Mountain Hemlock
Tsuga mertensiana
Pinaceae · coniferous tree · native
Last updated
Mountain hemlock is a native conifer of high elevations from southern Alaska to central California. It reaches 30-100 ft with an erect form and slow growth. It produces yellow flowers and brown cones with fine, gray-green foliage.
Mountain hemlock is hardy in zones 5a-8b and prefers partial shade with well-drained soil (pH 4.5-7.3). It requires high moisture and long cold winters with heavy snowpack. Growth is slow and takes decades to reach mature size. It is notably intolerant of low-elevation, warm summer conditions.
Quick Facts
Aecial Stage (Alternate Host)
Wound Infection Window
Crawler Emergence
Active Below-ground Growth
+ 2 more — see full disease and pest lists below
Phenological Calendar
| Stage | Typical Window |
|---|---|
| New growth flush BBCH 11 | May 15-Jun 15 |
| Pollen release BBCH 61 | Jun 1-Jun 30 |
| Pollen shed complete BBCH 69 | Jun 15-Jul 15 |
| Cone development BBCH 71 | Jun 1-Aug 31 |
| Cone maturity and seed shed BBCH 85 | Sep 1-Nov 30 |
Diseases: Regionally Documented (10)
Pests: Regionally Documented (5)
Cultivars (1)
Mountain hemlock is a high-elevation species that does not belong in the Puget Sound lowlands. It requires long cold winters with heavy snowpack, cool summers, and 42-140 inches of annual precipitation concentrated in the mountains. Planted at sea level in Kent, it will suffer from summer heat, inadequate cold dormancy, and disease pressure from root pathogens that are absent at its native elevation. If you want a hemlock for a lowland landscape, plant western hemlock instead. Mountain hemlock belongs above 3,000 feet.
— Chris Welch, ISA Certified Arborist