium

Frost Tolerance

hardy

Time to Maturity

20-50 years

Care & Maintenance

Care Guide

Plant in full sun to partial shade in well-drained to moist, acidic soil with a pH of 4.5–6.5. Prefers cool temperatures and high humidity — best suited to cooler, moister Pacific Northwest lowland gardens that approximate mountain conditions. Requires consistent moisture but excellent drainage. Slower-growing than western hemlock. The blue-green foliage color is most intense in full sun. Not suited to hot, dry lowland conditions.

Pruning

No pruning required for natural form. Lower branches can be removed as the tree matures to raise the crown. Do not top. Minimal maintenance required.

Pruning Schedule

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late spring

Maintenance Level

very low

⚠️ Toxicity Warning

Non-toxic